Green tea

Green tea is made from the leaves of the plant Camellia sinensis of the Theaceae family that can grow up to 16 meters tall ¹. Moreover, green, black, white and oolong teas all come from the same plant, Camellia sinensis, but are prepared using different methods. Green tea is made by processing fresh Camellia sinensis leaves using heat or hot steam immediately after collection, thus minimising any oxidation processes. White tea is produced with minimal fermentation from new buds and young Camellia sinensis leaves, which are harvested only once a year in early spring ². In black tea the leaves undergo several treatments, including withering by blowing air, preconditioning, ‘cut‐tear‐curl’, fermentation and final drying, which result in an oxidised tea ³. Oolong tea is made from wilted, bruised, and partially oxidized Camellia sinensis leaves, creating an intermediate kind of tea. Shortly after harvesting, tea leaves begin to wilt and oxidize. During oxidation, chemicals in the leaves are broken down by enzymes, resulting in darkening of the leaves and the well-recognized aroma of tea. This oxidation process can be stopped by heating, which inactivates the enzymes. The amount of oxidation and other aspects of processing determine a tea’s type. Black tea is produced when tea leaves are wilted, bruised, rolled, and fully oxidized. In contrast, green tea is made from unwilted leaves that are not oxidized. Oolong tea is made from wilted, bruised, and partially oxidized leaves, creating an intermediate kind of tea. White tea is made from young leaves or growth buds that have undergone minimal oxidation. Dry heat or steam can be used to stop the oxidation process, and then the leaves are dried to prepare them for sale.

Tea (Camellia sinensis) is the most highly consumed manufactured drink in the world, second only to water ⁴, ⁵. Approximately 20% of the world’s Camellia sinensis consumption is in the form of green tea; with black tea accounting for about 75 percent of the world’s tea consumption ⁶, ⁷. In the United States, United Kingdom and Europe, black tea is the most common tea beverage consumed; green tea is the most popular tea in Japan and China ⁸. Oolong and white tea are consumed in much lesser amounts around the world ⁸.

Green tea (Camellia sinensis L.) has been reported to show multiple bioactivities with health benefits, such as antioxidant, anti-inflammation, hepatoprotection, cardiovascular protection, neuroprotection, and anti-cancer ⁹, ¹⁰, ¹¹. The epidemiological studies showed that green tea consumption can result in a decreased risk of metabolic syndrome, but there is not enough evidence to draw a strong conclusion regarding tea and non-alcoholic fatty liver ¹². Moreover, accumulating in vivo (animal studies) evidence suggested that green tea showed hepatoprotective effects, which can ameliorate the liver injury induced by alcohol, cholesterol, chemicals, or drugs ¹³, ¹⁴. These benefits are mainly due to the richness of bioactive compounds like polyphenols, polysaccharides, and amino acids ¹⁵.

The active ingredients of green tea are composed of polyphenols, alkaloids (caffeine, theophylline, and theobromine), amino acids, carbohydrates, proteins, chlorophyll, volatile organic compounds (chemicals that readily produce vapors and contribute to the odor of tea), fluoride, aluminum, minerals, and trace elements ¹⁶, ¹⁷, ¹⁸. The polyphenols, most of which are flavonols, commonly known as catechins ¹⁹, are thought to be responsible for the health benefits that have traditionally been attributed to tea, especially green tea. Catechins are a part of the chemical family of flavonoids, a naturally occurring antioxidant, and a secondary metabolite in certain plants. Green tea contains higher amounts of catechins than black tea ²⁰ and green tea processing prevents oxidation ²¹. After fermentation from green to black tea, about 15% of catechins remain unchanged while the rest of the catechins are converted to theaflavins, which are polyphenol pigments and thearubigins ²². Brewing conditions, including water temperature and infusion time, influence the antioxidant capacity of green tea ²³. The active catechins and their respective concentrations in green tea infusions are listed in the Table 1 below.

Although iced and ready-to-drink teas are becoming popular worldwide, they may not have the same polyphenol content as an equal volume of brewed tea ²⁴. The polyphenol concentration of any particular tea beverage depends on the type of tea, the amount used, the brew time, and the temperature ¹⁶. The highest polyphenol concentration is found in brewed hot tea, less in instant preparations, and lower amounts in iced and ready-to-drink teas ¹⁶. As the percentage of tea solids (i.e., dried tea leaves and buds) decreases, so does the polyphenol content ²⁵. Ready-to-drink teas frequently have lower levels of tea solids and lower polyphenol contents because their base ingredient may not be brewed tea ²⁶. The addition of other liquids, such as juice, will further dilute the tea solids ²⁵. Decaffeination reduces the catechin content of teas ²⁷.

Dietary supplements containing green tea extracts are also available ⁷. In a U.S. study that evaluated 19 different green tea supplements for tea catechin and caffeine content, the product labels varied in their presentation of catechin and caffeine information, and some values reported on product labels were inconsistent with analyzed values ⁷.

Green tea catechins are well recognized for their essential anti-inflammatory, photo-protective, antioxidant, and chemo-preventive functions. The catechins found in green tea include epigallocatechin‐3‐gallate (EGCG), epigallocatechin (EGC), epicatechin‐3‐gallate (ECG) and epicatechin (EC), gallocatechins and gallocatechin gallate. Epigallocatechin-3-gallate (EGCG) is the most active, abundant and most studied catechin in green tea ²⁰, as it is a powerful antioxidant believed to be an important determinant of the therapeutic qualities of green tea ²⁸. It is suggested that EGCG works by suppressing the formation of new blood vessels (angiogenesis) and regulating their permeability, thereby cutting off the blood supply to cancerous cells ²⁹. Several in-vitro studies (test tube studies) and in-vivo animal and human studies demonstrated green tea catechin’s anti-inflammatory, antioxidant, photo-protective, and chemo-preventative effects after topical application and/or oral ingestion ³⁰, ³¹, ³², ³³.

Tea is brewed from dried leaves and buds (either in tea bags or loose), prepared from dry instant tea mixes, or sold as ready-to-drink iced teas. So-called herbal teas are not really teas but infusions of boiled water with dried fruits, herbs, and/or flowers.

• Green, black, white and oolong teas all come from the same plant, Camellia sinensis, but are prepared using different methods. To produce green tea, fresh leaves from the plant are lightly steamed and crushed.
• Tea has been used for medicinal purposes in China and Japan for thousands of years.
• Green tea is consumed as a beverage. Green tea is available loose or in bags and a typical cup (200 mL) contains 40–60 mg of caffeine, 8–25 mg of L-theanine, and 25–200 mg of catechins ³⁴. Additionally, green tea supplements (e.g., green tea extract, catechins, L-theanine) are available in the forms of pills, capsules, liquid, and powder.
• Current uses of green tea as a beverage or dietary supplement include improving mental alertness, relieving digestive symptoms and headaches, and promoting weight loss.
• Green tea and its extracts, such as one of its components, EGCG (epigallocatechin‐3‐gallate), have been studied for their possible protective effects against heart disease and cancer.
• It is also sold in liquid extracts, capsules, and tablets and is sometimes used in topical products (intended to be applied to the skin).
• White Tea is the least processed, and it has a light, sweet flavor. One laboratory study showed white tea sped up the breakdown of existing fat cells and blocked the formation of new ones. Whether it has the same effects in the human body remains to be seen.
• Black Tea is the type of tea that’s often served in Chinese restaurants and used to make iced tea. Black tea is produced when tea leaves are wilted, bruised, rolled, and fully oxidized– a process that allows it to change chemically and often increases its caffeine content. The tea has a strong, rich flavor. Whether it helps with weight loss isn’t certain. But research done on rats suggests substances called polyphenols in black tea might help block fat from being absorbed in the intestines.
• Oolong Tea is made by drying tea leaves in the hot sun. Like green tea, it’s a rich source of catechins. In one study, more than two-thirds of overweight people who drank oolong tea every day for six weeks lost more than 2 pounds and trimmed belly fat.
• Except for decaffeinated green tea products, green tea and green tea extracts contain substantial amounts of caffeine. The average cup of green tea, however, contains from 10-50 mg of caffeine and over-consumption may cause irritability, insomnia, nervousness, and tachycardia (fast heart rate).

Table 1. Catechin concentrations of green tea infusions

Catechin in Green Tea Infusion	Catechin Concentration (mg/L)*	Catechin Concentration (mg/8 fl oz)*
Epigallocatechin-3-gallate (EGCG)	117–442	25–106
Epigallocatechin (EGC)	203–471	49–113
Epicatechin-3-gallate (ECG)	17–150	4–36
Epicatechin (EC)	25–81	6–19

Abbreviations: *mg = milligram; L = liter; fl oz = fluid ounce.

[Source ³⁵ ]

Figure 1. Green tea leaves

Figure 2. Matcha powder

Figure 3. Green tea catechins chemical structure

[Source ³⁶ ]

What are antioxidants?

Antioxidants are substances that are found in many foods, including fruits and vegetables and in vitamin and mineral supplements, that can neutralize free radicals in your body and stop them damaging your health. But while they’re generally healthy for you, antioxidants can cause problems, too.

Antioxidants became prominent 20 years ago, because research suggested they would be able to prevent heart disease and many other chronic conditions. This research led many people to start taking antioxidant supplements.

The best known antioxidants are:

• Beta-carotene
• Lutein
• Lycopene
• Selenium
• Vitamin A
• Vitamin C
• Vitamin E

Your body also produces its own antioxidants.

Vegetables and fruits are rich sources of antioxidants. There is good evidence that eating a diet with lots of vegetables and fruits is healthy and lowers risks of certain diseases. But it isn’t clear whether this is because of the antioxidants, something else in the foods, or other factors. Official U.S. Government policy urges people to eat more vegetables and fruits. Concerns have not been raised about the safety of any amounts of antioxidants in food.

