Is soya the food it’s cracked up to be?
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The value of increasing soya in Western diets has been much debated. Is it healthy or harmful? It has its detractors, but this is a healthy food with some interesting qualities.
Soya is part of the legume family that includes peas, other beans and lentils. Soya has a good nutritional profile: it is low in saturated fat, is a good source of polyunsaturated fats (especially the heart healthy omega-3 fatty acids), and contains high-quality vegetable protein (similar in quality to meat and milk protein), fibre, various vitamins and minerals. Hence soya fits in well with general healthy eating guidelines and has now been included as a protein in the Eatwell Plate, the national public health teaching model for healthy eating. Soya is useful as a staple of the vegetarian diet since it contains some iron, zinc and calcium.
Foods containing soya include soya milk, yoghurts and desserts, tofu, miso, tempeh, natto, edamame (young soya beans), soya beans, soya nuts (roasted soya beans), soya mince and various meat alternatives such as veggie burgers and sausages.
There is little evidence to suggest that fermented soya foods (miso, tempeh and natto) are significantly superior to unfermented ones (tofu, soya milk, soya beans, soya nuts).
So what are the benefits of soya? To begin with, soya has a lower saturated fat content than other protein foods and this is valuable for heart health. In addition, numerous studies have shown that eating 15-40g of soya protein daily helps lower “bad” LDL cholesterol, thereby significantly reducing the overall risk of coronary heart disease.
As a result, cholesterol-lowering health claims for 25g of soya protein per day as part of an overall diet low in saturated fat are allowed in both the UK and USA, while the Word Health Organisation recommends legumes as part of the daily diet to combat risk of coronary heart disease. Soya beans contain phytoestrogens, natural plant compounds which have a similar chemical structure to the hormone oestrogen. Soya beans are one of the richest food sources of a group of phytoestrogens called isoflavones. Although isoflavones are weaker than oestrogen, they exert anti-oxidant, anti-inflammatory and anti-cancer effects.
Soya isoflavones would seem to offer several benefits to menopausal women. Research into their bone health has shown that a soya isoflavone intake of 90mg/day increases spinal bone mineral density.
Other research has shown that 30-100 mg/day of soya isoflavones can significantly reduce hot flushes and this has been endorsed as a non-prescription remedy by the North American Menopause Society. Soya protein on its own or associated with isoflavones can help lower blood cholesterol levels which characteristically become raised with menopause.
Soya can help in weight management. Protein induces satiety or fullness more effectively than fat or carbohydrate. The satiety effect of soya protein is similar to that of skimmed milk, and as part of a healthy balanced diet, can help reduce food intake and so aid weight reduction.
Soya is thought to be protective against breast and prostate cancer. Plant-based diets including legumes have been advocated by the World Cancer Research Fund. Research data from 10 European countries found that higher levels of the soya isoflavone genistein in the blood were associated with a 26 per cent lower risk of prostate cancer.
Because of the oestrogen-like effects of isoflavones, it was thought that soya might increase the risk of breast cancer. Some rat and laboratory tests have found that isoflavones stimulate the growth of breast cancer cells. However, animals metabolise soya in a different way to humans. Human studies have found no effect of soya isoflavones on either tissue density or breast cell proliferation (both markers for breast cancer risk).
Even in those women who have breast cancer, evidence such as data from the Shanghai Breast Cancer Survival Study involving 5,000 breast cancer patients suggests that soya in modest amounts (approximately 15.5g a day) is safe to consume. In fact soya has been associated with a reduced risk of breast cancer and eating it earlier in life appears to offer the greatest protection.
Some animal or test-tube studies have linked soya consumption to impaired fertility, but in addition to their limitations, these often use very high doses of pure isoflavones which do not reflect soya in its normal state. Furthermore clinical studies on humans show that soya does not affect sperm quality.
Although isoflavones have a chemical structure similar to oestrogen, the majority of human studies show no imbalance effects on hormone levels following soya consumption in either men or women. It is likely that soya isoflavones exert their benefits independently of hormone levels.
In order to gain the benefits of soya, the optimal amount of soya protein appears to be around 15-25g of soya protein a day which can be achieved in 2-3 servings of soya foods. Soya protein content will vary between brands
Soya has been shown to have considerable health benefits for those who have or who are at risk of developing a number of conditions. Above all, soya has its place as part of a healthy balanced diet for meat-eaters and vegetarians alike.
Joan Wides is a qualified dietician