skipping meals

Skipping meals

Skipping meals also known as “intermittent fasting”, “periodic fasting”, “time‐restricted feeding”, “alternate-day fasting” or “reduced meal frequency”, is when people restrict the time (e.g., 16–48 hours) during the day when they can eat 1). Intermittent fasting focuses on the timing of when you can consume meals either within a day or a week. For example, someone may eat only during a 12-hour time period, such as 7 a.m. to 7 p.m. Skipping meals commonly consists of a daily fast for 16 hours, a 24-hour fast on alternate days, or a fast 2 days per week on non-consecutive days 2). Intermittent fasting is not a diet plan. While it may have some of the same health benefits as a diet, it’s really an eating pattern. It means you fast (don’t eat) during a certain period of time each day (usually an extended period of time). You then eat during another period of time each day (usually a smaller period of time). While you’re fasting, you can drink beverages that don’t contain calories, including water, black coffee, and unsweetened tea.

Some popular approaches to intermittent fasting include:

  • Alternate-day fasting. Eat a normal diet one day and either completely fast or have one small meal (less than 500 calories) the next day.
  • 5:2 fasting. Eat a normal diet five days a week and fast two days a week.
  • Daily time-restricted fasting. Eat normally but only within an eight-hour window each day. For example, skip breakfast but eat lunch around noon and dinner by 8 p.m.

How fasting and eating is divided each day is called an eating schedule. One of the most common, easy-to-follow schedules is 16:8. This means you fast for a 16-hour period of time and eat your daily meals during an 8-hour period of time. For example, you may want to fast from 7 p.m. until 11 a.m. the next day. You would then eat a healthy lunch and dinner between 11 a.m. and 7 p.m. You wouldn’t eat anything after 7 p.m. until 11 a.m. the next day. This is just an example of times. You can select any 16-hour and 8-hour block of time that works best for your schedule. But it’s important to keep your eating window at the same every day.

Other intermittent fasting schedules include 18:6 (when you fast for 18 hours and eat for 6 hours) or alternating days. With alternating days, you fast for 24 hours, then eat a healthy diet for the next 24 hours, then fast again for the following 24 hours. This schedule continues using the every-other-day format. Another schedule option is 5:2. This is when you fast for two days a week, and eat a normal, healthy diet the other five days. This is a little different, though, as this schedule allows you eat one small meal of 500 to 600 calories on your fasting days.

The time you’re allowed to eat is called your eating window. During your eating window, focus on eating a healthy diet and maintaining portion control. Don’t eat too many calories and avoid junk food and fast food. While you don’t need to eat anything in particular, you do need to make sure you’re getting the nutrition you need. Some people choose to use the Mediterranean diet as a guideline for what to eat. This plan focuses on fruits, vegetables, beans, whole grains, and fish. You can also choose lean proteins and healthy fats.

During fasting, caloric consumption often ranges from zero to 25% of caloric needs 3). Alternative day fasting may consist of 24-hour fasts followed by a 24-hour eating period that can be done several times a week such as a 5:2 strategy when there are 2 fast days mixed into 5 nonrestrictive days 4). For time restricted fast programs, variations include 16-hour fasts with 8 hour feeding times, 20-hour fasts with 4-hour feed times or other similar versions 5). While both caloric restriction and intermittent fasting may result in overall decreased caloric intake, this is not integral to intermittent fasting. Intermittent fasting has been linked to better glucose control in both humans and animals 6). However, long-term adherence to caloric restriction is low while adherence to intermittent fasting may be more promising.

While researchers are still studying intermittent fasting, some research has shown it offers some health benefits. For starters, it’s common to lose weight when following intermittent fasting. That’s because your body is using fat—not glucose—as its energy source. Additionally, if you make wise food choices when you do eat, you’re likely eating fewer calories than before you started intermittent fasting. If you add exercise to the mix, it’s a great combination for not only weight loss, but also improved health benefits. Intermittent fasting may help people who have cardiovascular disease, neurological disorders, and some cancers. Intermittent fasting may also help lower your bad cholesterol and improve symptoms of arthritis.