Antioxidant molecules have been shown to counteract oxidative stress in laboratory experiments (for example, in cells or animal studies). However, there is also some concern that consuming antioxidant supplements in excessive doses may be harmful in some cases. For example, high doses of beta-carotene may increase the risk of lung cancer in smokers. High doses of vitamin E may increase risks of prostate cancer and one type of stroke. Antioxidant supplements may also interact with some medicines. To minimize risk, tell you of your health care providers about any antioxidants you use.

What are free radicals?

Inside the cells in your body, many chemical reactions take place. Free radicals are highly unstable molecules that are naturally formed when you exercise and when your body converts food into energy. Your body can also be exposed to free radicals from a variety of environmental sources, such as cigarette smoke, air pollution, and sunlight. Free radicals can cause “oxidative stress,” a process that can trigger cell damage. Oxidative stress is thought to play a role in a variety of diseases including some cancers, cardiovascular diseases, diabetes, Alzheimer’s disease, Parkinson’s disease, and eye diseases such as cataracts and age-related macular degeneration. But not all free radicals bad. Some are used by your body’s immune system to attack viruses or bacteria.

What is matcha tea?

Matcha is finely ground powder also called fine powdered tea of specially grown and processed green tea [Camellia sinensis] leaves ³⁷. Matcha tea is special in two aspects of farming and processing: the green tea plants for matcha are shade-grown for about three weeks before harvest and the stems and veins are removed in processing. During shaded growth, the plant Camellia sinensis slows down growth, stimulates an increase in chlorophyll levels, turns the leaves a darker shade of green, and causes the production of amino acids, in particular theanine and produces more caffeine. The powdered form of matcha is consumed differently from tea leaves or tea bags, and is dissolved in a liquid, typically water or milk.

Is green tea safe?

Tea as a food item is generally recognized as safe by the U.S. Food and Drug Administration. Green tea is believed to be safe when consumed as a beverage in amounts up to 8 cups per day ³⁸. Multiple meta-analyses have reported that green tea consumption is safe at moderate and regular amounts (3 to 5 cups per day, up to 1200 ml/day) and side effects are mild ³⁹, ⁴⁰, ⁴¹. Three drugs are known to interact with green tea: warfarin (also known as Coumadin™ and Jantoven™), anisindione (or Miradon™), and dicumarol. Green tea consumption may reduce the levels of folic acid in the body and interfere with iron absorption. Because green tea contains caffeine, pregnant women and people with cardiovascular problems or other health risks should consult their physician or healthcare provider about consuming green tea. Caffeine in green tea can also interact with some medications.

Green tea contains caffeine. Keep in mind that only the amount of added caffeine must be stated on product labels and not the caffeine that naturally occurs in green tea. The amount of caffeine present in tea varies by the type of tea; the caffeine content is higher in black teas, ranging from 64 to 112 mg per 8 fl oz (237 ml) serving, followed by oolong tea, which contains about 29 to 53 mg per 8 fl oz (237 ml) serving ¹⁹. Green and white teas contain slightly less caffeine, ranging from 24 to 39 mg per 8 fl oz (237 ml) serving and 32 to 37 mg per 8 fl oz (237 ml) serving, respectively ⁴². Decaffeinated teas contain less than 12 mg caffeine per 8 fl oz (237 ml) serving ⁴². As with other caffeinated beverages, such as coffee and colas, the caffeine contained in many tea products could potentially cause adverse effects, including tachycardia, palpitations, insomnia, restlessness, nervousness, tremors, headache, abdominal pain, nausea, vomiting, diarrhea, and diuresis (excessive production of urine) ⁴³. However, there is little evidence of health risks for adults consuming moderate amounts of caffeine (about 300 to 400 mg per day). A review by Health Canada concluded that moderate caffeine intakes of up to 400 mg per day (equivalent to 6 mg per kilogram [kg] body weight) were not associated with adverse effects in healthy adults ⁴⁴. Drinking green tea may be safe during pregnancy and while breastfeeding when consumed in amounts up to 6 cups per day (no more than about 300 mg of caffeine). Drinking more than this amount during pregnancy may be unsafe and may increase the risk of negative effects. Green tea may also increase the risk of birth defects associated with folic acid deficiency ³⁸. Caffeine passes into breast milk and can affect a breastfeeding infant. Research on the effects of caffeine in children is limited ⁴³. In general, caffeine doses of less than 3.0 mg per kg body weight have not resulted in adverse effects in children ⁴³. Higher doses have resulted in some behavioral effects, such as increased nervousness or anxiety and sleep disturbances ⁴⁴.

Aluminum, a neurotoxic element, is found in varying quantities in tea plants ⁴⁵. Studies have found concentrations of aluminum (which is naturally taken up from soil) in infusions of green and black teas that range from 14 to 27 micrograms per liter (μg/L) to 431 to 2239 μg/L ¹⁹. The variations in aluminum content may be due to different soil conditions, different harvesting periods, and water quality ¹⁹. Aluminum can accumulate in the body and cause osteomalacia and neurodegenerative disorders, especially in individuals with renal failure ¹⁹. However, it is not clear how much of the aluminum in tea is bioavailable, and there is no evidence of any aluminum toxicity associated with drinking tea ¹⁹.

Black and green tea may inhibit iron bioavailability from the diet ¹⁹. This effect may be important for individuals who suffer from iron-deficiency anemia ¹⁹. The authors of a systematic review of 35 studies on the effect of black tea drinking on iron status in the UK concluded that, although tea drinking limited the absorption of non-heme iron from the diet, there was insufficient evidence to conclude that this would have an effect on blood measures (i.e., hemoglobin and ferritin concentrations) of overall iron status in adults ⁴⁶. However, among preschool children, statistically significant relationships were observed between tea drinking and poor iron status ⁴⁶. The interaction between tea and iron can be mitigated by consuming, at the same meal, foods that enhance iron absorption, such as those that contain vitamin C (e.g., lemons), and animal foods that are sources of heme iron (e.g., red meat) ¹⁹. Consuming tea between meals appears to have a minimal effect on iron absorption ¹⁹.

Epigallocatechin-3-gallate (EGCG) supplements are considered safe for most people when taken at commonly used doses (300–400 mg/day) ⁴⁷, ⁴⁸, but high doses (800–1600 mg of EGCG per day) may negatively affect liver function ⁴⁹. Most safety data on long-term EGCG intake come from large meta-analyses of tea consumption, which have reported that side effects are mild [15][16] and can include nausea and upset stomach ⁴⁸, ⁵⁰.

Safety studies have looked at the consumption of up to 1200 mg of epigallocatechin-3-gallate (EGCG) in supplement form in healthy adults over 1- to 4-week time periods ⁵¹, ⁵². The adverse effects reported in these studies included excess intestinal gas, nausea, heartburn, stomach ache, abdominal pain, dizziness, headache, and muscle pain ⁵¹, ⁵². In a Japanese study, children aged 6 to 16 years consumed a green tea beverage containing 576 mg catechins (experimental group) or 75 mg catechins (control group) for 24 weeks with no adverse effects ⁵³. The safety of higher doses of catechins in children is not known.

Green tea is an ingredient in many over-the-counter weight loss products, some of which have been identified as the likely cause of rare cases of liver injury. Although uncommon, liver problems have been reported in a number of people who took concentrated green tea extracts in pill form ⁵⁴. More than 100 instances of clinically apparent liver injury attributed to green tea extract have been reported in the literature ⁵⁴. Liver injury typically arises within 1 to 6 months of starting the product but longer and shorter latencies (particularly with reexposure) have been reported. The majority of cases present with an acute hepatitis-like syndrome and a markedly hepatocellular pattern of serum enzyme elevations. Most patients recover rapidly upon stopping the green tea extract or the herbal and dietary supplements, although fatal instances of acute liver failure have been described ⁵⁴. Biopsy findings show necrosis, inflammation, and eosinophils in a pattern resembling acute hepatitis. Immunoallergic and autoimmune features are usually absent or minimal. A small number of similar cases have also been described after drinking green tea “infusions” rather than taking oral preparations of extracts of green tea. Experts suggest that green tea extracts should be taken with food, people with liver problems or liver disease should not take green tea extracts, and users should discontinue use and consult a health care provider if they develop symptoms of liver trouble, such as abdominal pain, dark urine, or jaundice (yellowing of the skin or eyes).

Based on the available data on the potential adverse effects of green tea catechins on the liver, the European Commission to the European Food Safety Authority (EFSA) and the Scientific Panel on Food Additives and Nutrient Sources added to Food ² concluded that there is evidence from interventional clinical trials that intake of green tea extracts at doses equal or above 800 mg EGCG/day taken as a food supplement for 4 months or longer has been shown to induce a statistically significant increase of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) (commonly found in liver damage or liver disease) in treated subjects (usually less than 10%) compared to control ².

Catechins in green tea extracts, either consumed as a beverage or in liquid or dry form as dietary supplements, may be more concentrated, may differ in composition and pattern of consumption compared to catechins from traditional green tea infusions and cannot be regarded as safe according to the presumption of safety approach, as exposure to green tea extracts at and above 800 mg EGCG/day in intervention studies causes elevated serum transaminases which is indicative of liver injury.

The European Commission to the European Food Safety Authority Panel concluded that it was not possible to identify an EGCG dose from green tea extracts that could be considered safe. From the clinical studies reviewed there is no evidence of hepatotoxicity (liver damage) below 800 mg EGCG/day up to 12 months ². However, hepatotoxicity (liver damage) was reported for one specific product containing 80% ethanolic extract at a daily dose corresponding to 375 mg EGCG ².

