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Soya: the healthy option for fighting disease

Sarah Schenker
BSc SRD PhD
Dietician
British Nutrition
Foundation
London

The effect of soya consumption on health has been linked with cancer, heart disease, osteoporosis, cognitive function and menopausal symptoms and is currently under investigation in a number of areas. Epidemiological evidence has shown that populations consuming high intakes of soya have a lower prevalence of the above diseases, and this has been attributed to the protective role of soya.(1)

What is soya?
Soya beans belong to the pulses category, which along with soya products plays an important role in the traditional diets in Asian countries and plays only a minor dietary role in Western countries. Those who do eat soya and soya products do so for a number of reasons. It is a valuable source of high-quality protein (meaning that it provides good amounts of essential amino acids) for vegans. It is also useful for those diagnosed with cows' milk allergy and lactose intolerance.
Soya beans are generally purchased dried and must be soaked and cooked before consumption, although canned soya beans are already cooked. Consumption of raw or undercooked soya beans can cause serious food poisoning due to the presence of lectins, which are destroyed by cooking.
Other soya products include soya "milk", textured vegetable protein (TVP), tofu, tempeh, miso, soya sauce and soya bean oil. As well as being a good source of protein, soya beans provide modest amounts of a range of vitamins and minerals, as well as fibre, and most of the fat present in soya beans is unsaturated.
Raw soya beans also contain bioactive substances, including trypsin inhibitors, lectin, phytate, oligosaccharides, saponins, sterols and isoflavones, many of which are destroyed during processing techniques used to produce soya products such as soya drinks, or by soaking and boiling before consumption. It is the oligosaccharides, isoflavones and sterols that have attracted considerable attention for their potential health benefits.
Oligosaccharides are classified as prebiotics. They resist digestion and pass into the large intestine where they promote the growth and colonisation of bifidobacteria ("friendly" bacteria), helping to improve the health of the colon.
 
Benefits
Soya consumption is known to be beneficial for men in reducing the risk of prostate cancer. The early stages of prostate cancer occur at fairly similar rates throughout the world. However, the rates of more advanced, clinically important prostate cancer in Western countries are much higher than in Asian countries. For example, cancer mortality in the USA is 17 times greater than in China, although latent prostate cancer rates are the same.(2) Migration studies show that males of Chinese and Japanese descent who move to Western countries and adopt Western eating habits have greater prostate cancer rates than their counterparts who remain in Asia.
These observations have been attributed to a protective effect of soya consumption. One study found consumption of soya to be more protective than any other dietary factor related to prostate cancer mortality across 42 countries.(3)
As well as being beneficial for men, it is thought that soya is useful for both premenopausal and postmenopausal women in helping to reduce the risk of hormone-related conditions associated with each stage, including breast cancer, osteoporosis and menopausal symptoms.
Soya contains isoflavones, which are a class of phytoestrogens and, as their name suggests, are similar in structure to the oestrogen hormone 17-beta-oestradiol. Isoflavones can behave as oestrogen mimics, but are much less potent in their effects than 17-beta-oestradiol, or can act as oestrogen antagonists. Like human hormones, isoflavones can bind to oestrogen receptors. Breast cancer risk is increased with continuous endogenous oestrogen exposure (early menarche, late or no pregnancy, and late menopause). Breast cancer risk is four to fives times lower in Asian women who eat a traditional diet consuming between 20mg and 50mg of isoflavones per day.(4) Isoflavones compete with the potent endogenous oestrogens to bind with the oestrogen receptors in the breast tissue, thus reducing exposure. Conversely, postmenopausal women suffer menopausal symptoms and are at increased risk of osteoporosis due to the fall in endogenous oestrogen levels. Once again, Asian women who eat a traditional diet have a much lower incidence of these conditions due to the ability of isoflavones to mimic oestrogen.(5) It is thought that both the reduced incidence of breast cancer and the lack of postmenopausal symptoms in Asian women are due to their lifetime exposure to isoflavones.(6)