The majority of studies of intermittent fasting in humans have considered whether intermittent fasting can be a potential strategy to reduce weight and correct adverse metabolic parameters amongst obese and overweight subjects 7). This is important since the problems of long term adherence to continuous calorie restriction for weight management are well known 8). Losing weight and being physically active help lower your risk of obesity-related diseases, such as diabetes, sleep apnea and some types of cancer. For these diseases, intermittent fasting seems to be about as beneficial as any other type of diet that reduces overall calories. Johnson et al. 9) undertook the first trial of intermittent fasting for weight loss amongst 10 obese subjects with asthma which tested alternate days of an 85% energy restricted low carbohydrate diet regimen. This study reported beneficial reductions in serum cholesterol and triglycerides, markers of oxidative stress (8-isoprostane, nitrotyrosine, protein carbonyls, and 4-hydroxynonenal adducts) and inflammation (serum tumor necrosis factor-α) 10). Circulating ketone levels were also elevated on the fasting days 11). In more recent 27 clinical trials, intermittent fasting resulted in weight loss, ranging from 0.8% to 13.0% of baseline body weight 12). Weight loss occurred regardless of changes in overall caloric intake. In the studies of 2 to 12 weeks’ duration, body mass index (BMI) decreased, on average, by 4.3% to a median of 33.2 kg/m². Therefore intermittent fasting shows promise as a primary care intervention for obesity, but little is known about long-term sustainability and health effects. Longer-duration studies are needed to understand how intermittent fasting might contribute to effective weight-loss strategies. Symptoms such as hunger remained stable or decreased, and no adverse events were reported. While intermittent fasting is a moderately successful strategy for weight loss, it shows promise for improving glycemic control, although it does pose a potential risk of hypoglycemia. However, to lose weight and keep it off, the best strategy is to adopt healthy eating and exercise habits that you enjoy so you can stick with them over time.

Ramadan is a culturally determined example of intermittent fasting for many Muslims. During the entire month of Ramadan, Muslims fast every day from dawn to sunset. Those who fast often do so for approximately 14 hours per day for 30 days, presenting a real-world opportunity for examining effects of fasting. Eight Ramadan studies examined weight loss in obese adults (n = 856) 13). Weight losses ranged from 0.1 kg 14) to 1.8 kg 15). Studies enrolling participants with diabetes saw a modest improvement in glycemic control 16).

To get the benefits of intermittent fasting, you need to fast for at least 12 hours. That’s how long it takes your body to switch from using glucose for energy to using fat for energy. Additionally, it will take your body a while to get used to this new eating schedule. So don’t expect results right away. You may need to wait between 2 and 4 weeks to see or feel any results.

It may take 2 to 4 weeks for your body to get used to eating on an intermittent fasting schedule. During those first few weeks, you may have headaches and feel hungry, grouchy, or tired. Know you may feel this way before you start and make a plan to push through these feelings. After a few weeks, your body will get used to this eating pattern and those symptoms should go away. In the end, many people say that feel better following an intermittent fasting lifestyle.

There is still much for scientists and doctors to learn about intermittent fasting. They’re researching how it affects the body and its systems. They’re also looking to see if there are long-term benefits to following this eating schedule for many years.

What happens when you skip meals?

Fasting for at least 12 hours changes how your metabolic system works. Most of the time, your body gets its energy from a sugar called glucose. Glucose is found in the foods you eat and beverages you drink. When you eat three meals during a day, your body maintains a steady glucose level because you’re eating and drinking frequently. However, when you fast for more than 12 hours, your body’s glucose levels start to dip because you’re not eating as frequently. When your body doesn’t have the glucose it needs for energy, it taps into your body’s fat for energy. When this happens, your body go into ketosis — a process that occurs when your body doesn’t have enough sugar for energy, so it breaks down stored fat instead, the fatty acids in your body are absorbed into your blood. They produce a chemical called ketones. Your body then uses the ketones as its energy source. This is called a metabolic switch. Your body is switching from glucose to ketones. When your body uses ketones instead of fat, you may lose weight. But, behind the scenes, the ketones also may be having a positive effect on your body’s organs and cells.

