soybean

What is a soybean

Soybean or soya bean [Glycine max (L.) Merrill, family Leguminosae, subfamily Papilionoidae] is a species of legume (a plant in the pea family) native to East Asia, probably in north and central China, has been common in Asian diets for thousands of years 1). Soy is also present in modern Western diets as a food and food ingredient. The main countries growing soybeans are the United States (32% of world total), Brazil (31%) and Argentina (18%).

Soybeans are, primarily, an industrial crop, cultivated for oil and protein. Despite the relatively low oil content of the seed (about 20% on moisture-free basis), soybeans are the largest single source of edible oil and account for roughly 50% of the total oilseed production of the world.

With each ton of crude soybean oil, approximately 4.5 tons of soybean oil meal with a protein content of about 44% are produced. For each ton of soybeans processed, the commercial value of the meal obtained usually exceeds that of the oil. Thus, soybean oil meal cannot be considered a by-product of the oil manufacture. The soybean is, in this respect, an exception among oilseeds.

It can be calculated that, the quantity of protein in the yearly world production of soybeans, if it could be totally and directly utilized for human consumption, would be sufficient for providing roughly one third of the global need for food protein. This makes the soybean one of the largest potential sources of dietary protein. However, the bulk of soybean oil meal is used in animal feeds for the production of meat and eggs. Despite considerable public and commercial interest in soybean products as food,the proportion of soybean protein consumed directly in human nutrition is still relatively small.

Traditional non-fermented food uses of soybeans include soy milk from which tofu and tofu skin are made. Fermented soy foods include soy sauce, fermented bean paste, natto and tempeh.

In addition to its food uses, soy is available in dietary supplements, in forms such as tablets, capsules, and powders. Soy supplements may contain soy protein, isoflavones (compounds that have effects in the body similar to those of the female hormone estrogen), or other soy components.

Isoflavones (genistein, daidzein, and glycitein) are bioactive compounds with mildly estrogenic properties and often referred to as phytoestrogen. These are present in significant quantities (up to 4–5 mg·g−1 on dry basis) in legumes mainly soybeans, green beans and mung beans 2). In grains (raw materials) isoflavones are present mostly as glycosides, which are poorly absorbed on consumption. Thus, soybeans are processed into various food products for digestibility, taste and bioavailability of nutrients and bioactives. Main processing steps include steaming, cooking, roasting, microbial fermentation that destroy protease inhibitors and also cleaves the glycoside bond to yield absorbable aglycone in the processed soy products, such as tempeh, miso, natto, soy milk, tofu and increase shelf lives.

Processed soy food products have been an integral part of regular diets in many Asia–Pacific countries for centuries, e.g. China, Japan and Korea. However, in the last two decades, there have been concerted efforts to introduce soy products in western diets for their health benefits with some success.

Figure 1. Soybean

soybean

Soybean nutrition facts

The composition of soybeans may vary somewhat according to variety and growing conditions.Through plant breeding it has been possible to obtain protein levels between 40% and 45%, and lipid levels between 18 and 20%. Usually, an increase of 1% in protein content is accompanied by a decrease of 0.5% in oil. Incidentally, this negative correlation between protein and oil is one of the reasons for the lack of interest in high-protein varieties, since the production of these varieties does not result in increased income per hectare cultivated.

In 100 grams, raw soybeans supply 446 calories and are 9% water, 30% carbohydrates, 20% total fat and 36% protein (Table 1).

Soybeans are an exceptional source of essential nutrients, providing in a 100 gram serving (raw, for reference) high contents of the Daily Value (DV) especially for protein (36% DV), dietary fiber (37%), iron (121%), manganese (120%), phosphorus (101%) and several B vitamins, including folate (94%) (table). High contents also exist for vitamin K, magnesium, zinc and potassium (Table 1).

For human consumption, soybeans must be cooked with “wet” heat to destroy the trypsin inhibitors (serine protease inhibitors). Raw soybeans, including the immature green form, are toxic to all monogastric animals 3).