Lowering cholesterol
Soya consumption is strongly associated with reduced risk of coronary heart disease via two mechanisms: the ability to lower blood cholesterol levels and the ability to reduce LDL-cholesterol oxidation.
Many clinical trials have shown that soya consumption can lower both total and LDL-cholesterol levels. A meta-analysis of 38 clinical studies involving more than 700 subjects has demonstrated that, in comparison with control diets, substitution of soya protein resulted in significant reductions in total cholesterol (9.3%), LDL-cholesterol (12.9%) and triglycerides (10.5%), with a small but significant increase (2.4%) in HDL-cholesterol.(7)
Because of soya's effectiveness in lowering blood lipids, the US Food and Drug Administration has authorised the use, on food labels, of health claims on the association between soya protein intake and reduced risk of coronary heart disease. The soya must be eaten as part of a low-saturates, low-cholesterol diet. The response in the UK has been slightly more muted, allowing health claims on packs associating soya protein intake and reduced levels of blood cholesterol. In practice, this means that foods containing over 6.25g of soya protein per serving can use the claim, with the recommendation that they are eaten four times a day.
Isoflavones are a subgroup of the flavonoids and so have the ability to act as antioxidants. In one study it was found that subjects who ate 54mg of isoflavones for 17 days showed significantly reduced levels of LDL oxidation.(8) Isoflavones are also thought to increase arterial compliance (a measure of stiffness that relates to risk of cardiovascular disease).
Soya isoflavones together with soya protein appear to have a greater cholesterol-lowering effect than soya protein alone. In fact, a reduction in plasma cholesterol is more likely in those with initially raised cholesterol levels. The amount of soya necessary to impact on cholesterol levels is considered high, typically about 25g, when compared with estimated habitual intakes in the UK, on average 1g per day. Therefore, to make use of this strategy to reduce blood cholesterol levels, substantial dietary changes would be needed at each meal. For example, soya milk could be used on breakfast cereal (150ml, 5g protein), and TVP could be used to make lasagne (100g portion, 15g protein) followed by a soya yoghurt (150g pot, 5g protein). These substitutions would mean the balance of the diet would be altered, possibly causing effects on nutritional status, particularly in relation to iron and calcium.
 
Soya use in infants
One area of concern is over the use of soya infant formulas. Soya infant formulas can provide essential nutrients required for growth and normal development of babies. They are of particular use for babies diagnosed as needing a lactose-free diet.
Soya formulas are sometimes used for babies who have developed an allergy to cows' milk protein, although a very small proportion of these babies will also react to soya protein. Therefore, as a safeguard, dietitians and other health professionals often prescribe a hydrolysed formula for infants with cows' milk protein allergy. Infants are exposed to isoflavones present in soya infant formula, and it has been suggested that this could be harmful in some way, possibly upsetting hormonal balance, but so far hazards have not been reported clinically.
There are currently no reports of abnormal growth, pubertal or reproductive development in those fed soya formulas during infancy. However, the long-term effects of soya-based infant formulas are currently under review by a Food Standards Agency specialist group.
Lastly, there have been reports of a relationship between soya and cognitive function. A positive association was found in middle-aged white men,(9) and the effects of a 10-week soya-rich diet on memory in healthy student volunteers, both male and female, also showed a positive link.(10)
Currently, intervention trials of soya protein/isoflavones on cognitive function in postmenopausal women are underway. Baseline data from one study suggest that habitual phytoestrogen intake in older Dutch women is associated with improved memory function. In another 12-week soya supplement trial, significant improvements in memory, mental flexibility and ability to plan were observed in those taking the supplement compared with those taking a placebo.(11)
 
Conclusion
Overall soya beans are a useful source for a range of nutrients, and the beans themselves and other soya products can be nutritious components of a healthy diet.
The cholesterol-lowering effects of soya are now well established and, in addition, current research is suggestive of a series of other potential health-promoting effects. However, the evidence is still relatively weak, and much more work on the risks and benefits of isoflavones is required, including whether the form of exposure matters, such as in a food or supplement, and the amount required.

References

  1. Setchell KD, Cassidy A. J Nutr 1999;129:S758-67.
  2. Hsing AW, Tsao L, Devesa SS. Int J Cancer 2000;85(1):60-7.
  3. Hebert JR, et al. J Natl Cancer Inst 1998;90:1637-47
  4. Wu AH, et al. Am J Clin Nutr 1998;68 Suppl 6:S1437-43.
  5. Cassidy A, et al. Am J Clin Nutr 1994;60:333-40.
  6. Cassidy A, Faughnan M. Proc Nutr Soc 2000;59:489-96.
  7. Anderson JW, et al. N Engl J Med 1995;333:276-82.
  8. Sanders TA, et al. Am J Clin Nutr 2002;76:373-7.
  9. White LR, et al. J Am Coll Nutr 2000;19:242-55.
  10. File SE, et al.Psychopharmacol (Berl) 2001;157:430-6.
  11. Kreijkamp-Kaspers S, et al. JAMA 2004;292(1):65-74.