Fasting also affects metabolic processes in your body. These processes trigger a number of responses, including decreased inflammation, improved blood sugar regulation and better response to physical stress. Research in animals has shown that calorie reduction can slow down aging and prevent some diseases. Some research suggests that intermittent fasting may be more beneficial than other diets for reducing inflammation and improving conditions associated with inflammation, such as:

  • Alzheimer’s disease
  • Parkinson’s disease
  • Arthritis
  • Asthma
  • Multiple sclerosis
  • Stroke

However, more research needs to be done in humans before the health benefits—and risks—are fully understood.

In laboratory rats and mice intermittent fasting has profound beneficial effects on many different indices of health and, importantly, can counteract disease processes and improve functional outcome in experimental models of a wide range of age-related disorders including diabetes, cardiovascular disease, cancers and neurological disorders such as Alzheimer’s disease, Parkinson’s disease and stroke 17). Numerous physiological indicators of health are improved in laboratory rats and mice maintained on intermittent fasting diets (including alternate day fasting and time-restricted feeding). Among such responses to intermittent fasting are: reduced levels of insulin and leptin which parallel increases in insulin and leptin sensitivity; reduced body fat; elevated ketone levels; reduced resting heart rate and blood pressure, and increased heart rate variability (resulting from increased parasympathetic tone); reduced inflammation; increased resistance of the brain and heart to stress (e.g., reduced tissue damage and improved functional outcome in models of stroke and heart attack); and resistance to diabetes 18). Intermittent fasting can delay onset and slow the progression of neuronal dysfunction and degeneration in animal models of Alzheimer’s disease, Parkinson’s disease and Huntington’s disease. Emerging findings are revealing cellular and molecular mechanisms by which intermittent fasting increases the resistance of cells, tissues and organs to stress and common diseases associated with aging and sedentary, overindulgent lifestyles. The results of human studies in which various health indicators are measured at baseline and after periods of intermittent fasting of 2–6 months or more, suggest that intermittent fasting can protect against the metabolic syndrome and associated disorders including diabetes and cardiovascular disease. Recent small trials of intermittent fasting in patients with cancer 19) or multiple sclerosis 20) have generated promising results that provide a strong rationale for moving forward with larger clinical trials in patients with a range of chronic age- and obesity-related disorders.

The cellular and molecular mechanisms by which intermittent fasting improves health and counteracts disease processes involve activation of adaptive cellular stress response signaling pathways that enhance mitochondrial health, DNA repair and autophagy 21). Periodic fasting (intermittent fasting with periods of fasting or fasting mimicking diets lasting from 2 to as many as 21 or more days) also promotes stem cell-based regeneration as well as long-lasting metabolic effects. Intermittent fasting could provide a primary strategy to improve metabolic syndrome and reduce the incidence of metabolic syndrome-induced cancer 22). Fasting during activity hours in contrast to the inactivity hours of the day appears to be important for the optimization of anticancer effect and gene expression. Mice with no access to food during the activity phase (dark phase) of a 12-hours light/12-hours dark cycle had a significantly slower tumor progression and higher survival compared with mice that had no access to food during the inactivity phase (light phase), and mice that had access to food ad libitum 23).

Studies of intermittent fasting (e.g., 60% energy restriction on 2 days per week or every other day), periodic fasting (e.g., a 5 day diet providing 750–1100 kcal) and time-restricted feeding (limiting the daily period of food intake to 8 hours or less) in normal and overweight human subjects have demonstrated effectivenss for weight loss and improvements in multiple health indicators including insulin resistance and reductions in risk factors for cardiovascular disease 24). A 30-day intermittent fasting from dawn to sunset (the human activity phase), was associated with an anticancer serum proteome response and upregulated several key regulatory proteins that play a key role in tumor suppression, DNA repair, insulin signaling, glucose, and lipid metabolism, circadian clock, cytoskeletal remodeling, immune system, and cognitive function 25). Several human studies showed beneficial effects of intermittent fasting (e.g., Ramadan fasting) 26) and time-restricted eating 27) in subjects with metabolic syndrome.

