The soya bean has been part of the human diet for thousands of years and the Ancient Chinese believed it to be one of the five sacred grains vital for life, (alongside rice, wheat, barley and millet.)
Soya is particularly important in a vegan diet, as it is one of the few plant products which provides protein that is nutritionally equivalent to meat.
However, despite soya being a longstanding natural source of nutrition, there are elements of controversy regarding the benefit and safety thereof.
The first point to establish in this debate is that different sources of soya may have different health implications. The widespread genetic modification of agricultural soya has been associated with negative health and environmental consequences, but this must not be confused with soya that has been naturally farmed.
The next point, which makes research so complex, is that not all sources of information are equally credible, if credible at all. There are many unsubstantiated claims, (both positive and negative), which can send unsuspecting information seekers into a state of overwhelming confusion! Further on the topic of evidence based research, there are areas in which different studies, (on the same topic), conflicting results and there are still areas where no research has been done at all, or not enough to provide reliable answers.
To clarify the issues, we’ve looked at position statements of the American Dietetic Association, the FDA, and the US National Library of Medicine, (amongst other sources).
The areas of health where soya has been suggested to provide benefit include aspects of heart disease, reduction of menopausal symptoms, prevention of hormone dependent cancers, (breast, prostate and endometrial), and bone health.
What makes soya healthy?
Soya contains healthy phytochemicals (plant compounds), called isoflavones. There are various types of isoflavones, but two specifically, (genistein and diadzen), have been studied closely and found to be very similar in structure to the hormone oestrogen, which thus mimic the activities of oestrogen in the body. These so-called ‘plant hormones’ are much weaker than true hormones, yet seem to have a positive influence on oestrogen balancing in the body and lowering LDL cholesterol. Phytoestrogens found in soya foods also act as antioxidants, carcinogen blockers or tumor suppressors and may exert a protective effect against hormone related cancers by binding at oestrogen receptor sites.
Further studies suggest that plant based estrogens may reduce the incidence of vasomotor symptoms, (hot flashes) of menopause and lastly that they may protect women against osteoporosis by the action of genestein, which stimulates osteoblasts, (bone forming cells).
Are there side effects to soya?
As long as you are choosing good quality soya products, that have not been genetically modified, there is very little reason to avoid soya with the obvious exception being soya allergy.
There have been references to limiting soya in order to combat gout. (All protein foods contain substances called purines, which yield uricacid as a by-product of metabolism, and could thus supposedly aggravate acid build up and the symptoms of gout.) However, although soya,(being a protein), does have a moderate purine content, it is much lower than that of many high purine animal proteins. Further, a vegetarian diet, being high in carbohydrates and generally lower in total protein than a non-vegetarian diet would most likely have a low total purine content, regardless of moderate soya intakes.
It is also worth noting that improvements in the efficacy of gout medication have largely replaced the need for rigid dietary restriction of purines in recent times.
What about supplements?
There is a big difference between soya based foods and soya supplements, which contain a much higher concentration of isoflavones. There are many theories regarding the proposed benefits of soya supplements, but there is unfortunately not enough accurate scientific evidence to support the use of supplements and no documentation on the potential side effects of mega- dosing with isoflavones.
As with many healthful nutritional ingredients, more is not necessarily better – a balanced diet, (applicable to macro-and micronutrients), remains the professional prescription for optimal health.
Conclusions
The only specific recommendation available is from the FDA, which advises 25g soya protein per day for adults to potentially reduce the risk of heart disease. Although exact amounts of soya foods needed to reduce disease risks in other areas remains undefined scientifically, adding soya foods to the diet on a regular basis could indeed promote better health.
Adding to this, we recommend choosing soya that is GM free! (which is the start of another story, altogether).
Caryn Davies RD(SA)
Consultant Dietician
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