The protein content of soybeans is 36% by weight11. A serving of soybeans (125 ml, 100g cooked) provides 16,6g protein or 30% of the recommended dietary intake (RDI) for protein for a 70-kg adult14.Although soybeans are recognised as being high in protein, the quality of soybean protein is often underestimated12. Until recently the protein-efficiency ratio, based on the growth of rats, was the standard method of evaluating protein quality. Rats have a methionine requirement that is 50% higher than that of humans. Consequently, because soybean proteins are relatively low in sulphur amino acids (SAAs), the protein-efficiency ratios of soybeans are quite low12. However, the World Health Organization (WHO) and the US Food and Drug Administration (FDA) have adopted an alternative method for evaluating protein quality, namely the protein digestibility corrected amino acid score (PDCAAS)15. This method uses the amino acid score (based on the Food and Agriculture Organization (FAO) estimated amino acid requirement for 2-5-year-old children) and a correction factor for digestibility to arrive at a value for protein quality. The PDCAASs of most beans are reasonably good, although their overall value is reduced somewhat by their lower digestibility16. Some types of soy protein products have PDCAASs of close to one, the same score as that of casein and egg protein4. Interestingly, the relatively low SAA content of beans may actually provide an advantage in terms of calcium retention. It has been estimated that every gram of protein consumed causes urinary loss of 1mg calcium5 (which may appear to be a trivial amount, but may increase dietary calcium requirements markedly, because the average calcium absorption from foods is 30%). The hypercalciuric effect of protein is likely to be at least partially due to the metabolism of SAAs. The skeletal system serves as one of the main buffering systems in the body; as a result, the hydrogen ions produced from the metabolism of SAAs cause demineralisation of bone and excretion of calcium in the urine5. According to Messina 12 human studies showed that the consumption of soy protein is associated with a markedly lower urinary calcium excretion compared with the consumption of similar amounts of whey protein or a mixture of animal proteins.
Although some soyfoods are relatively high in fat, they may still be lower in total fat than the foods they frequently replace, such as meats and cheeses. Soyfoods are certainly lower in saturated fat and are cholesterol-free. The consumption of full-fat soyfoods contributes significantly to alpha-linolenic acid intake, a n-3 fatty acid not commonly found in plant foods (alpha-linolenic acid makes up 7-8% of the total fat) 12. Furthermore, sterols in soybeans inhibit cholesterol absorption in the small intestine, thereby decreasing serum cholesterol concentrations17. Plant sterol- or stenol-enriched table spreads (margarine) have recently been launched in Westernised countries as a heart-health product. The structures of plant sterols are similar to that of cholesterol with an extra methyl or ethyl group and a double bond in the ring structure. Saturated plant sterols, referred to as stanols, have no double bond in the ring structure. Free plant sterols are esterified to increase solubility in the spreads. Esterified plant sterols and stanols in spreads lower total cholesterol and low-density lipoprotein cholesterol (LDLC) by about 8% and 13%, respectively 17.
The total carbohydrate content of soybeans is about 30% and is primarily complex structural and storage polysaccharides 10,11. The main storage carbohydrate is starch, with small amounts of monosaccharides and disaccharides such as sucrose. The oligosaccharides – raffinose, stachyose and verbascose – are not hydrolysed in the small intestine because there is no alpha-galactosidase in the human intestinal mucosa and are fermented to short chain fatty acids (SCFAs) and (methane, hydrogen and carbon dioxide) gas in the colon 10. It is generally accepted that many of the beneficial effects of starch and fibre can be attributed to the production of these SCFAs. Because of the discomfort and social embarrassment associated with flatulence, some people opt to avoid beans entirely 12. Soy flour derived from a new variety of soybeans that are naturally low in indigestible oligosaccharides produces significantly less gas than that derived from conventional soybeans 18. However, the beneficial effects associated with oligosaccharide consumption will then be diminished. The role of the oligosaccharides found in soybeans in the promotion of bifidobacteria development in the colon is still under study. Favourable effects on gastrointestinal function (faecal bulking and production of SCFAs) 7 as well as on metabolism (reduction of serum cholesterol, improved glucose tolerance 7 and mineral absorption 19) have been found. Because of their potential health benefits, soy oligosaccharides are available as commercial sweeteners in Japan 20. Oligosaccharides can be classified as prebiotics 7. Prebiotics are defined as nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, and thus improve host health. Favourable bacterial populations, such as bifidobacteria, can promote health by inhibiting pathogenic bacteria such as Clostridium perfringens and Escherichia coli7. Most soy foods, including soybeans, soy flour and textured soy protein, are rich in dietary fibre (non-starch polysaccharides). However, isolated soy protein does not contain dietary fibre.10 Insoluble fibre components aid in gastrointestinal function due to the bulking properties, water-holding capacity and fermentability 10. The soluble fibre components measurably lower the postprandial increase in serum glucose concentrations 10.
The complex carbohydrate and dietary fibre content of soybeans contribute to the very low glycaemic index (GI) of 18 21. The GI ranks foods on the basis of their measured blood glucose response to a specific food 2. Diets with a low GI may decrease the risk of Type 2 diabetes (non-insulin dependent diabetes mellitus), reduce mean blood glucose concentrations, reduce insulin secretion and reduce serum triglycerides in individuals with hypertriglyceridaemia 22.