Type 2 diabetes

There are minimal data on the effects of intermittent fasting versus calorie restriction on glucose homeostasis amongst overweight or obese individuals with type 2 diabetes. Ash et al. 28) reported that a four day intermittent fasting led to comparable reductions in percentage body fat and reductions in HbA1c to an isocaloric calorie restriction. Mean reduction for the overall group was 1.0 (8.4)%, although this small study may have been underpowered to show significant differences 29). Williams et al. 30) assessed the effect of enhancing a standard 25% calorie restriction diet with periods of intermittent fasting, (75% calorie restriction either 5 days/week every 5 weeks or 1 day/week for 15 weeks). Predictably, additional periods of calorie restriction increased weight loss. The 5 days/week every 5 weeks intervention resulted in the greatest normalization of hemoglobin A1c (HbA1c), independent of weight loss suggesting a potential specific insulin-sensitizing effect of this pattern of intermittent fasting added to calorie restriction 31).

The two studies of a 2 days/week intermittent fasting have reported greater reductions in insulin resistance versus calorie restriction amongst overweight and obese non-diabetic subjects 32), 33). In the first study the subjects on the intermittent fasting diet exhibited a 25% greater reduction in insulin resistance compared to the CER group when measured on the morning after five normal feeding days, with a further 25% reduction in insulin resistance compared with CER on the morning after the two energy restricted days. These differences in insulin sensitivity occurred despite comparable reductions in body fat between the groups 34). A follow up study reported greater reductions of insulin resistance with the 2 days/week low carbohydrate intermittent fasting compared to calorie restriction, which this time was associated with a greater loss of fat with the intermittent fasting regimen 35).

Three studies have assessed the effects of 2–3 weeks of intermittent fasting with alternating 20–24 hour periods of a total fast interspersed with 24–28 hours periods of excessive eating (175–200% of normal intake) and were designed to ensure there was no overall energy deficit or weight loss. Results have been variable between the studies. Halberg et al. 36) reported improvements in insulin-mediated whole body glucose uptake and insulin-induced inhibition of adipose tissue lipolysis when measured after two normal feeding days, whereas Soeters et al. 37) failed to replicate these findings. Heilbronn 38) assessed 3 weeks of alternate day fasting (24 hours total fast and 24 hours ad lib feeding) amongst 16 normal and overweight men and women. Glucose uptake during a test meal (peripheral insulin sensitivity) was assessed on the morning after a fasting day, i.e. after a 36 hours fast; interestingly, insulin sensitivity was increased in men but decreased in women. The latter observation may be a benign observation linked to greater fluxes of free fatty acids amongst fasting women 39) and likely to be a normal physiological adaptation to fasting rather than a cause for concern 40). Thus, intermittent fasting has been reported to have variable effects on peripheral and hepatic insulin sensitivity which may be different in obese and normal weight subjects and may be gender-specific. Further studies are required using more robust measures of insulin sensitivity e.g. insulin clamp or other techniques.

Cardiovascular disease

Varady et al. 41), 42) performed several different studies to evaluate the effects of modified alternate day fasting on cardiovascular risk factors in overweight and obese subjects. In one study 43), alternate day fasting for 2 months resulted in decreases in resting heart rate, and circulating levels of glucose, insulin and homocysteine, all of which are favorable with regards to the risk of cardiovascular disease. In another study, 2 months of alternate day fasting reduced fat mass, total cholesterol, LDL cholesterol (low density lipoprotein or “bad” cholesterol) and triglyceride concentrations 44). However, there have been few studies that have evaluated the relative effects of intermittent fasting and calorie restriction on cardiovascular risk markers. The randomized comparisons of intermittent fasting and calorie restriction have reported equivalent reductions in blood pressure 45) and triglycerides 46) and increased LDL particle size 47). Whilst Hill et al. reported a greater reduction in serum cholesterol with intermittent fasting (14%) versus calorie restriction (6%) 48).

Cancer

There are no data on the effects of intermittent fasting on cancer rates in humans. Weight control is likely to reduce the incident risk of thirteen cancers which have been linked to obesity 49), although the role of weight management after diagnosis on the outcome of obesity related cancers is not known 50). Surrogate evidence that intermittent fasting may reduce cancer risk can be derived from its effects on a number of cancer risk biomarkers such as insulin, cytokines, and the inflammation-related molecules leptin and adiponectin, which are thought to mediate the effects of adiposity and excessive energy intake on the development and growth of cancers in humans 51).