Green tea at high doses has also been shown to reduce blood levels and therefore the effectiveness of the drug nadolol, a beta-blocker used for high blood pressure and heart problems. It may also interact with other medicines.

Green tea health benefits

Due to the high content of antioxidant compounds, tea has long been regarded as an aid to good health and many believe it can prevention of chronic diseases and reduce the risk of cancer ⁵⁵, ⁵⁶, as well as possible beneficial effects on cardiovascular disease, insulin sensitivity and lipid profiles ⁵⁷, ⁵⁸, ⁵⁹. A small number of studies suggests that both green and black tea might have beneficial effects on some heart disease risk factors, including blood pressure and cholesterol. The research has limitations though, including how the data was evaluated and differences in study populations, so no definite conclusions have been reached.

Most studies of tea and cancer prevention have focused on green tea ⁶⁰. Although tea and/or tea polyphenols have been found in animal studies to inhibit tumorigenesis (formation of a tumor or tumors) at different organ sites, including the skin, lung, oral cavity, esophagus, stomach, small intestine, colon, liver, pancreas, and mammary gland ⁶¹, the results of human studies—both epidemiologic and clinical studies—have been inconclusive. Studies of green tea and cancer in people have had inconsistent results. Overall, findings from experimental and nonexperimental epidemiological studies yielded inconsistent results, thus providing limited evidence for the beneficial effect of green tea consumption on the overall risk of cancer or on specific cancer sites. Based on current evidence, it isn’t possible to say whether green tea helps to prevent cancer ⁴¹. A beneficial effect of green tea consumption on cancer prevention remains unproven so far. Caution is advised regarding supplementation with high‐dose green tea extracts due to the possible adverse effects ⁴¹. The National Cancer Institute does not recommend for or against using green tea to reduce the risk of any type of cancer ⁴⁵. Furthermore, Current research shows that green tea and the polyphenol epigallocatechin-3-gallate (EGCG), an antioxidant component of green tea, can impact the pharmacokinetics (the activity of drugs in the body over a period of time, including the processes by which drugs are absorbed, distributed in the body, localized in the tissues, and excreted) or pharmacodynamics (the intensity of a drug effect in relation to its concentration in a body fluid, usually at the site of drug action, ‘what the drug does to the body’) of certain drugs, thus impacting the metabolism and effectiveness of anticancer drugs ⁶².

Drinking coffee or green tea is associated with many health benefits, such as better cardiovascular health, lower inflammation levels, and a reduced risk for developing chronic disease ⁶³. A study published online Oct. 20, 2020, by BMJ Open Diabetes Research & Care ⁶³ suggests tea and coffee consumption are also linked to a reduced risk for early death among people with diabetes. Researchers evaluated the health and self-reported lifestyle habits — including diet — of about 5,000 people in Japan over five years. Compared with people who didn’t drink any coffee or green tea, people who drank the most had much lower risks of premature death from any cause: about 40% lower for people who drank either two cups of coffee or four cups of tea per day; and 63% lower for people who drank both four cups of tea plus two cups of coffee per day ⁶³. The study is only observational and doesn’t prove that drinking coffee or green tea keep people alive longer.

Table 2. Clinical trials for Camellia sinensis

Study (Author, Year, Country)	Study Design	Sample Size	Population	Type of Plant	Intervention	Duration of Treatment	Results
DIABETES
Alves Ferreira et al., 2017, Brazil 64	Randomized, double-blind, placebo-controlled study	120	Women (20–45 years) abnormal glucose values	Green tea capsules	Group 1: control (cellulose) Group 2: green tea (1 g) Group 3: metformin (1 g) Group 4: green tea (1 g) + metformin (1 g)	12 weeks	Improving glycemic and lipid profile ↓ Fasting glucose ↓ Total cholesterol and LDL
Lasaite et al., 2014, Lithuania 65	Randomized double-blind placebo-controlled study	56	Patients (37–78 years) with diabetes mellitus type 2 and diabetic retinopathy, nephropathy or neuropathy	Green tea extract	Group 1: placebo Group 2: Gingko biloba dry extract Group 3: green tea extract For extracts: one capsule twice a day (9 months) and one capsule three times a day (9 months)	18 months	No statistically significant differences in HbA1c level, antioxidant state, and psychological data
Mahmoud et al., 2016, Kuwait 66	Randomly assigned	34	Male and female type 2 diabetics	Black tea infusions	Group 1: three cups black tea daily (600 mL) Group 2: one cup black tea daily (200 mL)	12 weeks	↓ HbA1c levels ↑ Regulatory T cells ↓ Pro-inflammatory
Spadiene et al., 2014, Lithuania 67	Randomized, double-blind, placebo-controlled study	45	Patients (35-80 years) with diabetes mellitus type 2 and diabetic retinopathy, nephropathy or neuropathy	Green tea extract	Group 1: green tea extract Group 2: placebo	9–18 months	↓ Lipid peroxidation
Vaz et al., 2018, Brazil 68	Randomized, double-blind, placebo-controlled study	60	Patients with diabetes	Green tea extract	Group 1: green tea extract (two capsules/day, containing 560 mg of polyphenols/each) Group 2: cellulose (two capsules/day)	20 weeks	No effect on total antioxidant capacity, glycemic control markers, and renal function ↑ SOD activity
HYPERCHOLESTEROLEMIA
Imbe et al., 2016, Japan 69	Randomized, double-blind, placebo-controlled trial	155	Healthy volunteers High LDL cholesterol levels Aged 20–80 years	“Benifuuki” green tea	Group 1: “Benifuuki” Group 2: “Yabukita” Group 3: barley infusion drinker	12 weeks	↓ LDL cholesterol levels ↓ Lectin-like oxidized LDL receptor-1 containing LAB level
Orem et al., 2017, Canada 70	Randomized, double-blind, placebo-controlled study	125	Subjects 25–60 years hypercholesterolemia	Black tea	Group 1: placebo Group 2: instant black tea Group 3: functional black tea	4 weeks	Functional black tea: ↓ Total cholesterol ↓ LDL ↓ Oxidative stress index ↑ Total antioxidant status
Troup et al., 2015, United States 71	Randomized, double-blind, crossover trial	57	45–65 years, hypercholesterolemia	Black tea	Group 1: controlled low flavonoid diet plus five cups per day of black tea Group 2: Placebo	4 weeks	↓ LDL/HDL ratio ↓ Total cholesterol
HYPERTENSION
Alkerwi et al., 2015, Luxembourg 72	National cross-sectional stratified sample	1352	18–69 years	Tea	Group 1: nonconsumers Group 2: ≤ 3-dL/d consumers (tea/coffee) Group 3: > 3-dL/d consumers (tea/coffee)	–	↓ Systolic BP and pulse pressure
METABOLIC SYNDROME
Yang et al., 2014, China 73	–	134	Metabolic syndrome	Green tea extract	Group 1: green tea extract (500 mg). Two capsules/time/day Group 2: control (water)	45 days	↑ Adiponectin serum concentrations ↓ Visfatin levels
OBESITY
Chen et al., 2016, Taiwan 74	Randomized, double-blind trial	102	Women BMI ≥ 27 kg/m² Waist circumference ≥ 80 cm	EGCG	Group 1: placebo Group 2: high dose green tea	12 weeks	↓ Weight ↓ Waist circumference ↓ Total cholesterol and LDL plasma levels
Dostal et al., 2016, USA 75	Randomized, double-blind, placebo-controlled clinical trial	937	Postmenopausal women aged 50–70 with high breast density and overweight/obese	Green tea extract	Group 1: placebo Group 2: EGCG (843 mg), four capsules daily	12 months	No ↓ adiposity No improvements in BMI ↓ Tissue fat and gynoid fat
Huang et al., 2018, Taiwan 76	Randomized, double-blind, crossover, placebo-controlled	90	Women (18 – 65 years) BMI  ≥  27 kg/m² LDL-C  ≥  130 mg/dL	Green tea extract	Group 1: placebo Group 2: one capsule 30 min after meal, three times a day, green tea extract	6 weeks	↑ Leptin ↓ LDL
Janssens et al., 2015, The Netherlands 77	Randomized, placebo-controlled, single-blind design	60	Caucasian men and women with body mass index from 18 kg/m², age: 18–50	Green tea extract	Group 1: placebo Group 2: green tea (capsules > 0.06 g EGCG and 0.03–0.05 g caffeine)	12 weeks	No effect on fecal energy content, fecal fat content, resting energy expenditure, respiratory quotient, and body composition
Mielgo-Ayuso et al., 2014, Spain 78	Randomized, double-blind, parallel design	83	Obese (30 kg/m². BMI, 40 kg/m²) premenopausal women	EGCG	Group 1: placebo (lactose) Group 2: EGCG (300 mg/d)	12 weeks	No changes in body weight No changes in adiposity
Nicoletti et al., 2019, Brazil 79	Longitudinal interventional study	11	Women (18–60 years) (BMI) > 40 kg/m²	EGCG	Group 1: eutrophic women Group 2: decaffeinated green tea capsules with 450.7 mg of EGCG, two capsules/day	8 weeks	↑ RICTOR ↑ HIF1-α expression
OSTEOPOROSIS
Amorim et al., 2018, Brazil 80	Double-blind, randomized, controlled clinical trial	35	≥ 18 years old Diabetes for more than 5 years.	Green tea extract	Group 1: cellulose Group 2: 1120 mg of green tea extract contains 560 mg of polyphenols/day	10 and 20 weeks	↑ Bone mineral content

Green tea for cancer

There is no real evidence that green tea can help treat cancer. Several clinical trials have investigated the role of tea and tea polyphenols in cancer prevention ⁸¹, ⁸², ⁸³, ⁸⁴, ⁸⁵, ⁸⁶, ⁸⁷, ⁸⁸, ⁸⁹.