Soy protein

Soybean protein is a well-known botanical protein that is regarded as a kind of complete protein, highly valuable in promoting health 4). The cardioprotective effects of soybean protein have been proven by evaluating the association between dietary protein source, protein level, and serum lipid profile in male rats 5). It was found that the total serum triglyceride level was significantly lowered after long-term intake of soybean protein, indicating the possibility of reducing the risks of atherosclerosis. It was also reported that soybean protein possessed cardioprotective effects, partially by improving serum lipids via modifying the expression of sterol regulatory element-binding protein-2 and its downstream genes (hydroxymethylglutaryl-coenzyme A reductase and LDL receptor), and increasing the antioxidant activities of SOD and catalase 6).

Most soy protein is a relatively heat-stable storage protein. This heat stability enables soy food products requiring high temperature cooking, such as tofu, soy milk and textured vegetable protein (soy flour) to be made.

Soy is a good source of protein, amongst many others, for vegetarians and vegans or for people who want to reduce the amount of meat they eat. According to the US Food and Drug Administration: Soy protein products can be good substitutes for animal products because, unlike some other beans, soy offers a ‘complete’ protein profile 7). Soy protein products can replace animal-based foods—which also have complete proteins but tend to contain more fat, especially saturated fat—without requiring major adjustments elsewhere in the diet.

The Protein Digestibility Corrected Amino Acid Score (PDCAAS) of soy protein is the nutritional equivalent of meat, eggs, and casein for human growth and health. Soybean protein isolate has a biological value of 74, whole soybeans 96, soybean milk 91, and eggs 97 8).

Soy protein is essentially identical to the protein of other legume seeds and pulses 9). Moreover, soybeans can produce at least twice as much protein per acre than any other major vegetable or grain crop besides hemp, five to 10 times more protein per acre than land set aside for grazing animals to make milk, and up to 15 times more protein per acre than land set aside for meat production 10).

Carbohydrates

The principal soluble carbohydrates of mature soybeans are the disaccharide sucrose (range 2.5–8.2%), the trisaccharide raffinose (0.1–1.0%) composed of one sucrose molecule connected to one molecule of galactose, and the tetrasaccharide stachyose (1.4 to 4.1%) composed of one sucrose connected to two molecules of galactose 11). While the oligosaccharides raffinose and stachyose protect the viability of the soybean seed from desiccation they are not digestible sugars, so contribute to flatulence and abdominal discomfort in humans and other monogastric animals, comparable to the disaccharide trehalose. Undigested oligosaccharides are broken down in the intestine by native microbes, producing gases such as carbon dioxide, hydrogen, and methane.

Since soluble soy carbohydrates are found in the whey and are broken down during fermentation, tempeh, soy concentrate, soy protein isolates, tofu, soy sauce, and sprouted soybeans are without flatus activity. On the other hand, there may be some beneficial effects to ingesting oligosaccharides such as raffinose and stachyose, namely, encouraging indigenous bifidobacteria in the colon against putrefactive bacteria.

The insoluble carbohydrates in soybeans consist of the complex polysaccharides cellulose, hemicellulose, and pectin. The majority of soybean carbohydrates can be classed as belonging to dietary fiber.

Fats

Raw soybeans are 20% fat, including saturated fat (3%), monounsaturated fat (4%) and polyunsaturated fat, mainly as linoleic acid (Table 1 and 4).

Within soybean oil or the lipid portion of the seed is contained four phytosterols: stigmasterol, sitosterol, campesterol, and brassicasterol accounting for about 2.5% of the lipid fraction; and which can be converted into steroid hormones. Additionally soybeans are a rich source of sphingolipids 12).