From Table 2 it is clear that soybeans are low in sodium but are excellent sources of iron, and phosphorus. Soybeans also provide riboflavin, folate and magnesium 10,11,13. However, iron availability from legumes is poor and thus their value as a source of iron is limited 23. Zinc and calcium availability, on the other hand, are relatively good at 25% and 20%, respectively 12.
It was recently recognised that the human diet contains, in addition to essential macro- and micronutrients, a complex array of naturally occuring bioactive non-nutrients called phytochemicals (plant chemicals) that may confer long-term health benefits 10. This concept resulted in the development of the relatively new field of functional foods. Functional foods are foods that, by virtue of physiologically active food components, provide health benefits beyond basic nutrition 24. Functional foods are the ‘hottest’ topic in the food industry and nutrition today 25. Consumers are concerned about maintaining good health and are becoming more educated in ways to achieve this through their diets. Soybeans provide a wide variety of phytochemicals that may have a significant role in reducing risk of several chronic diseases. Traditionally some of these components have been considered to be antinutrients, such as trypsin inhibitors, phytate (inositol hexaphosphate) and saponins 10, but latest research indicates the contrary as summarised in the section below.
Some of the major nutrients provided by soybeans are shown in the table.
* MRC Food Composition Tables 13
Trypsin inhibitors from soybeans can certainly interfere with protein digestion, cause pancreatic enlargement and enhance chemically induced pancreatic tumours in some animal species 12. However, boiling beans generally reduces the trypsin inhibitor content by 80-90% 12. In humans, harmful effects have only been reported in instances where the beans were not properly cooked 12. In contrast to the trypsin inhibitor, the chymotrypsin and trypsin inhibitor (Bowman-Birk inhibitor) found in soybeans has anticarcinogenic activity in various tissues in animal models 26.
Phytate is largely responsible for the poor iron bioavailability from soybeans 27. On average, the phytate content of beans is 1-2% 12. However, phytate has antioxidant effects and has been known to lower the risk of colon cancer in vitro and animal studies .28.
The saponins in soybeans are triterpene glycosides, which are very poorly absorbed by humans 12. Most saponins form insoluble complexes with 3-ß-hydroxysteroids and are known to form large, mixed micelles with bile acids and cholesterol 12. Although saponins were shown to lower cholesterol in some animal species, the hypocholesterolaemic effects of saponins in humans are more speculative 29. Saponins may have anticancer properties, as suggested by a study in mice,30.
Isoflavones are unique groups of phyto-oestrogens (plant chemicals) found almost exclusively in soybeans. The isoflavones are strikingly similar in chemical structure to mammalian oestrogens 31 and share some (but not all) of the physiological properties of oestrogen. They are currently being studied for their potential role in the prevention and treatment of a range of hormone-dependent conditions, including cancer, menopausal symptoms, cardiovascular disease and osteoporosis31. These potential health benefits are consistent with epidemiologic evidence that these health problems are less common among populations with diets that are traditionally high in soy products. The primary isoflavones in soybeans are genistein and daidzein and their respective ß-glycosides genistein and daidzein 12. Smaller amounts of glycitein and its glycoside, glycitin are present. Although isoflavones are weak oestrogens, the current hypothesis is that isoflavones exert anti-oestrogenic effects in a high-oestrogen environment, such as exists in premenopausal women, and oestrogenic effects in a low-oestrogen environment, such as exists in postmenopausal women 12. The anti-oestrogenic effects of soy isoflavone consumption may lower breast cancer risk in premenopausal women whereas oestrogenic effects may benefit the cardiovascular system, bone and vasomotor systems in peri- and postmenopausal women. Isoflavones control the growth and regulation of many different types of cells by affecting the activity of certain enzymes and by affecting the level of specific growth factors 31. Intestinal microflora play a key role in the metabolism and bioavailability of the isoflavones. After ingestion, the isoflavones are hydrolysed by intestinal glucosidases, which release daidzein, genistein and glycitein. These may be absorbed or further metabolised to many specific metabolites including equol 31. The extent of this metabolism appears to be highly variable among individuals and is influenced by other components in the diet. A high carbohydrate milieu, which causes increased intestinal fermentation, results in more extensive biotransformation of phyto-oestrogens, with greatly increased formation of equol 31. The importance of microflora in the metabolism of phyto-oestrogens is evident from the illustration that antibiotic administration blocks metabolism and infants fed soy formulas in the first four months of life, when gut microflora are underdeveloped, cannot form appreciable amounts of equol 31.
Is there a recommended amount of isoflavones to consume everyday?
The total amount of isoflavones in soy products varies with the type of soybean, geographic area of cultivation and processing 10. Products that contain most of the bean, such as roasted soybeans, soy flour, soy flakes, textured soy protein and traditional soyfoods such as tofu and soy drinks are excellent sources, but alcohol-extracted products such as soy protein concentrate, have much lower amounts 10.
Are the isoflavones potentially toxic?
Isoflavones are now being extracted to provide the consumer with commercial phytochemical supplements as an alternative to a soy protein diet. There is a distinct possibility of risk associated with the use of these compounds as uncontrolled over-the-counter pharmacological agents 31. Two recent studies 34,35 suggest that the purified isoflavones do not have the same effects on plasma lipid concentration as isoflavones in the presence of soy protein. The question whether the purified isoflavone pills have any benefits on cancer or bone remains unanswered.
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