People with cancer also drink green tea because they believe it might:

• boost their immune system which might help them fight their cancer
• improve health, energy levels and well being
• get rid of toxins in the body
• give them some control over their cancer and its treatment
• treat their cancer if no other conventional treatment can

The media has also promoted black tea as an anti cancer agent. But at the moment the evidence is not strong enough to know this for sure.

Two randomized trials evaluated the effects of tea extracts on premalignant oral lesions ⁸⁸, ⁸⁹. One of the trials was a double-blind interventional trial involving 59 people with leukoplakia, which is a putative precursor lesion for oral cancer ⁸⁸. The trial’s participants were randomly assigned to receive either 3 grams of a mixed tea product, given both orally and topically, or a placebo. After 6 months, 38 percent of the participants in the treatment group had partial regression of their oral lesions compared with 10 percent of the participants in the placebo group. In addition, fewer participants in the treatment group than in the placebo group had an increase in lesion size (3 percent in the treatment group versus 7 percent in the placebo group). Furthermore, mucosal cell proliferation decreased in the treatment group, suggesting a possible protective effect of tea on the development of oral cancer. In contrast, in the second trial, 39 people with high-risk premalignant oral lesions were randomly assigned to receive one of three doses of a green tea extract—500 mg per square meter of body surface area (mg/m²), 750 mg/m², or 1000 mg/m²—or a placebo three times daily for 12 weeks ⁸⁹. At the end of the trial, no differences in lesion responses or histology were found between the groups.

Two other randomized trials examined the effects of tea on urine levels of 8-hydroxydeoxyguanosine (8-OHdG), a biomarker of oxidative DNA damage that may be a predictor of increased cancer risk. Urinary 8-OHdG levels are higher in individuals with lung cancer than in control subjects, and human breast, lung, liver, kidney, brain, stomach, and ovarian tumor tissue has a higher content of 8-OHdG than adjacent nontumor tissue ⁸². In one trial, 133 adult heavy smokers were randomly assigned to drink 4 cups of one of the following beverages each day for 4 months: decaffeinated green tea, decaffeinated black tea, or water ⁸². Among those who drank green tea, there was a statistically significant 31 percent decrease in urinary levels of 8-OHdG; in the black tea group, there was no change in urinary 8-OHdG levels ⁸². In the second trial, 124 individuals at increased risk of liver cancer due to hepatitis B virus infection and aflatoxin exposure took a placebo or 500 mg or 1000 mg of a green tea polyphenol supplement daily ⁸³. The two supplement doses were reported to be equivalent to 2 or 4 cups, respectively, of green tea infusions. No other tea or tea products were consumed. Compared with those in the placebo group, individuals who took the green tea supplement at either dose for 3 months had substantially lower urinary 8-OHdG levels ⁸³. Although these trials indicate that green tea polyphenols from tea or supplements can reduce urinary 8-OHdG levels, it is unclear if reduced 8-OHdG levels are associated with reduced cancer risk.

Additional trials have investigated whether green tea catechins or green tea extracts alter prostate cancer risk. In a double-blind, placebo-controlled study, 60 men took 200 mg of green tea catechin or a placebo three times daily for 1 year ⁸¹. These men had high-grade prostatic intraepithelial neoplasia, which is thought to be a precursor of prostate cancer. After 1 year, fewer prostate cancers were detected in the green tea catechin group (1 cancer in 30 men) compared with the placebo group (9 cancers in 30 men) ⁸¹. Two other clinical trials, both uncontrolled studies, investigated the use of green tea extracts to reduce prostate-specific antigen levels in men with prostate cancer and found no evidence of such a reduction ⁸⁵, ⁸⁴.

Another trial examined the effect of tea polyphenols on serum pepsinogen levels in 163 individuals with high serum pepsinogen levels ⁸⁶. Serum pepsinogen is a biomarker of gastric atrophy and an indicator of increased risk for stomach cancer. The participants in this trial were given either one or six 100-mg capsules of tea polyphenols daily for 1 year. Each capsule was the equivalent of about 1.7 cups of tea. After 1 year, no decrease in serum pepsinogen levels was observed in either treatment group ⁸⁶.

In yet another trial, a possible role for green tea supplements in treating precancerous lesions of the esophagus was investigated ⁸⁷. In the trial, 200 Chinese participants with such lesions were treated with 5 mg of a decaffeinated green tea extract daily or a placebo. After 12 months, lesion histopathology was scored as improved, unchanged, or deteriorated. The trial found no difference between the treatment and placebo groups with regard to changes in the esophageal lesions or in abnormal cell proliferation ⁸⁷.

Green tea for weight loss

Obesity (body mass index ≥ 30) and overweight (body mass index ≥ 25) are increasing due to an excessive intake of energy-dense foods and sedentary lifestyle; obesity affects more than 1.9 billion adults worldwide ⁹⁰. Obesity causes about 4 million deaths globally and it is related to other prevalent pathologies including hypertension, type 2 diabetes mellitus, stroke, obstructive sleep apnea, and several cancers ⁹¹. Although many studies have been done on green tea and its extracts, definite conclusions cannot yet be reached on whether green tea is helpful for most of the purposes for which it is used including its use for weight loss. There are just not enough reliable data to determine whether produce a meaningful weight loss in adults who are overweight or obese ⁹². Green tea extracts also haven’t been shown to help people maintain a weight loss. Be wary of weight-loss claims. Some advertisements claim that tea can speed weight loss, but research on the effects of green tea and fat reduction have shown little promise of weight loss benefits. Moreover, it’s best to skip any so-called “diet” teas that may contain potentially harmful substances such as laxatives.

In a well-conducted 2012 Cochrane Review ⁹² the study authors looked at 15 weight loss studies and three studies measuring weight maintenance where some form of a green tea preparation was given to one group and results compared to a group receiving a control. Neither group knew whether they were receiving the green tea preparation or the control. Green tea preparations used for losing weight are extracts of green tea that contain a higher concentration of catechins and caffeine than the typical green tea beverage prepared from a tea bag and boiling water. A total of 1945 participants completed the studies, ranging in length from 12 to 13 weeks. In summary, the loss in weight in adults who had taken a green tea preparation was statistically not significant, was very small and is not likely to be clinically important. Similar results were found in studies that used other ways to measure loss in weight (body mass index, waist circumference). Studies examining the effect of green tea preparations on weight maintenance did not show any benefit compared to the use of a control preparation.

Green tea preparations appear to induce a small, statistically non-significant weight loss in overweight or obese adults ⁹². Because the amount of weight loss is small, it is not likely to be clinically important. Green tea had no significant effect on the maintenance of weight loss. Of those studies recording information on adverse events, only two identified an adverse event requiring hospitalisation. The remaining adverse events were judged to be mild to moderate.

Most adverse effects, such as nausea, constipation, abdominal discomfort and increased blood pressure, were judged to be mild to moderate and to be unrelated to the green tea or control intervention. No deaths were reported, although adverse events required hospitalisation. One study attempted to look at health-related quality of life by asking participants about their attitudes towards eating. Nine studies tracked participants’ compliance with green tea preparations. Studies did not include any information about the effects of green tea preparations on morbidity, costs or patient satisfaction.

In another study conducted in 2008, randomised blinded controlled trials that compared catechins (in green tea or capsules) – epigallocatechin-3-gallate (EGCG) + caffeine mixture, showed a small positive effect on weight loss and weight maintenance. However, the authors stated that further research was needed to assess whether (and to what extent) people are genetically predisposed to the effect of epigallocatechin gallate-caffeine mixtures. And due to the limitations in the review methodology mean that the overall effect size estimate was unlikely to be reliable and the conclusions should be treated with caution.

To get the same amount of EGCG used in the research, you’d need to drink about six to seven cups of your typical green tea every day. You could also try a green tea extract, but it might be risky. Though rare, high-dose tea extracts found in some weight-loss supplements have been linked to serious liver damage.

Recent studies confirm that a mixture of green tea catechins and caffeine has a beneficial effect on body-weight management through sustained energy expenditure, fat oxidation, and preservation of fat-free body mass ⁹³. Polyphenols such as EGCG can increase lipolysis, in addition to possessing anti-adipogenic effects through being a fatty-acid synthase, which is a possible therapeutic target for appetite and weight control ⁹⁴. Polyphenols have “exercise mimetic” properties through exercise-inducible pathways ⁹⁵.

Since the 2012 Cochrane Review, more clinical studies evaluating the weight loss activity of green tea and its main secondary metabolite epigallocatechin-3-gallate (EGCG) have been done. These clinical trials were mainly randomized, double-blind and placebo-controlled, except one which was single-blind and another one which was longitudinal. The duration of these clinical trials was mainly weeks (6, 8, and 12 weeks), although there is a study with 12-month intervention. There is also variability in the administered doses from 300 mg/day of EGCG to 856.8 mg/day. Analyzing the different clinical trials, it has been observed that high doses of EGCG (856.8 mg for 6–12 weeks) do have anti-obesity activity, reducing weight in women and men with a body mass index (BMI) ≥ 27 kg/m² ⁷⁴, ⁷⁶. However, in another study in which a dose of 843 mg of EGCG was administered for 12 months, it had no effect on reductions in adiposity nor body mass index (BMI). Compared with the previous cited studies, this lack of efficacy can be attributed to the fact that this last study has only been performed with postmenopausal obese women ⁹⁶. On the other hand, low doses of EGCG (300 mg/day and 560 g/day for 12 weeks) do not have any beneficial effect on weight control ⁷⁷. Finally, another clinical trial elucidated that the possible mechanism of action by which EGCG exerts its anti-obesity activity is by increasing HIF1-alpha (HIF1-α) and rapamycin-insensitive companion of mTOR (RICTOR) ⁷⁹.