Table 1. Soybeans green (raw) nutrition facts

NutrientUnitValue per 100 g
Approximates
Waterg67.5
Energykcal147
EnergykJ614
Proteing12.95
Total lipid (fat)g6.8
Ashg1.7
Carbohydrate, by differenceg11.05
Fiber, total dietaryg4.2
Minerals
Calcium, Camg197
Iron, Femg3.55
Magnesium, Mgmg65
Phosphorus, Pmg194
Potassium, Kmg620
Sodium, Namg15
Zinc, Znmg0.99
Copper, Cumg0.128
Manganese, Mnmg0.547
Selenium, Seµg1.5
Vitamins
Vitamin C, total ascorbic acidmg29
Thiaminmg0.435
Riboflavinmg0.175
Niacinmg1.65
Pantothenic acidmg0.147
Vitamin B-6mg0.065
Folate, totalµg165
Folic acidµg0
Folate, foodµg165
Folate, DFEµg165
Vitamin B-12µg0
Vitamin A, RAEµg9
Retinolµg0
Vitamin A, IUIU180
Vitamin D (D2 + D3)µg0
Vitamin DIU0
Lipids
Fatty acids, total saturatedg0.786
14:00:00g0.006
16:00:00g0.57
18:00:00g0.21
Fatty acids, total monounsaturatedg1.284
16:1 undifferentiatedg0.011
18:1 undifferentiatedg1.262
20:01:00g0.011
Fatty acids, total polyunsaturatedg3.2
18:2 undifferentiatedg2.823
18:3 undifferentiatedg0.376
Fatty acids, total transg0
Cholesterolmg0
Phytosterolsmg50
Amino Acids
Tryptophang0.157
Threonineg0.516
Isoleucineg0.57
Leucineg0.926
Lysineg0.775
Methionineg0.157
Cystineg0.118
Phenylalanineg0.586
Tyrosineg0.464
Valineg0.576
Arginineg1.042
Histidineg0.348
Alanineg0.582
Aspartic acidg1.508
Glutamic acidg2.433
Glycineg0.539
Prolineg0.607
Serineg0.721
Other
Caffeinemg0
Theobrominemg0
Flavones
Luteolinmg0
Flavonols
Kaempferolmg1.2
Myricetinmg0
Quercetinmg0
Isoflavones
Daidzeinmg20.34
Genisteinmg22.57
Glyciteinmg7.57
Total isoflavonesmg48.95
Biochanin Amg0
Formononetinmg0
Coumestrolmg0
[Source: United States Department of Agriculture Agricultural Research Service 13)]

Table 2. Soybeans mature seeds (raw) nutrition facts

[Source: United States Department of Agriculture Agricultural Research Service 14)]

Table 3. Soybeans mature seeds sprouted (raw) nutrition facts

[Source: United States Department of Agriculture Agricultural Research Service 15)]

Soybean Uses

Among the legumes, the soybean is valued for its high (38–45%) protein content as well as its high (approximately 20%) oil content. Soybeans are the second-most valuable agricultural export in the United States, behind corn. Approximately 85% of the world’s soybean crop is processed into soybean meal and soybean oil, the remainder processed in other ways or eaten whole 16).

Soybeans can be broadly classified as “vegetable” (garden) or field (oil) types. Vegetable types cook more easily, have a mild, nutty flavor, better texture, are larger in size, higher in protein, and lower in oil than field types. Tofu and soy milk producers prefer the higher protein cultivars bred from vegetable soybeans originally brought to the United States in the late 1930s. The “garden” cultivars are generally not suitable for mechanical combine harvesting because there is a tendency for the pods to shatter upon reaching maturity.

Soybean oil

Soybean seed contains 18-19% oil 17). To extract soybean oil from seed, the soybeans are cracked, adjusted for moisture content, rolled into flakes and solvent-extracted with commercial hexane. The oil is then refined, blended for different applications, and sometimes hydrogenated. Soybean oils, both liquid and partially hydrogenated, are exported abroad, sold as “vegetable oil”, or end up in a wide variety of processed foods.

