SOYA & HEALTH - POISON OR PANACEA?
Sandra Hood, Dietician
Published in The Vegan, Summer 2006
Soya continues to exercise the minds of many in the media, who extol its virtues or bewail its impact on both the environment and health. As soya is a food many vegans eat regularly, it’s hardly surprising that it raises a lot of questions.
Most of the attention is focused on soya bean isoflavones, a class of phytoestrogens - hormone-like chemicals that occur in small amounts in many plants, seeds and grains. As more is learned about the biological activity of these isoflavones, concerns are raised as to whether they are harmful or helpful.
These phytoestrogens can act in a similar way to the human hormone oestradiol but have a weak oestrogenic effect. Accordingly, the most notable concerns are around infants consuming soya and women with oestrogen positive breast tumours. Evaluating the health benefits of phytoestrogens is complex as these compounds have been shown to have many different effects. The problem comes when trying to determine whether it is safe to eat and if so, how much.
Unfortunately, there is lack of data on the effect of intake of soya in the UK and although internationally populations consuming high intakes of soya have a lower prevalence of certain diseases, there is insufficient data to attribute this protective role to soya. However, neither is there enough evidence to suggest that soya is harmful to health. In 2002 the British Nutrition Foundation produced a comprehensive briefing paper entitled ‘Soya and Health’ which looked at the various health scares and claims surrounding this food. It concluded that ‘soya beans are a useful source of a range of nutrients and the beans themselves and foods derived from them can be useful components of a healthy diet.’
What is soya?
The soya bean belongs to the legume family, which includes all types of beans, peas and lentils, the edible seeds that come from pod-bearing leguminous plants, also known as pulses. A staple food in Asian countries for centuries, soya is a nutritious alternative to meat in that it provides a high quality source of protein. Naturally low in fat, it is also rich in fibre and provides essential fatty acids, mostly omega-6 and some omega-3, as well as a variety of vitamins and minerals including iron, potassium and folic acid. A portion of soya counts towards the recommended five portions of fruits and vegetables per day.
It is now well established that soya as part of a healthy eating diet has a cholesterol lowering effect and that people with raised cholesterol can benefit from 25g of soya protein per day as part of a diet low in saturated fat (British Nutrition Foundation 2002). As a guide, 200ml of soya milk provides around 6g of protein, 200ml soya yoghurt around 10g and 100g of tofu around 8g.
The cholesterol lowering effect of soya is not caused by the presence of phytoestrogens alone but is likely to be the result of a combination of the components in soya which affect cholesterol metabolism (Jenkins et al 2006) and the overall composition of diets with a high soya content (Rosell et al 2004).
The safety of soya for infants and children
Concerns have been expressed regarding the suitability of soya infant formula or soya milk for children because it contains isoflavones.
In 2003, the controversy surrounding phytoestrogens led the UK Committee on Toxicity of Chemicals in Food (COT) to produce a 440 page report considering the exposure to phytoestrogens generally in the diet, including those in soya. When it researched soya-based infant formulae it concluded: ‘studies do not provide definitive evidence that phytoestrogens present in soya-based infant formulae can adversely affect the health of infants.’ Allergies aside, there has not been a single study demonstrating adverse effects of soya in infants. In addition, soya-based infant formulae have been used in the UK since the 1960s and during this time there have been no reports of abnormal growth or reproductive problems.
Soya foods, particularly fortified versions, make a useful contribution to vegan diets, providing convenient alternatives to animal foods.
In the UK the average vegetarian eats around 5 grams of soya protein per day and the average vegan around 15 grams. Daily intakes of soya in Asian countries are about 10 grams. With 25 grams necessary to lower cholesterol, any intake between 10 and 25 grams can be considered safe and healthy (Scott 2004).
Soya contains several components that traditionally have been considered to be anti-nutritional e.g. phytates and trypsin inhibitors. Phytates are said to act as anti-nutrients because they have the ability to bind to minerals and possibly reduce the assimilation of calcium, copper, iron, magnesium and zinc. Trypsin is an enzyme needed to break down proteins to make them available; therefore inhibitors prevent this happening.