The effect of intermittent fasting on total and bioavailable insulin-like growth factor 1 (IGF-1) in human studies has been variable. This reflects the fact that, in contradistinction to animal studies, circulating levels of total IGF-1 and bioactive IGF-1 (determined by measuring IGF-binding proteins 1,2 and 3) are poor markers of the effects of energy restriction and weight loss in humans 52) and do not relate well to IGF-1 bioactivity at the tissue level 53). Harvie et al 54) reported no change in total circulating IGF-1 alongside weight loss with intermittent fasting or calorie restriction. Intermittent fasting and calorie restriction both increased IGF binding protein 1 (26% and 28%) and IGFBP-2 (22% and 36%), but did not change serum bioavailable IGF-1 (ultrafiltered) when measured after feeding days. There is a further acute 17% increase in IGF binding protein 2 on the morning after the two restricted days of a 70% calorie restriction, but no measurable change in total or serum bioavailable IGF-1 (ultrafiltered) 55). Reductions in IGF-1 (−15%) have been reported in normal and overweight subjects who had followed an intermittent fasting diet which involved 5 days per month of a low protein, low energy diet (~0.25g protein/kg weight, 34–54% of normal energy intake) interspersed with normal intake for the remaining 25 days of the month. These reductions were observed after 5 days of normal eating after three months alongside modest reductions in body weight (−2%) 56). The aforementioned effects of intermittent fasting in relation to insulin resistance and diabetes risk may therefore have an important role in protecting against obesity-related cancers 57).

Adipose tissue exhibits increased leptin and decreased adiponectin production with increasing adiposity, which is thought to have a role in cancer development and progression via effects on insulin sensitivity, inflammation, and direct effects on cell proliferation and apoptosis 58). Adiponectin levels only increase in overweight humans following calorie restriction when there are large reductions in body and visceral fat (>10%) 59). Some studies of intermittent fasting have reported increases in adiponectin with more modest weight loss. i.e. a 30% increase in plasma adiponectin on both restricted and feeding days, alongside modest reductions in weight (−4%) and body fat (−11%) 60). Harvie et al 61) reported a tendency for a greater increase in adiponectin with intermittent fasting than calorie restriction despite a comparable reductions in weight and adiposity. These observations are interesting; however our follow up intermittent fasting study reported no change in adiponectin levels with either intermittent fasting or calorie restriction 62). Intermittent fasting brings about large and comparable reductions in leptin as calorie restriction (both 40%) and the leptin: adiponectin ratio 63). Weight loss with calorie restriction reduces circulating levels of C reactive protein (CRP) by 2–3% for every 1% weight loss, whereas TNF-α and IL-6 are reduced by around 1–2% per 1% weight loss 64). Reductions of inflammatory markers with intermittent fasting are comparable to calorie restriction for a given weight loss 65). Thus, although limited, the available biomarker data suggest that intermittent fasting leads to comparable changes in most cancer risk biomarkers to calorie restriction, with the possible exceptions of insulin resistance and adiponectin which require further study using robust methodologies.

Is skipping meals bad?

Skipping meals is safe for many people, but it’s not for everyone, especially those with health conditions like diabetes, kidney disease, kidney stones, gastroesophageal reflux, heart-related issues or other medical problems. Skipping meals may not be the best way to manage your weight if you’re pregnant or breast-feeding. Intermittent fasting should be approached individually and with the help of a dietitian or physician. As with all diets, you should talk with your doctor or dietitian before you try it. Your doctor or dietitian can review your health history and medication list and help you decide which eating plan and exercise schedule is best for you.

It is important to note that intermittent fasting can have unpleasant side effects, but they usually go away within a month. Side effects may include:

  • Hunger
  • Fatigue
  • Insomnia
  • Nausea
  • Headaches

No serious adverse events were reported in the 27 intermittent fasting clinical trials 66). Fasting-related safety concerns include mood-related side effects and binge eating, among other symptoms. Obese participants observing a fast every second day did not develop binge-eating patterns or purgative behavior 67) and reported improved body image and less depression 68). During the 6-month study by Harvie et al 69), 32% of participants reported less depression and increased positive mood and self-confidence. Study participants also occasionally reported dizziness, general weakness, bad breath, headache, feeling cold, lack of concentration, sleep disturbance, nausea and constipation 70). When compared with baseline, these symptoms were unchanged with fasting 71).