Table 4. Soybeans oil nutrition facts

[Source: United States Department of Agriculture Agricultural Research Service 18)]

Soybean meal

Soybean meal, or soymeal, is the material remaining after solvent extraction of oil from soybean flakes, with a 50% soy protein content. The meal is ‘toasted’ (a misnomer because the heat treatment is with moist steam) and ground in a hammer mill. Ninety-seven percent of soybean meal production globally is used as livestock feed. Soybean meal is also used in some dog foods.

Livestock feed

One of the major uses of soybeans globally is as livestock feed, predominantly in the form of soybean meal. Spring grasses are rich in omega-3 fatty acids, whereas soy is predominantly omega-6. The soybean hulls, which mainly consist of the outer coats of the beans removed before oil extraction, can also be fed to livestock, as well as whole soybean seeds after processing.

Food for human consumption

In addition to their use in livestock feed, soybean products are widely used for human consumption. Common soybean products include soy sauce, soy milk, tofu, soy meal, soy flour, textured vegetable protein, tempeh, soy lecithin and soybean oil. Soybeans may also be eaten with minimal processing, for example in the Japanese food edamame (edamame), in which immature soybeans are boiled whole in their pods and served with salt.

In China, Japan, and Korea, soybean and soybean products are a common part of the diet. Tofu is thought to have originated in China, along with soy sauce and several varieties of soybean paste used as seasonings.[citation needed] Japanese foods made from soya include miso, nattō, kinako and edamame, as well as products made with tofu such as atsuage and aburaage. In Korean cuisine, soybean sprouts (kongnamul) are used in a variety of dishes, and are the base ingredient in doenjang, cheonggukjang and ganjang. In Vietnam, soybeans are used to make soybean paste (tương) in the North with the most popular products are tương Bần, tương Nam Đàn, tương Cự Đà as a garnish for phở and gỏi cuốn dishes, as well as tofu, soy sauce, soy milk and tofu sweet soup.

Flour

Soy flour refers to soybeans ground finely enough to pass through a 100-mesh or smaller screen where special care was taken during desolventizing (not toasted) to minimize denaturation of the protein to retain a high protein dispersibility index, for uses such as food extrusion of textured vegetable protein. It is the starting material for production of soy concentrate and soy protein isolate.

Soy flour is made by roasting the soybean, removing the coat, and grinding into a flour. Soy flour is manufactured with different fat levels.[113] Alternatively, raw soy flour omits the roasting step.

  • Defatted soy flour is obtained from solvent extracted flakes, and contains less than 1% oil.
  • “Natural or full-fat soy flour is made from unextracted, dehulled beans, and contains about 18% to 20% oil.” Its high oil content requires the use of a specialized Alpine Fine Impact Mill to grind rather than the usual hammer mill. Full-fat soy flour has a lower protein concentration than defatted flour.
  • ow-fat soy flour is made by adding some oil back into defatted soy flour. Fat levels range from 4.5% to 9%.
  • High-fat soy flour can also be produced by adding back soybean oil to defatted flour, usually at the level of 15%.

Soy lecithin can be added (up to 15%) to soy flour to make lecithinated soy flour. It increases dispersibility and gives it emulsifying properties.

Soy flour has 50% protein and 5% fiber. It has higher levels of protein, thiamine, riboflavin, phosphorus, calcium, and iron than wheat flour. It does not contain gluten. As a result, yeast-raised breads made with soy flour are dense in texture. Among many uses, soy flour thickens sauces, prevents staling in baked food, and reduces oil absorption during frying. Baking food with soy flour gives it tenderness, moistness, a rich color, and a fine texture.

Soy grits are similar to soy flour except the soybeans have been toasted and cracked into coarse pieces.

Kinako is a soy flour used in Japanese cuisine.

Soy-based infant formula

Soy-based infant formula is sometimes given to infants who are not being strictly breastfed; it can be useful for infants who are either allergic to pasteurized cow milk proteins or who are being fed a vegan diet. It is sold in powdered, ready-to-feed, and concentrated liquid forms.