Green tea for high cholesterol

Hypercholesterolemia which is high blood cholesterol values > 200 mg/dL affects over 39% of people worldwide, Europe and America being the most affected continents ⁶⁶. Green tea can be used as anti-hypercholesterolemic agent by acting in several ways, such as enhancing hepatic excretion of cholesterol or inhibiting absorption of cholesterol in the alimentary tract ⁹⁷. This is accompanied by increasing fecal bile acids and cholesterol excretion, resulting in the lowering of the plasma cholesterol ⁹⁸ and the lowering of low-density lipoprotein (LDL or “bad” cholesterol) oxidation by increasing cellular antioxidant status or inhibiting oxidizing enzyme activities in the arterial wall ⁹⁹. Other possible mechanisms include diminishing the levels of α-ketoglutarate and pyruvate dehydrogenases enzymes, which are vital in the cholesterol biosynthesis ¹⁰⁰ or inhibition of the rate limiting enzyme of cholesterol biogenesis, squalene epoxidase ¹⁰¹. Green tea also causes a prolongation in LDL oxidation lag time by flavonoids ¹⁰², inhibition of serum triglyceride elevation ¹⁰³ and prevention of fat storage in the liver, lowering blood lipids and increasing fecal excretion of triglycerides ¹⁰⁴.

Green tea extracts have demonstrated in in-vivo animal studies reduced total cholesterol, low-density lipoprotein (LDL or “bad” cholesterol), and tryglicerides ¹⁰⁵, ¹⁰⁶, ¹⁰⁷ which is mainly attributed to epigallocatechin gallate and flavonols ¹⁰⁸, ¹⁰⁹. Moreover, Chungtaejeon aqueous extracts, which is a Korean fermented tea, has shown to decrease cholesterol, total serum cholesterol, and LDL cholesterol in high fat atherogenic Wistar rats ¹¹⁰.

A good-quality review from 2013 ¹¹¹ of 11 studies involving 821 people found daily consumption of green and black tea (as a drink or a capsule) could help lower cholesterol and blood pressure thanks to tea and its catechins. The authors of the review caution that most of the trials were short term and more good quality long-term trials are needed to back up their findings.

Another good-quality review from 2011 ¹¹² found drinking green tea enriched with catechins led to a small reduction in cholesterol, a main cause of heart disease and stroke. However, it’s still not clear from the evidence how much green tea you’d need to drink to see a positive effect on your health, or what the long-term effects of drinking green tea are on your overall health.

Clinical trials on the anti-hypercholesterolemia action of black tea and green tea were investigated in patients with high cholesterol levels in randomized, double-bind, and placebo studies. The cholesterol-lowering effect of tea extracts was evaluated by measuring biochemical parameters (i.e., LDL content and total cholesterol) and antioxidant content. Both clinical studies with black tea demonstrated its effectiveness of reducing LDL/HDL ratio, total cholesterol, apolipoprotein B, and oxidative stress. In one of these clinical trials, the effective dose was 2.5 g black tea and phytosterol mixture which contains 1 g plant sterols for 4 weeks ⁷⁰. However, for the other study, a specific dose is not specified, but five cups of black tea per day for two 4-week treatment periods ⁷¹. On the other hand, the consumption of “Benifuuki” green tea, which is rich in methylated catechins (3 g of green tea extract/three times daily for 12 weeks) contributed significantly to reduce serum total cholesterol and serum LDL cholesterol compared to “Yabukita” green tea or barley infusion (placebo tea) consumers ⁶⁹.

Green tea for high blood pressure

High blood pressure or hypertension (blood pressure of ≥ 130/85 mm Hg) is one of the most common cardiovascular diseases which affects around 1.13 billion people worldwide. Endocrine hypertension occurs when there is a hormone imbalance as example in Cushing syndrome, primary aldosteronism, and pheochromocytoma ¹¹³.

Fresh green tea fermented under nitrogen gas produces gamma-aminobutyric acid (GABA)-rich tea, which was proved to prevent the occurrence of hypertension in salt-sensitive rats ¹¹⁴. Theanine (a unique amino acid of green tea) exerted a significant decline in blood pressure in humans following a dose-dependent manner ¹¹⁵. Epigallocatechin-3-gallate (EGCG) and epigallocatechin (EGC) were found to inhibit dopa decarboxylase enzymes in a concentration- and time-dependent manner, which is a known target for drugs used in hypertension ¹¹⁶. EGCG that was chronically infused in the hypothalamic paraventricular nucleus (PVN) attenuates hypertension by chronic inhibition of ROS (reactive oxygen species), in addition to regulating the balance of neurotransmitters, as well as cytokines, in the hypothalamic paraventricular nucleus ¹¹⁷. Green tea extract was found to prevent high angiotensin 2 dose-induced hypertension and the accompanied organ damage by preventing or scavenging superoxide anion generation ¹¹⁸. Decaffeinated green tea extract also reduces the metabolic syndrome through reduction of the formation of ROS, which results in lowered blood pressure ¹¹⁹.

Angiotensin-1 converting enzyme converts angiotensin 1 into angiotensin 2 (vasoconstrictor properties). Infusions and decoctions of four black tea samples (Doors tea, Siliguri tea, Guwahati tea, and Nilgiri tea) (15 μg/mL) were investigated for their ability to inhibit angiotensin 1 converting enzyme. In general, decoctions were more active than infusions and Nilgiri tea showed the highest inhibitory activity. Antihypertension properties are mainly attributed to thearubigin and theaflavin ¹²⁰. In another in vitro study, pretreatments with black tea extract (0.3–5 μg/mL) and theaflavin-3,3’-digallate (0.03–0.5 μg/mL) for 30 min improved endothelium dependent relaxations in homocysteine (endoplasmic reticulum stress inductor) treated cultured rat aortic endothelial cells ¹²¹. Moreover, San Cheang et al. ¹²¹ also investigated the effect of black tea extract (15 mg/kg/day for 2 weeks) in a rat model of angiotensin 2. This study revealed that black tea extract prevented elevated plasma homocysteine levels and downregulated endoplasmic reticulum stress markers. Furthermore, Nomura et al. ¹²² investigated the protective effect of three different cultivars of Camellia sinensis (“Yabukita”, “Sofu” and “Sunrouge”) in a model of hypertensive rats fed with a high salt diet. All these tea cultivars reduced urinary NO metabolite and, moreover, “Yabukita” and “Sofu” increased soluble guanylate cyclase expression.

Finally, a single clinical trial has been identified in which the effect of tea, compared with coffee, on blood pressure was evaluated. This study (1352 subjects aged 18–69 years) stratified population in three groups (non-consumers, ≤3 dL/day consumers, and >3 dL/day consumers of tea or coffee). Results showed that consumption of 1 dL/day of tea was associated with lower systolic blood pressure (by 0.6 mm Hg) and lower pulse pressure (by 0.5 mm Hg) ⁷².

Green tea for heart disease

Green tea could improve the risk factors for heart disease ¹²³, as it significantly reduces total cholesterol, low density lipoprotein (LDL) cholesterol, and blood pressure ³⁹. Green tea also improves microvascular function and skin oxygen tension in both older and younger populations ¹²⁴. Nonfermented Chinese green tea is considered to be an ideal beverage to prevent the incidence of coronary heart disease ¹²⁵. Its consumption, together with its catechin-rich fractions, lowers the risk of coronary heart diseases through delaying atherogenesis by significantly preventing endothelial cell induced LDL oxidation and foam cell formation ¹²⁵. The inhibition of advanced glycation end products formation in collagen represents another important mechanism for the protective effects of green tea catechins against cardiovascular diseases ¹²⁶.

The consumption of green tea decreases the risk of myocardial infarction (heart attack) in a dose-dependent manner up to ≥4 cups/day ¹²⁷, as it reduces cardiac hypertrophy, improves systolic and diastolic dysfunction, restores the antioxidant enzyme activity, and stimulates the glucose pathway and mitochondrial function with reduced apoptosis after heart attack ¹²⁸. High dietary intake of green tea may be useful in the reduction and prevention of cardiac injury following ischemia ¹²⁹.

The flavonoids that exist in green tea perform its cardio-protective effect by improving the reserve in coronary flow velocity ¹³⁰. EGCG is able to reduce both arsenic- and doxorubicin-induced cardiotoxicity ¹³¹. It reduces the inflammation and preserves the cardiac function, with lowered mortality rate ¹³². Concerning epicatechin (EC), it may be involved in treating cardiac arrhythmia ¹³³. Meanwhile, epicatechin supplementation has a cardioprotective effect without changing blood pressure, arterial stiffness, or the blood lipid profile ¹³⁴.

Regarding green tea anti-thrombotic activity, it was found that unprocessed tea extracts can significantly reduce the levels of thromboxane-B2 and then eliminate the aggregation of platelets to produce microthrombi, while processed ones are unable to form any inhibition, significantly owing to the presence of a heat-labile compound ¹⁰⁰. Possibly, green tea has a fibrinolytic effect ¹²³. In addition, the catechins inhibit induced platelet aggregation in vitro in a dose-dependent manner, without changing the coagulation parameters ¹³⁵. Green tea can also be used to prevent red blood cell hemolysis ¹³⁶. Different green tea extracts have different degrees of inhibition of dehydration of stored sickle cells, and this inhibitory activity increases by increasing the number of hydroxyl groups ¹³⁷.