Skipping meals to lose weight

Many people choose skipping meals (intermittent fasting) as a way to lose weight. Losing weight and being physically active help lower your risk of obesity-related diseases, such as diabetes, sleep apnea and some types of cancer. For these diseases, intermittent fasting seems to be about as beneficial as any other type of diet that reduces overall calories. A systematic review shows that intermittent fasting may be effective in reducing weight when compared to a diet that is available at all times (ad libitum feeding) and may be as effective as continuous energy restriction 72). In 27 clinical trials involving 944 participants, intermittent fasting resulted in weight loss, ranging from 0.8% to 13.0% of baseline body weight (see Table 1 below) 73). Weight loss occurred regardless of changes in overall caloric intake 74). In the 16 studies of 2 to 12 weeks’ duration that measured body mass index (BMI), BMI decreased, on average, by 4.3% to a median of 33.2 kg/m² 75). Waist circumference decreased by 3 cm to 8 cm in studies longer than 4 weeks that recorded it 76).

Twelve studies used calorie-restricted diets as a comparator to intermittent fasting and found equivalent weight loss in both groups 77). Study duration was 8 weeks to 1 year, with a combined total of 1206 participants (527 undergoing intermittent fasting, 572 using calorie restriction, and 107 control participants) and demonstrated weight loss of 4.6% to 13.0% 78). Adherence appears similar for both weight loss strategies 79). The largest study comparing intermittent fasting with calorie restriction was by Headland et al 80) in 2019 of 244 obese adults who achieved a mean 4.97-kg weight loss over 52 weeks versus a mean weight loss of 6.65 kg with calorie-restricted diets. All of the 11 other comparisons of intermittent fasting and calorie-restriction diets also found similar results between both groups 81). In several of these studies, those in the intermittent fasting group consumed the same amount of calories 82) or less 83) than those in the calorie-restriction group. Four studies combined fasting and calorie restriction on the non-fasting days and found comparable weight loss to other studies (3.4% to 10.6%) 84). In a direct comparison of 88 participants over 8 weeks, intermittent fasting combined with restricting calories to 30% less than their calculated energy requirements led to greater weight loss versus intermittent fasting alone 85).

Most of the weight loss with intermittent fasting is fat loss 86). A 2011 study by Harvie et al 87) calculated that 79% of weight loss was owing to loss of fat specifically (level I evidence). Participants regained some weight during follow-up after intervention, although average body weight remained statistically significantly lower than baseline levels 88). Weight regain did occur after 6 months. Five studies followed participants for 6 months or longer after completing intermittent fasting interventions of 8 weeks to 1 year and most studies saw body weight increase by 1% to 2% of their weight nadir 89). Catenacci et al 90) found a mean 2.6-kg regain over 6 months and Schübel et al 91) and Trepanowski et al 92) each found a regain of 2% of baseline body weight. The year-long study by Carter et al 93) of 137 participants was the exception, demonstrating a maintained weight loss. Zuo et al 94) saw a BMI increase of less than 1% during a year-long follow-up period after 12 weeks of intermittent fasting. In 6 comparisons of intermittent fasting and calorie restriction, the amount of weight regained after intermittent fasting and calorie restriction was similar 95). The 2016 study by Catenacci et al 96) showed differing patterns of weight regain. In the 11 intermittent fasting patients who completed follow-up, this was limited to lean body mass, while the 10 calorie-restricted patients who completed follow-up regained both fat and lean body mass 97).

The practical length of a fast to effect changes in weight appears to be 16 hours. In intermittent fasting studies with a daily fasting intervention, a total of 120 participants were able to maintain a minimum daily fast of about 16 hours (15.8 to 16.8 hours), with an 8-hour eating window each day 98). Arnason et al 99) found that participants were able to fast for an average of 16.8 hours per day, rather than the 18- to 20-hour goal they had set. Combining exercise with intermittent fasting improved weight loss in a 2013 study by Bhutani et al of 64 obese patients. They found weight loss doubled (6 kg) when exercise was added to intermittent fasting (level I evidence) 100). In 2019, Cho et al 101) found no improvement in weight loss when exercise was added to intermittent fasting (n = 31) (level I evidence). There were high dropout rates (≥ 25%) in several intermittent fasting studies, which compare poorly to the 12% to 14% dropout rates of other long-term diets: Atkins, Zone, LEARN (Lifestyle, Exercise, Attitudes, Relationships, and Nutrition), and Ornish 102). In direct comparisons of intermittent fasting to calorie restriction, the 2 have similar dropout rates 103). Across the intermittent fasting studies, adherence to fasting ranged from 77% to 98% (n = 265) 104). In a 2009 study, Varady et al 105) found weight loss was directly related to percentage of adherent days per week (level II evidence).