Some reviews have expressed the opinion that more research is needed to determine what effect the phytoestrogens in soybeans may have on infants 19). Diverse studies have concluded there are no adverse effects in human growth, development, or reproduction as a result of the consumption of soy-based infant formula 20). One of these studies, published in the Journal of Nutrition, concludes that there are 21):

“no clinical concerns with respect to nutritional adequacy, sexual development, neurobehavioral development, immune development, or thyroid disease. SBIFs provide complete nutrition that adequately supports normal infant growth and development. FDA has accepted SBIFs as safe for use as the sole source of nutrition.”

Meat and dairy alternatives and extenders

Soybeans can be processed to produce a texture and appearance similar to many other foods. For example, soybeans are the primary ingredient in many dairy product substitutes (e.g., soy milk, margarine, soy ice cream, soy yogurt, soy cheese, and soy cream cheese) and meat alternatives (e.g. veggie burgers). These substitutes are readily available in most supermarkets. Soy milk does not naturally contain significant amounts of digestible calcium. Many manufacturers of soy milk sell calcium-enriched products, as well. Soy is also used in tempeh: the beans (sometimes mixed with grain) are fermented into a solid cake.

Soy products also are used as a low-cost substitute in meat and poultry products 22), 23). Food service, retail and institutional (primarily school lunch and correctional) facilities regularly use such “extended” products. Extension may result in diminished flavor, but fat and cholesterol are reduced. Vitamin and mineral fortification can be used to make soy products nutritionally equivalent to animal protein; the protein quality is already roughly equivalent. The soy-based meat substitute textured vegetable protein has been used for more than 50 years as a way of inexpensively extending ground beef without reducing its nutritional value 24).

Soy nut butter

The soybean is used to make a product called soy nut butter which is similar in texture to peanut butter 25).

Sweetened soybean

Sweet boiled beans are popular in Japan and Korea and the sweet boiled soybeans are called as “Daizu no Nimame (ja)” in Japan and Kongjorim in Korea. Sweet boiled beans are even used in sweetened buns.

The boiled and pasted edamame, called Zunda, is used as one of the Sweet bean pastes in Japanese confections.

Coffee substitute

Roasted and ground soybeans can be used as a caffeine-free substitute for coffee. After the soybeans are roasted and ground, they look similar to regular coffee beans or can be used as a powder similar to instant coffee, with aroma and flavor of roasted soybeans 26).

Other products

Soybeans with black hulls are used in Chinese fermented black beans, douchi, not to be confused with black turtle beans.

Soybeans are also used in industrial products, including oils, soap, cosmetics, resins, plastics, inks, crayons, solvents, and clothing. Soybean oil is the primary source of biodiesel in the United States, accounting for 80% of domestic biodiesel production.[130] Soybeans have also been used since 2001 as fermenting stock in the manufacture of a brand of vodka.[131] In 1936, Ford Motor Company developed a method where soybeans and fibers were rolled together producing a soup which was then pressed into various parts for their cars, from the distributor cap to knobs on the dash board. Ford also informed in public relation releases that in 1935 over five million acres (20,000 km2) was dedicated to growing soybeans in the United States.[132]

Soybean benefits

Soybeans are a common vegetable that can be used to extract soybean oil and make soy milk. Polyphenols, mainly including phenolic acid and flavonoids like flavones and flavonols, are among the most important bioactive components extracted from soybeans 27). It was reported that phenolic acid mainly contributed to the antioxidant capacities of many natural products 28). Many researchers suggested that polyphenols possessed biological effects like antioxidation and anti-inflammation, which in turn provided cardiovascular protection 29). In an in test tube study, phenolic-rich extracts from soybeans were found to inhibit the activities of α-amylase, α-glucosidase, and angiotensin-I converting enzyme (ACE), which are key enzymes linked to diabetes and hypertension 30). Thus, researchers came to the conclusion that soybeans have health-promoting effects including anti-diabetes and anti-hypertension. Another study investigated the effects of saponin (2-phenyl-benzopyrane), a soybean flavonoid, on glucose tolerance and risk factors for atherosclerosis 31). In saponin-treated animals, the LDL “bad” cholesterol/triglyceride ratio was increased, and triglyceride, very low-density lipoprotein cholesterol (VLDL-C), lipid hydroperoxide, and total cholesterol/HDL “good” cholesterol ratio were decreased. However, no effects were found on glucose tolerance, LDL “bad” cholesterol, superoxide dismutase (SOD), and glutathione peroxidase (GPx) in the experimental groups. These observations indicated that saponin from soybeans might improve the serum lipid profile due to direct antioxidant activity.