Green tea for diabetes

Diabetes mellitus is a chronic metabolic disease that causes abnormally high levels of blood sugar (hyperglycemia) due to a failure in insulin production by pancreas or when the body cannot use insulin effectively ¹³⁸. In 2021, approximately 537 million adults (20-79 years) are living with diabetes. The total number of people living with diabetes is projected to rise to 643 million by 2030 and 783 million by 2045 ¹³⁹. 3 in 4 adults with diabetes live in low- and middle-income countries such as those of South-East Asia and Western Pacific ¹³⁹. There are three diabetes mellitus types: type 1, type 2, and gestational. Type 1 diabetes mellitus also called insulin-dependent diabetes, is an autoimmune condition that commonly affects individuals during childhood and accounts for around 5% of diabetes mellitus diagnosed cases ¹⁴⁰. Type 2 diabetes mellitus also called adult onset diabetes, is the most common of the diabetes types (90%–95% of all diagnosed cases worldwide) and it is mainly associated with excess body fat, sedentary lifestyle, and aging ¹⁴¹. Gestational diabetes mellitus occurs during pregnancy (second or third trimester) because of glucose intolerance; the main risk factors for gestational diabetes mellitus include obesity, ethnicity, age at childbearing, and family history of type 2 diabetes mellitus ¹⁴².

Most in vitro (test tube) diabetes studies with Camellia sinensis are based on the ability of their isolated compounds and extracts to inhibit alpha-amylase and alpha-glucosidase activity. In addition, there are several in vitro studies with cellular models, mouse 3T3-L1 pre/adipocytes and HepG2 cell lines being the most common. Moreover, for in vivo studies, preclinical diabetic animal models (Kunming mice, Sprague-Dawley, and Wistar rats) commonly used to investigate the anti-diabetic properties of tea are streptozotocin and alloxan-induced diabetic animals ¹⁴³, ¹⁴⁴.

The ability of different types of teas and their bioactive compounds to inhibit the enzymes alpha-amylase and alpha-glucosidase has been extensively studied in recent years. The enzyme alpha-amylase, found in saliva and pancreas, catalyzes the hydrolysis of alpha 1–4 bonds of glycogen and starch to form simple sugars (oligosaccharides and disaccharides). Then, alpha-glucosidase enzyme catalyzes alpha 1–4 bonds of oligosaccharides and disaccharides to form glucose in the small intestine. Both enzymes are a therapeutic target for diabetes mellitus treatment ¹⁴⁵. Yang and Kong ¹⁴⁶ investigated the α-glucosidase inhibitory activity of green tea, black tea, and oolong tea, oolong tea having the lowest IC50 value (1.38 µg/mL). Moreover, Oh et al. ¹⁴⁷ compared α-glucosidase inhibitory activity of tea water extracts and tea pomace extracts obtained from green, oolong, and black tea; this research demonstrated that there were no differences between tea water extracts and tea pomace extracts and that green tea was the most active of all assayed type teas (IC50 = 2040 µg/mL for tea water extracts and IC50 = 1950 µg/mL for tea pomace extracts). Furthermore, the aqueous extract of black tea leaves inhibited α-glucosidase enzyme activity (IC50 = 2400 µg/mL for sucrose and IC50 = 2800 µg/mL for maltase) but not α-amylase activity ¹⁴⁸. Additionally, black and green teas inhibited α-amylase activity with IC50 = 589.86 μg/mL and IC50 = 947.80 μg/mL, respectively, and α-glucosidase activity with IC50 = 72.31 μg/mL and IC50 = 100.23 μg/mL, respectively. Differing chemical composition of these three teas may explain, at least in part, their different effects on diabetes-related enzyme activity. Oolong tea stands out for having dimeric flavan-3-ols (theasinensins), green tea has epigallocatechin-3-gallate as major catechin, and black tea is rich in theaflavins and thearubigins ¹⁴⁹. Moreover, the differences in activity for the same type of tea may be due to the fact that the chemical composition is highly influenced by the nature of the green shoots and the procedures to manufacture tea in the producing countries ¹⁵⁰. Apart from studies on black, green, and oolong tea, different ages of pu-erh tea (post-fermented tea) polysaccharide have demonstrated inhibition of α-glucosidase activity, specially 3-year old and 5-year old tea (IC50 = 0.583 and 0.438 μg/mL, respectively), however no inhibitory activity was found against α-amylase ¹⁵¹. In a similar work, Xu et al. ¹⁵² found that pu-erh tea polysaccharides with aging for 3 years and 5 years resulted in inhibition of α-glucosidase enzyme activity with same potency as acarbose (3 years aging) and three times more potently than acarbose (5 years aging). Besides, water extract of pu-erh tea moderately inhibited sucrose activity (IC50 = 14.4 μg/mL) and maltase (IC50 = 11.4 μg/mL), the compound epigallo-catechin-3-O-gallate having the greatest inhibitory activity with IC50 = 32.5 μM against sucrose and IC50 = 1.3 μM against maltase ¹⁵³. In another study, the ethyl acetate fraction from Qingzhuan tea extracts showed significant α-glucosidase inhibitory potential (IC50 = 0.26 μg/mL), attributing this activity to the compounds epigallocatechin gallate and epicatechin gallate. Epicatechin gallate has shown to inhibit α-amylase activity (IC50 = 45.30 μg/mL) and α-glucosidase activity (IC50 = 4.03 μg/mL) and epigallocatechin gallate inhibited α-glucosidase with IC50 = 19.5 μM ¹⁵⁴. Moreover, the isolated compound amelliaone A from YingDe black tea inhibited more potently α-glucosidase enzyme activity (IC50 = 10.2 μM) than the reference compound acarbose (IC50 = 18.2 μM) ¹⁵⁵. Furthermore, Hua et al. ¹⁵⁶ investigated the inhibitory activity of flavone and flavone glycosides of green tea (Lu’an GuaPian) on α-glucosidase and α-amylase enzymes; 7 kaempferol monoglycoside was the most active against α-glucosidase (IC50 = 40.02 µM) and kaempferol diglycoside against α-amylase (IC50 = 0.09 µM). Based on IC50 values of the isolated compounds, epigallocatechin gallate and 7 kaempferol monoglycoside resulted as the most promising α-glucosidase inhibitory agents and kaempferol diglycoside the most interesting α-amylase inhibitor.

Oxidative stress (reactive oxygen species/antioxidant imbalance) contributes to the development of diabetes mellitus and its associated complications. Black tea aqueous extract (2.5%) reduced lipid peroxidation levels and increased GSH content in diabetic rats ¹⁴⁴. Moreover, tea polysaccharides from green tea (200, 400, and 800 mg/kg b.w. per day for 4 weeks) increased superoxide dismutase (SOD) and glutathione peroxidase (GPX) activities in diabetic Kunming mice ¹⁵⁷. Furthermore, in another study epigallocatechin-3-gallate demonstrated reduction of lipid peroxidation, protein oxidation, and superoxide level and increased antioxidant enzymatic activity and GSH content in diabetic rats ¹⁴³.

Several studies have identified changes in relevant biomarkers for diabetes mellitus after tea extract supplementation. Hence, epigallocatechin-3-gallate (2 mg/kg, p.o., alternative days, 1 month) reduced glucose levels and glycosylated hemoglobin and increased insulin ¹⁴³. Moreover, green tea powder (10%) and ethanolic extract of green tea (5%) for 8 weeks reduced glucose levels in Sprague-Dawley rats ¹⁵⁸. Furthermore, green tea extract and pu-erh tea extract (both at doses of 0.8 g/kg with a content of 30% catechin and 10% caffeine) but not epigallocatechin-3-gallate (at a dose of 0.24 g/kg) reduced blood glucose levels in BALB/c mice which suggests that caffeine is essential in the hypoglycemic effect of tea ¹⁵⁶. The doses and time treatments could explain the differences in the effectiveness of epigallocatechin-3-gallate ¹⁵⁶. Finally, both black and green teas suppressed the increased production of advanced glycosylation end products in 3T3-L1 preadipocytes ¹⁵⁹.

Clinical trials were randomized, double-blind, and placebo-controlled and they evaluated the hypoglycemic effect of green tea (mainly) and black tea. Most of these works included patients of both sexes (except one with overweight women) and aged between 30 and 80 years. The duration of the treatments varied from weeks to months and the doses/day administered were also different in each clinical trial (i.e., 1 g/day; 2.5 g/three times day; 560 mg tea polyphenols/two times day; 200 mg tea extract/day). The parameters measured were different, being analyzed from biochemical parameters such as blood glucose levels to oxidative stress markers. Doses of 1 g of dry extract of green tea and 2.5 g/three times day of black tea for 12 weeks were effective to improve glycemic control even better than the reference drug metformin ⁶⁴. Moreover, both 560 mg tea polyphenols/two times day for 20 weeks and 200 mg tea extract/day for 9–18 months had an antioxidant effect as evidenced in an increase of superoxide dismutase activity and a decrease of lipid peroxidation ⁶⁸.

Green tea for osteoporosis

Osteoporosis is a health condition that weakens bones, making them fragile and more likely to break especially hip and vertebral fractures ¹⁶⁰. Osteoporosis develops slowly over several years and is often only diagnosed when a fall or sudden impact causes a bone to break (fracture). Osteoporosis is a multifactorial disease, with age being the most common risk. Other risk factors include environmental (i.e., alcohol consumption, smoking, vitamin D and calcium deficiencies, low physical activity), metabolic (estrogen deficiency), and genetic factors (i.e., cathepsin K, sclerostin, chloride channel 7, high-risk ethnic groups) ¹⁶¹. Osteoporosis affects around 200 million people worldwide (30% of women, 12% of men) ¹⁶¹.