However, research suggests that this may be an oversimplification. For example, boiling beans removes most of the trypsin inhibitors and these are also inactivated by sprouting and fermentation. There is suggestion that the human digestive system is stimulated to reject anti-nutrients and the body can adequately deal with these ‘non-nutritive’ components without any detriment to nutritional states. There are many factors affecting the absorption of nutrients but few will alter the body’s normal mineral status if a person is physically well and following a balanced diet.
Effects on the thyroid
The thyroid gland is responsible for the production of hormones necessary for growth and development and for regulating metabolism. It has been hypothesised that phytoestrogens interact with the thyroid gland by a number of potential mechanisms, raising concerns that the phytoestrogens in soya may affect thyroid function. This has only been suggested in adults where iodine intake was inadequate. These concerns remain theoretical and there is no evidence that soya foods adversely affect thyroid function in healthy, iodine-replete children and adults. Dairy products provide about half the iodine consumption in the UK. Vegan sources include seaweeds and foods made from them such as kelp, Vecon, a vegetable stock, and iodised salt.
Numerous studies continue to suggest that soya isoflavones help with improved mental processes and a reduction in hot flushes. However, the results are inconclusive. The isoflavones found in soya bind to oestrogen receptors and prevent the binding of the body’s own oestrogen to the receptor and may help replace oestrogen in postmenopausal women. In the American Family Physician Journal this year an article appeared suggesting patients who do not wish to take hormonal medicines should try other options including soya, red clover and black cohosh (Carroll 2006).
Many tumours, especially breast cancers, are stimulated by human oestrogen. Therefore the fact that isoflavones act in a similar way to mammalian hormones supports a potential role in breast cancer prevention. It is believed that these phytoestrogens may counteract some of the hormone’s cancer-causing potential. On the other hand, there are also concerns that these phytoestrogens may have a negative effect in relation to breast cancer risk. Again this is hypothetical and there is a lack of studies to conclusively support either of these theories. Women should be reassured that, whatever their health status, there seems no reason to avoid a moderate intake of soya (COT 2003).
The influence of diet on bone health is complex. There are intriguing suggestions that phytoestrogens may have the potential to protect bones (Arjmandi et al 2002) but studies remain controversial and no conclusions can be drawn. Whilst too much protein can be detrimental to bone health, too little can also be damaging. The most important dietary aspect in bone status is a healthy balanced diet with adequate calcium (found in nuts, seeds, pulses, figs, grains and fortified non-dairy milks), vitamin D (from the action of sunlight on skin and found in margarines and fortified non-dairy milks) and vitamin K and potassium (for which green leaf vegetables are an excellent source).
Soya is a good source of protein and can reduce cholesterol levels. Other claimed effects, both positive and negative, remain controversial and unproven. We all know that diet and lifestyle impact on our health and that it is unlikely that any one food is the panacea of all health and disease. A balanced vegan diet, with or without soya, will always be preferable to one based on animal foods for a multitude of reasons – nutritional, ethical and ecological.
Arjmandi, B.H. and Smith, B.J. (2002) ‘Soy isoflavones’ osteoprotective role in postmenopausal women: mechanism of action’. J Nutr Biochem 13 130-337
British Nutrition Foundation (2002) Soya and Health Briefing Paper, November 2002
Carroll, D.G. (2006) ‘Non-hormonal therapies for hot flashes in menopause’, Am Fam Physician Feb 1 73(3) 396, 398
COT Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (2003) Phytoestrogens and Health. The Food Standards Agency
Jenkins DJ et al (2006) Assessment of the longer term effects of a dietary portfolio of cholesterol-lowering foods in hypercholesterolemia Am J Clin Nutr Mar 83(3) 582-91
Rosell MS, Appleby PN, Spencer EA, Key TJ (2004) Soy intake and blood cholesterol concentrations: a cross-sectional study of 1033 pre- and postmenopausal women in the Oxford arm of the European Prospective Investigation into Cancer and Nutrition Am J Clin Nutr 80 1391-6
Scott, L (2004) The Great Soya Saga in Veggiehealth Viva Spring 4 16-17
Acknowledgements: Tanya Carr RD R Nutr.
Plant Based Nutrition & Health by Stephen Walsh