It was reported that soybean products could be enhanced in nutritional value after fermentation 32). For instance, doenjang was more effective at preventing diet-induced visceral fat accumulation than non-fermented soybeans in rats, by stimulating carnitine palmitoyltransferase-1 activity and suppressing fatty acid synthase activity, possibly due to the higher content of aglycone isoflavones 33).

It was also reported that soybeans contain considerable phytoestrogens, like isoflavones (mainly genistein and daidzein) and lignans, which are safe and natural estrogen receptor modulator alternatives to hormone therapy and possess antioxidant and cardioprotective effects 34).

Phytoestrogens are nonsteroidal plant compounds of diverse structure that are found in many fruits, vegetables and grains 35). The most common types of phytoestrogens are coumestans, lignans and isoflavones. These compounds structurally resemble oestradiol (E2) and are shown to have weak oestrogenic activity 36). When ingested in relatively large quantities, dietary phytoestrogens have been shown to have significant biological effects in several animal species 37) and in humans 38). In humans, they appear to have both oestrogenic and anti-oestrogenic effects, depending on the concentrations of circulating endogenous oestrogens and oestrogen receptors 39).

Isoflavones are among the most oestrogenically potent phytoestrogens; the major dietary isoflavones, genistein and daidzein, are found almost exclusively in legumes such as soybean, chick peas, lentils and beans 40). Urinary excretion of equol, a weak oestrogen, in humans eating soy-supplemented diets can greatly exceed the concentration of urinary endogenous oestrogens; this enhances the plausibility of human physiological health effects 41). Other classes of phytoestrogens—lignans and prenylated flavonoids—also have potent oestrogenic activity but are not as well studied 42).

Soy, a particularly abundant source of isoflavones, is a staple ingredient in the traditional Asian diet. It is postulated that high intake of soy among Asian women may account for lower rates of some menopausal symptoms in this group. Asian populations, such as those in Japan, Taiwan and Korea, are estimated to consume 20 to 150 mg per day of isoflavones, with a mean of about 40 mg from tofu (soy bean curd) and miso (soy bean paste). Soy includes such products as tofu, miso, aburage (fried thin tofu) and fermented tempeh or boiled soy beans. Further evidence that soy might be beneficial is suggested by a cohort study of Japanese women 43), which found a significant inverse association between frequency of flushes and higher levels of soy consumption. However, the findings of this study are contradicted by data from a cross-sectional study, which found that women who frequently consumed soy products were not less likely to report hot flushes or night sweats than women who never consumed soy products 44). Thus it is not clear whether frequent soy consumption explains the lower rate of hot flushes among different ethnic groups.

Potential adverse effects of phytoestrogens have included deficits in sexual behaviour in rats and impaired fertility in livestock 45). No specific examples of toxicity among humans have been noted in countries in which soy is consumed regularly 46). It is generally considered difficult for humans to consume the quantity of isoflavones from natural soy foods needed to reach toxicological levels that induce pathological effects, as recorded in animals.

Soybean’s isoflavones were hailed as magical natural component that attribute to prevent some major prevailing health concerns. Consumption of soy products have been linked to reduction in incidence or severity of chronic diseases such as cardiovascular, breast and prostate cancers, menopausal symptoms, bone loss, etc. Although there have been many studies on soy products, there are still uncertainties about soy’s health effects.