Redox imbalance is also involved in the pathogenesis of bone loss. Reactive oxygen species (ROS) also called oxygen radical or free radicals, are highly reactive chemicals formed from oxygen (O₂) and that easily reacts with other molecules in a cell. Elevated formation of the different ROS leads to molecular damage, denoted as ‘oxidative distress’, may cause damage to DNA, RNA, and proteins, and may cause cell death. Overproduction of reactive oxygen species ROS increases osteoclast activity and inhibits bone mineralization ¹⁶². Flavones from tea have demonstrated to act as antioxidants. Particularly, epicatechin isolated from Huangshan Maofeng tea (green tea produced in Anhui province of China) has shown to protect against oxidative stress in a hydrogen peroxide-induced model on C2C12 mouse myoblast cells ¹⁶³.

Furthermore, in vivo (animal studies) evidence has demonstrated that tea exerts a protective effect on osteoporosis as evidenced in relevant biomarkers. Hence, green tea extracts (dose of 370 mg/kg for 13 weeks) increase cortical and trabecular bone mass in ovariectomized female Wistar rats ¹⁶⁴. Furthermore, green tea polyphenols supplementation (4 months) improved bone properties (alleviate bone loss and favored bone microstructure restructuring) in obese rats fed with a high fat diet and a high fat diet followed by a caloric restricted diet ¹⁶⁵.

Patients with diabetes mellitus have low bone mass which increase fracture risk. Therefore, de Amorim et al. ⁸⁰ conducted a double-blind, randomized, placebo-controlled clinical trial to evaluate the effect of green tea extract on bone mass of diabetic patients. This clinical trial revealed that those subjects with diabetes who received 1120 mg of green tea extract containing 560 mg of polyphenols/day for 20 weeks increased their bone mineral content.

Green tea antibacterial and antiviral activity

Many studies reported the antibacterial activity of green tea. Green tea is effective against Staphylococcus epidermidis, Staphylococcus aureus, and Vibrio cholerae O1, owing to the bactericidal catechins that primarily cause defection in the bacterial membranes ¹⁶⁶. Green tea is also effective against various bacteria that cause tooth decay, such as Escherichia coli, Streptococcus salivarius, and Streptococcus mutans ¹⁶⁷. EGCG and gallocatechin gallate (GCG) markedly inhibit the secretion of extracellular Vero toxins from enterohemorrhagic Escherichia coli cells into the culture supernatant fluid ¹⁶⁸. Raw extract of green tea, especially gallocatechin gallate (GCG), is able to suppress 1-deoxy-d-xylulose 5-phosphate reductoisomerase activity, which is an antimicrobial target ¹⁶⁹. Furthermore, it can also reduce the lethality of ricin toxin ¹⁷⁰, but it has a poor activity against Babesia divergens that infect cattle ¹⁷¹.

EGCG can inhibit the activity of Salmonella typhimurium type III, and thus reduce the bacterial invasion into host cells ¹⁷². Green tea extract is bactericidal against Gram positive bacteria and bacteriostatic against Gram negative ones, with less antifungal activity against Aspergillus niger and Penicillium chrysogenum ¹⁷³.

In addition, tea polyphenols have the ability to inhibit the development and growth of bacterial spores, as in case of Bacillus stearothermophilus and Clostridium thermoaceticum, due to their ability to decrease the heat resistance of these bacterial spores when added at high temperature ¹⁷⁴. However, chlorogenic acid induces the apoptotic markers through excessive potassium efflux and an apoptotic volume reduction, which induces cytosolic calcium uptake and cell cycle arrest in Candida albicans, in addition to its ability to induce caspase activation and DNA fragmentation ¹⁷⁵.

Tea exhibits antiviral activity against human viruses and serves as a diet-derived immune-modulatory chemopreventive agent, as many of its contained flavonoids stimulate NK cell activity, so it may be used in the treatment and prevention of viral diseases ¹⁷⁶. Recently, it was found that EGCG is capable of inhibiting the Brazilian strain of Zika virus entry ¹⁷⁷. Additionally, topical application of EGCG can be used to prevent the sexual transmission of HIV, as it disaggregates existing amyloid fibrils termed semen-derived enhancers of viral infection fibers and inhibits the formation of new ones ¹⁷⁸. In 2006, the United States Food and Drug Administration (FDA) approved the marketing of the sinecatechins ointment as a prescribed Botanical Drug in USA ¹⁷⁹, which has been marketed in EU countries and USA under the trademark of Veregen® ¹⁸⁰. FDA approved the topical treatment of external genital warts and perianal warts with sinecatechins 15% in immuocompetent patients from the age of 18 years for three times daily until complete the clearance of warts or for up to 16 weeks ¹⁸¹.

Green tea neuroprotective activity against Alzheimer’s disease

Green tea contains several compounds that may have beneficial properties, including caffeine, L-theanine, and green tea catechins (e.g., EGCG) ¹⁸². Green tea extract has a protective effect on the ischemia/reperfusion-induced brain injury and behavior deficit. It also reduces the number of ischemia/reperfusion-induced apoptotic neuronal cells ¹⁸³. EGCG could theoretically promote brain health, but no studies have evaluated whether it can protect from cognitive decline or dementia. Green tea polyphenols, epigallocatechin-3-gallate (EGCG), epigallocatechin, epicatechin-3-gallate and epicatechin are able to protect synaptosomes from induced lipid peroxidation damage ¹⁸⁴. EGCG alone has a protective effect against stress-induced neural injuries ¹⁸⁵. EGCG was shown to be easily absorbed from the digestive tract and penetrate the brain, reaching levels similar to those found in lung, liver, kidney, and others ¹⁸⁶. It has a neuroprotective effect against neuronal damage following transient global ischemia in the gerbils acting by different mechanisms as angiogenesis in the early stage of ischemic stroke promoting ¹⁸⁷. Regarding neurotoxicity, L-theanine (a unique amino acid of green tea) has a protective effect against cadmium-induced neurotoxicity by reducing brain cadmium levels and oxidative damage, which lead to neurodegenerative diseases ¹⁸⁸.

In addition, green tea polyphenols can be considered therapeutic agents to alter brain aging processes by serving as neuroprotective agents in major neurodegenerative disorders, such as Parkinson’s disease and Alzheimer’s disease ¹⁸⁹. In addition, green tea catechin intake may be useful in the improvement of the morphologic and functional changes that occur naturally in the accelerated senile brains ¹⁹⁰. EGCG and epigallocatechin were found to be inhibitors of dopa decarboxylase enzyme in a concentration- and time-dependent manner ¹¹⁶. Green tea containing high levels of EGCG prevents the loss of tyrosine hydroxylase-positive cells in the substantia nigra. Both tea and EGCG, when used alone or with Parkinson’s disease’s inducers, can decrease the neuronal nitric oxide synthase (nNOS) expressions in the substantia nigra, which provides a neuroprotective effect ¹⁹¹.

Several observational studies and clinical trials suggest that greater green tea consumption is associated with lower dementia risk, but no clinical trials have tested whether green tea can prevent age-related cognitive decline or dementia. No studies have reported whether green tea can improve cognition or slow decline in people with dementia. In a clinical trial, markers of oxidative stress were decreased in Alzheimer’s patients who consumed a beverage that included green tea extracts for eight months ¹⁹². However, it is unknown whether this beverage helps patients. A different antioxidant therapy (not containing green tea extract) was reported to lower oxidative stress in Alzheimer’s patients but accelerated cognitive decline ¹⁹³.

Greater green tea consumption was associated with lower risk of dementia in two studies conducted in Japan, with the larger study reporting 27% lower risk in people who drank at least 5 cups a day ¹⁹⁴, ¹⁹⁵. Tea drinking was also associated with higher verbal fluency in elderly Chinese people (i.e., 80–115 years old) ¹⁹⁶.

Two double-blind randomized controlled trials have evaluated the effects of green tea extract on cognitive functions. One trial in 91 patients with mild cognitive impairment reported that the combination of green tea extract and L-theanine for 16 weeks resulted in significant improvements in memory and attention, particularly in patients who had relatively severe baseline impairment ¹⁹⁷. The second trial examined the acute effects of a drink containing 27.5 g of green tea extract and reported that the drink increased brain connectivity associated with working memory and the degree of connectivity correlated with the magnitude of improvement in working memory ¹⁹⁸.

Preclinical studies have found a wide range of actions of EGCG, including chelating metals, reducing inflammation, scavenging free radicals, improving mitochondrial function, and preventing death of brain cells ¹⁹⁹. EGCG may also inhibit enzymes that break down the neurotransmitter acetylcholine ²⁰⁰, which is important for memory retrieval. Other preclinical studies found that EGCG treatment improved function or reduced damage in the brain ²⁰¹, ²⁰², but these effects have not been confirmed in human trials.

Two small trials suggest that EGCG treatment may very modestly improve memory functions, inhibitory control, and adaptive behavior in people with Down syndrome, though this protective effect was evident in only 3 out of 24 cognitive tests ²⁰³. In healthy adults, a single dose of EGCG has had little influence on cognition. One double-blind randomized controlled trial reported that a single treatment did not affect cognitive performance or mood ²⁰⁴. A second trial reported increased calmness and reduced stress ²⁰⁵. EGCG also increased brain waves, which have been associated with relaxation, focused attention, and quiet wakefulness ²⁰⁶. Long-term studies in healthy people are needed to evaluate the effects of EGCG on cognitive health.

Green tea Vs. Green Tea Extract: Which one is better for you?