Intermittent fasting studies generally find that hunger levels remain stable or decrease during intermittent fasting. A study of 30 participants over 12 weeks by Varady et al 106) found reports of hunger during intermittent fasting were no higher than with unrestricted consumption (level I evidence). Kroeger et al 107) found that among those with the highest weight losses over 12 weeks of intermittent fasting, hunger decreased and fullness increased. In the study by Harvie et al 108), 15% of participants reported hunger. Sundfør et al  109) saw higher reported hunger in the intermittent fasting group compared with those in the calorie restriction group.

Table 1. Summary of intermittent fasting studies (intermittent fasting participants = 944)

STUDYNumber of participants
POPULATION*LEVEL OF EVIDENCEDURATIONWEIGHT LOSS, % OF BASELINE WEIGHTINTERVENTION†KEY RESULT
Anton et al 110)10Obese, > 65 yearsII4 weeks2.216-hours daily fast; self-reportedIntermittent fasting is feasible in older adults and leads to weight loss
Antoni et al 111)41Overweight and obeseIUntil 5% weight loss is reached5.32-days fast (25% of caloric needs) and 5-days ad libitum calorie intake vs calorie restriction; self-reported59 days to achieve 5% weight loss with intermittent fasting; not statistically different in calorie restriction group (73 days)
Arnason et al 112)10Obese, type 2 diabetesII2 weeks1.418- to 20-hours daily fast as a goal, but average fast was 16.8 hours; 2-weeks follow-up; self-reportedShort-term intermittent fasting might be safe in patients with type 2 diabetes and might improve glycemic control
Bhutani et al 113)64ObeseI12 weeks3.2Alternated 25% of caloric needs with ad libitum calorie intake vs usual diet with or without exercise; self-reportedIntermittent fasting in combination with exercise is more effective than either method alone
Bowen et al 114)136Overweight and obeseI16 weeks10.63-days fast, 3-days calorie restriction, and 1-day ad libitum intake vs calorie restriction; 8-weeks maintenance; self-reportedCalorie restriction combined with intermittent fasting does not improve on weight loss of calorie restriction alone
Carter et al 115)51Obese, type 2 diabetesI12 weeks5.92-days fast (1670 to 2500 kJ/d) and 5-days usual diet vs calorie restriction; self-reportedIntermittent fasting is a viable alternative to calorie restriction for weight loss and glycemic control in type 2 diabetes
Carter et al 116)137Obese, type 2 diabetesI52 weeks6.82-daysfast (25% of usual calorie intake) and 5-days usual diet vs calorie restriction; 1-year follow-up; self-reportedSimilar decrease in HbA1c level and weight with intermittent fasting or calorie restriction; weight is stable and HbA1c level climbs in follow-up
Catenacci et al 117)26ObeseI8 weeks8.7Alternated 0% usual calorie intake with ad libitum intake vs calorie restriction; 24-weeks follow-up; monitoredIntermittent fasting is a safe weight-loss strategy; no increase in risk of weight regain
Cho et al 118)31Overweight and obeseI8 weeks5Alternated 25% usual calorie intake with ad libitum intake vs usual diet with or without exercise; self-reportedExercise does not improve weight loss for intermittent fasting alone
Corley et al 119)41Obese, type 2 diabetesII12 weeks0.82-days fast (2 small snacks, 1 light meal) and 5-days ad libitum intake; self-reportedIntermittent fasting safe in type 2 diabetes; promotes weight loss and glycemic control
Coutinho et al 120)35ObeseI12 weeks133-days fast (25% of caloric needs) and 4-days full caloric needs vs calorie restriction; self-reportedSimilar weight losses result from intermittent fasting and calorie restriction
Eshghinia and Gapparov 121)26Obese womenII4 weeks4.93-days fast (25% to 40% of usual caloric intake) and 4-days calorie restriction (10% decrease in usual caloric intake) per week; self-reportedShort-term intermittent fasting with calorie restriction is a viable weight-loss strategy in obesity