  • Consuming soy protein in place of other proteins may lower levels of LDL (“bad”) cholesterol to a small extent 47). Regular consumption of 1 to 2 servings of soy protein daily (15 to 30 g) was associated with a significant improvement in lipoprotein risk factors for coronary heart disease.
  • Soy isoflavone supplements may help to reduce the frequency and severity of menopausal hot flashes, but the effect may be small 48), 49).
  • Soybean isoflavones, especially genistein and daidzein, are common phytoestrogens recognized as selective estrogen receptor modulators that possess cardioprotective effects in vitro and in vivo, but there is a lack of promising outcomes in clinical trials. In a six-month randomized controlled trial, purified daidzein did not exhibit significant effects on body weight, body mass index, waist and hip circumferences, waist to hip ratio, body fat percentage, fat mass, and free fat mass in equol-producing postmenopausal women with prehypertension 50). In the same study, it was found that purified daidzein had no significant effect on blood pressure and vascular function 51). However, in the above two studies urinary isoflavones suggested good compliance of the patients with the interventions.
  • It’s uncertain whether soy supplements can relieve cognitive problems associated with menopause 52).
  • Current evidence suggests that soy isoflavone mixtures do not slow bone loss in Western women during or after menopause 53).
  • Diets containing soy protein may slightly reduce blood pressure 54).
  • Cardiovascular disease is a major cause of morbidity and mortality in developed and developing countries. Evidence is increasing that the consumption of soy protein instead of animal protein lowers blood cholesterol levels and may lower the risk of cardiovascular disease. A meta-analysis showed that the substitution of soy protein for animal protein significantly lowered total cholesterol, low-density lipoprotein (LDL “bad”) cholesterol, and triglycerides without affecting high-density (HDL “good”) lipoprotein cholesterol 55). Since then, well-controlled studies of soy protein and soy-derived isoflavones have demonstrated that the impact of soy protein consumption on LDL cholesterol levels was small 56), 57). Dietary soy may be beneficial to cardiovascular health because of its high polyunsaturated fat, fiber, vitamin, and mineral content combined with its low saturated fat content 58).
  • Results from clinical trials also indicated that a combination of isoflavones and soybean protein might not be an effective intervention to prevent cardiovascular disease 59), 60). In a randomized controlled trial, isoflavone soybean protein supplementation did not result in a significant reduction of subclinical atherosclerosis progression in postmenopausal women 61). While subgroup analysis indicated that isoflavone soybean protein supplementation could reduce subclinical atherosclerosis in healthy young women (median age: 53 years) less than five years postmenopausal who were at low risk for cardiovascular diseases. In a double-blind randomized, placebo-controlled trial conducted among 180 postmenopausal Chinese women, soybean protein combined with isoflavones at the provided dosage (15 g soybean protein, 100 mg isoflavones) had no significant effect on measured cardiovascular risk factors, including serum HDL “good” cholesterol, LDL “bad” cholesterol, total cholesterol, triglyceride and highly sensitive C-reactive protein 62).
  • There is a fair amount of population-based evidence showing that soy isoflavones can protect against prostate cancer 63). Unfortunately, the mechanisms supporting this association have not yet been resolved. Nonetheless, mechanistic explanations for how soy isoflavones can reduce cancer risk or disrupt the biology of transformed cells are increasing. Although the role of soy isoflavones in prostate cancer has traditionally been linked with the suppression of proliferation and the induction of apoptosis, recently there is a compelling evidence that soy isoflavones regulate other cancer-related cellular processes. For example, a reasonable number of reports suggests that the antioxidant actions of soy isoflavones are worth exploring for cancer prevention.
  • There’s not enough scientific evidence to determine whether soy supplements are effective for any other health uses.

Soybean side effects

Except for people with soy allergies, soy is believed to be safe when consumed in normal dietary amounts. However, the safety of long-term use of high doses of soy extracts has not been established.