Green tea is made by processing fresh Camellia sinensis leaves using heat or hot steam immediately after collection, thus minimising any oxidation processes. The active ingredients of green tea are composed of polyphenols, alkaloids (caffeine, theophylline, and theobromine), amino acids, carbohydrates, proteins, chlorophyll, volatile organic compounds (chemicals that readily produce vapors and contribute to the odor of tea), fluoride, aluminum, minerals, and trace elements ¹⁶, ¹⁷, ¹⁸. The polyphenols, most of which are flavonols, commonly known as catechins, epigallocatechin-3-gallate (EGCG), epigallocatechin, epicatechin-3-gallate and epicatechin, are thought to be responsible for the health benefits that have traditionally been attributed to tea, especially green tea. A single cup (200 ml) of green or white tea contains 25–200 mg of epigallocatechin-3-gallate (EGCG) ²⁰⁷. Black tea contains much lower levels of EGCG (~20 mg) as it is converted during the oxidation process to thearubigin, a different type of polyphenol ³⁴. Many studies have demonstrated that the catechins and phenolic acids are responsible for the antioxidant properties of green tea, which has protective effects against many diseases, such as diabetes, cancer, hypertension, and cardiovascular diseases ²⁰⁸, ²⁰⁹, ²¹⁰, ²¹¹. However, there is potential liver toxicity induced by the overdose of EGCG ²¹², ⁴⁹.

The mean daily intake of EGCG resulting from the consumption of green tea infusions ranges from 90 to 300 mg/day while exposure by high‐level consumers is estimated to be up to 866 mg EGCG/day ². Experts concluded that catechins from green tea infusion, prepared in a traditional way, and reconstituted drinks with an equivalent composition to traditional green tea infusions, are in general considered to be safe ².

Green tea extracts are supposed to contain the same amount of catechins as contained in green tea leaves they are extracted from. But, the process of extraction can leave out some content in the leaves only. Different manufacturers create the green tea extracts differently. The process of manufacturing green tea extracts typically involves extraction of the leaves and stems of Camellia sinensis with water and/or mixtures of water with other organic solvents, most often alcohols, such as methanol or ethanol ²¹³. If the process also includes decaffeination, non-toxic supercritical carbon dioxide and toxic solvents such as chloroform and dichloromethane may be used. Extracts may be further refined by filtration through a synthetic resin absorbent to remove unwanted residues ²¹⁴, ²¹⁵. Such processes result in green tea extract with much higher concentrations of catechins compared to traditional green tea beverages and may also increase the concentration of potential contaminants such as pesticide residues, toxic elemental impurities, or pyrrolizidine alkaloids ²¹³.

Green tea extracts contain much higher concentrations of catechins than brewed green tea beverages. Green tea aqueous extracts can be further concentrated or purified to obtain a high-catechin fraction with some claiming to contain 80–95 % EGCG by weight ²¹⁶. The high-catechin fraction can be prepared by extracting the leaves with organic solvents such as 80 % acetone or 70 % ethanol with or without prior extraction with water ²¹⁷. In the early 2000s, one of the most popular commercial green tea extract containing products in France and Spain was Exolise, an 80 % ethanolic dry extract standardized at 25 % catechins expressed as EGCG. Weight loss products containing Exolise reportedly caused liver injury and were subsequently banned in France and Spain in 2003 ²¹⁸.

Several studies indicate that green tea extracts manufactured by extraction with organic solvents may contain solvent residues in the final product. In 2011, an independent monitoring group that tested 28 green tea extracts and 32 finished products (supplements) from different countries (unpublished; courtesy Taiyo Kagaku Co. Ltd.) found trace amounts of different solvent residues in both green tea extracts and finished products ²¹³. Of the 28 green tea extracts tested, 16 contained traces of chloroform ranging from 0.01–3.8 ppm, and two of the 28 extracts contained traces of a mixture of solvents (e.g., chloroform, dichloromethane, and ethyl acetate). Of the 32 finished products tested (green tea extract supplements), 17 contained traces of chloroform ranging from 0.01 to 1.6 ppm ²¹³.

Studies have demonstrated that pesticides can be transferred to the finished product during the tea brewing process ²¹⁹. A study that tested 18 green tea commercial samples reported significant amounts of pesticide residues in some products ²²⁰. Compliance to pesticide residue limits in tea ingredients varies by country. According to the Global MRL Database (https://www.bryantchristie.com), the U.S. has maximum residue levels (MRLs) for 34 pesticides for tea leaves as a raw agricultural commodity. In addition, the U.S. has MRLs for 7 pesticides as processed tea leaf food products (e.g., instant tea, packaged or bagged nonperishable processed food). All others must be “non-detectable”. By comparison, Canada has MRLs for 18 pesticides in the commodity tea (dried leaves) (Health Canada MRL Database, https://pr-rp.hc-sc.gc.ca/ls-re/index-eng.php). All other pesticides must meet the general 0.1 ppm MRL as specified in the Canadian Food and Drug Regulations. Other regions have very different requirements, including Japan and the EU, which permit 247 and 450 pesticide residues, respectively ²²¹).

green tea extract intake amounts ranged from 500 mg to 3000 mg which is about 250−1800 mg EGCG daily). The median intake amount was estimated at 720 mg/day (delivering 623 mg of EGCG daily). In most cases, the green tea extract had been taken daily for two or more weeks before onset of the acute liver injury, which in some cases occurred up to a month after stopping the intake of green tea extract. Most subjects involved in the DILI cases (21 out of 35) were using green tea extract for weight loss and likely had reduced food intake as well. This may be significant given the increased hepatotoxicity of EGCG preparations in fasted compared to fed dogs [107]. Individuals trying to lose weight are also likely be eating less food which may inadvertently mimic fasted conditions resulting in significantly increased bioavailability of EGCG.

The information reviewed by the 2020 United States Pharmacopeia (USP), together with their previous review of 34 case reports ²²² other publications on hepatotoxicity associated with products containing green tea ²²³, including a recent review by Teschke and Xuan, 2019 ²²⁴ and other reviews of case reports ²²⁵, indicate that green tea extract can cause liver injury. What remains unclear is the exact mechanism(s) by which green tea extract induces liver injury, and the level below which no liver injury occurs.

In 2017, Health Canada reviewed the potential risk of hepatotoxicity associated with green tea extract and strengthened the cautionary risk statement in their green tea extract monographs ²²⁶. Norway also reviewed possible safety concerns associated with consumption of green tea extracts and concluded that intake of more than 0.4 mg EGCG/kg body weight per day as a bolus may cause adverse biological effects and noted that there is increased susceptibility to toxicity when green tea extract or green tea infusion is taken following fasting ²²⁷.

The 2018 EFSA review of catechins in green tea extract concluded that clinical studies indicate that intake of green tea extract as a food supplement delivering EGCG amounts equal or above 800 mg daily increased serum transaminases, which is indicative of liver injury. The review further concluded that it was not possible to identify an EGCG dose from green tea extracts that could be considered safe because in one case, intake of 375 mg of 80 % ethanolic extract resulted in hepatotoxicity ²²⁸.

In conclusion, anything ingested in its purest form is more beneficial to the body. A warm cup of freshly brewed green tea is better than popping a supplement pill. But, if you have tried and just can’t build a taste for green tea, you may give its extract a try. These capsules give you almost the same health benefits as drinking green tea would, only if you exercise caution. You should not take high doses of the EGCG and other tea catechins before talking to your doctor for recommendations.

Green tea summary

Green tea extracts haven’t been shown to produce a meaningful weight loss in overweight or obese adults. They also haven’t been shown to help people maintain a weight loss.

Desirable green tea intake is 3 to 5 cups per day (up to 1200 ml/day), providing a minimum of 250 mg/day catechins. If not exceeding the daily recommended allowance, those who enjoy a cup of green tea should continue its consumption. Drinking green tea appears to be safe at moderate, regular and habitual use.

If you like a cup of tea with your morning toast or afternoon snack or on its own, enjoy it. It’s safe to drink as long as you don’t add sugar or artificial sweeteners and the caffeine doesn’t make you feel jittery and shaky; interfere with sleep; and cause headaches. And the limited evidence currently available suggests that both green and black tea might have beneficial effects on some heart disease risk factors, including blood pressure and cholesterol.

Just don’t expect miracles to come in a teacup toward your weight-loss goals. Real weight loss requires a whole lifestyle approach that includes diet changes and activity.

EGCG and other tea catechins are also available as dietary supplements, often in doses of 200–300 mg per serving, which are comparable to doses deemed safe in clinical trials ⁴⁷, ⁴⁸. Clinical trials examining the effects of EGCG on cognitive function have used doses ranging from 9 to 300 mg/day ²⁰⁵.

However, liver problems have been reported in a small number of people who took concentrated green tea extracts. Although the evidence that the green tea products caused the liver problems is not conclusive, experts suggest that concentrated green tea extracts be taken with food and that people discontinue use and consult a health care provider if they have a liver disorder or develop symptoms of liver trouble, such as abdominal pain, dark urine, or jaundice.

Tea has long been regarded as an aid to good health, and many believe it can help reduce the risk of cancer. Most studies of tea and cancer prevention have focused on green tea ²²⁹. Although tea and/or tea polyphenols have been found in animal studies to inhibit tumorigenesis at different organ sites, including the skin, lung, oral cavity, esophagus, stomach, small intestine, colon, liver, pancreas, and mammary gland, the results of human studies—both epidemiologic and clinical studies—have been inconclusive. Therefore we have to say that at present, with respect to tea and green tea and cancer prevention, the evidence regarding the potential benefits of tea consumption in relation to cancer is inconclusive.

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