Eshghinia and Mohammadzadeh 122)15Obese womenII6 weeks7.13-days fast (25% to 30% of caloric needs), 3-days usual diet, and 1-day ad libitum intake; self-reportedShort-term intermittent fasting is a viable weight loss strategy in obesity
Gabel et al 123)46ObeseI12 weeks3.216-hours fast daily vs usual-diet historical controls; self-reportedIntermittent fasting leads to weight loss compared with baseline and control group
Harvie et al 124)107Obese womenI24 weeks7.92-days fast (very low-calorie intake) and 5-days usual diet vs calorie restriction; self-reportedIntermittent fasting is as effective as calorie restriction for weight loss and insulin sensitivity
Headland et al 125)244ObeseI52 weeks5.62-days fast (25% of usual calorie intake) and 5-days usual diet vs calorie restriction; self-reportedIntermittent fasting and calorie restriction have similar weight loss results at 1 year
Hoddy et al 126)59ObeseI8 weeks4.2Alternated daily 25% of baseline caloric needs with ad libitum caloric intake; self-reportedIntermittent fasting is a safe weight-loss strategy; no increased risk of disordered eating; might decrease insulin resistance
Hutchison et al 127)88Overweight and obese womenI8 weeks4.63-days fast (32%–37% of energy requirements) and 4 days at 100% or 145% of energy requirements vs calorie restriction and control group; self-reportedCombining calorie restriction and intermittent fasting is more effective for weight loss than either alone
Kahleova et al 128)54Obese, type 2 diabetesI12 weeks3.916-hours daily fast vs calorie restriction; self-reportedIntermittent fasting is more effective than calorie restriction for weight loss and glycemic control in type 2 diabetes
Klempel et al 129)54Obese womenII8 weeks3.41-day fast (very low-calorie intake) and 6-days calorie restriction; self-reportedIntermittent fasting combined with calorie restriction promotes weight loss in obese women
Klempel et al 130)32Obese womenII8 weeks4.5Alternated 25% of usual calorie intake with 125% of usual calorie intake; high-fat vs low-fat diet; self-reportedIntermittent fasting is effective for weight loss with a high-fat or low-fat diet composition
Schübel et al 131)150ObeseI12 weeks6.42-days fast (25% of calorie requirements) and 5-days usual diet vs calorie restriction and control group; 12-weeks maintenance; 26-weeks follow-up; self-reportedWeight loss and maintenance is similar in intermittent fasting and calorie restriction
Sundfør et al 132)112ObeseI26 weeks8.42-days fast (20% of calorie requirements) and 5-days usual diet vs calorie restriction; 26-weeks maintenance; self-reportedWeight loss and maintenance are similar in intermittent fasting and calorie restriction
Trepanowski et al 133)79ObeseI24 weeks6Alternated 25% of usual calorie intake with 125% of usual calorie intake vs calorie restriction and control group; 24-weeks follow-up; self-reportedIntermittent fasting promotes weight loss and weight maintenance similar to calorie restriction
Varady et al 134)16ObeseII8 weeks5.8Alternated 25% of energy needs with ad libitum caloric intake; self-reportedIntermittent fasting is a viable option for weight loss in obese individuals
Varady et al 135)30ObeseI12 weeks6.5Alternated 25% of baseline energy needs with ad libitum caloric intake vs usual diet; monitoredIntermittent fasting is effective for weight loss in obese individuals
Zuo et al 136)40ObeseII12 weeks101-day fast (430 kcal) and 6-days high-protein diet; 52-weeks follow-up; monitored

Footnotes:

*Where sex is not specified, both men and women were enrolled.
Self-reported indicates participants reported consumption in food diaries; monitored indicates investigators monitored participants’ consumption.

Abbreviations: HbA1c = hemoglobin A1c

[Source 137) ]

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