  • The most common side effects of soy are digestive upsets, such as stomach pain and diarrhea.
  • Antinutritional factors: Several of the soybean proteins have been found to exert specific physiological effects. These are the trypsin inhibitors and the hemagglutinins (lectins). Protease inhibiting proteins are widespread in nature, but the trypsin inhibitors of soybeans are the best known and most thoroughly studied. Inhibition of trypsin by raw soybeans has been reported more than 50 years ago. Soybeans contain two types of trypsin inhibitors. Both bear the names of scientists who first isolated and characterized them. They are respectively known as the Kunitz inhibitor with a molecular weight in the range of 20000, and the Bowman-Birk inhibitor which is a much smaller polypeptide 64). It has been known for a long time that raw soybeans or unheated soybean meal will impair growth when fed to young rats or chicks. This effect is completely eliminated when the soybean component is properly heated. Since trypsin inhibitors are also heat labile, it was concluded that their presence in the diet is responsible for the suppression of growth. In fact, growth is retarded if the inhibitors are added to diets containing heat-treated soybean meal. A logical explanation for the harmful effect of the inhibitors could be that the inhibition of trypsin in the digestive track of the animal impairs protein digestibility and utilization. This hypothesis had to be abandoned, however, when it was observed that trypsin inhibitor preparations did impair growth when fed with diets containing completely pre-digested proteins. Inhibition of trypsin is not the only physiological effect of the trypsin inhibitors. It has been observed that their ingestion can result in increased pancreatic secretion and hypertrophy of the pancreas. Increased secretion of enzymes into the digestive tube represents an internal loss of protein. Since the proteins excreted by the pancreas are particularly rich in sulphur containing amino acids, this internal loss could be specially important if the diet is marginal in methionine/cystine.Are soybean trypsin inhibitors toxic to humans ? The bulk of the available information on their biochemical, physiological and nutritional properties stems from experimentation with animals or from in vitro investigations. There is no direct evidence as to the physiological effect of the inhibitors on humans. Nevertheless, it has become customary to take the necessary precautions for the removal or inactivation of trypsin inhibitors from soybean products intended for human consumption.The lectins, formerly known as hemagglutinins, are proteins which possess the ability to agglutinate red blood cells. They are widely distributed in plants and some, such as the castor bean lectin ricin, are highly toxic. The lectin found in raw soybeans has, apparently, no observable dietary effect, good or bad. Furthermore, it too is easily inactivated by heat.
  • Long-term use of soy isoflavone supplements might increase the risk of endometrial hyperplasia (a thickening of the lining of the uterus that may lead to cancer). Soy foods do not appear to increase the risk of endometrial hyperplasia.
  • Current evidence indicates that it’s safe for women who have had breast cancer or who are at risk for breast cancer to eat soy foods 65). However, it’s uncertain whether soy isoflavone supplements are safe for these women 66).
  • A prospective cohort study (Shanghai Women’s Health Study) was conducted in 66,832 Chinese women (aged 40–70 y) who had no cardiovascular disease or cancer at baseline, found the women who habitually consume soy isoflavones over 10 years may be associated with a modest but significant increase in risk of ischemic stroke 67).  However to date, limited epidemiologic studies that stroke risk associated with phytoestrogens have been mixed and inconclusive. Isoflavone intake was not associated with stroke incidence in Dutch women 68) but showed a positive, although nonsignificant, association with stroke mortality in 2 studies of US women 69), 70). In Asian countries, where soy consumption is traditionally common and relatively high (median isoflavone intake: 15–40 mg/d in Asian populations vs. 0.3–0.4 mg/d in Western populations), increased isoflavone intake was associated with lower stroke incidence in Japanese women 71) but not associated with stroke mortality in women from another Japanese cohort 72) or in women in the Singapore Chinese Health Study 73). The discrepant findings from these studies may be partly explained by differences in the amount of consumption, type of soy products consumed, population characteristics, assessment of isoflavone exposure, and variations in isoflavone metabolism.

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