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Spotlight on soya and its benefits for health

Lynne Garton
BSc(Hons) RD
Nutrition Consultant
Soya Protein Association

No longer exclusive to vegetarians or allergy sufferers, an increasing number of soya products are now appearing in mainstream supermarket shelves, with the total market in the UK estimated to be worth £200m and growing at a rate of 20% per year. Much of this growth is as a result of the increasing interest in the potential health benefits of soya, including its effect on reducing cholesterol, decreasing the risk of certain types of cancer and relieving symptoms of the menopause.

The nutrient-packed bean
Soya is high in protein, provides all the essential amino acids, and its quality is considered equivalent to that of animal and milk proteins. As well as being an excellent source of protein, it is a good source of polyunsaturated fats (in particular omega-3 fatty acids), fibre and a number of vitamins and minerals important for health. Recently other compounds found in soya beans, such as isoflavones, have been identified as being potentially beneficial to health.

Soya and cholesterol
In addition to a healthy diet, certain foods have been acknowledged as having a significant role in lowering blood cholesterol. A meta-analysis of 38 studies by Professor Anderson in 1995 found that the consumption of soya compared with the control group showed significant reductions in total cholesterol (-9.3%), LDL cholesterol (-12.9%) and triglycerides (-10.5%), and a nonsignificant increase in HDL cholesterol (2.4%).(1) The higher the initial cholesterol levels, the greater the decrease following the consumption of soya.
Since the publication of this paper, a number of other studies have supported this finding, albeit with differing levels of cholesterol reduction.(2) As a result, the UK Joint Health Claims Initiative (JHCI) has approved the following claim to be used for certain foods containing soya protein:
"The inclusion of at least 25g of soya protein per day, as part of a diet low in saturated fat, can help reduce blood cholesterol levels."(3)
Furthermore, a diet containing a portfolio of cholesterol-lowering foods, including soya protein foods, has recently been found to reduce serum cholesterol levels nearly as much as a statin drug.(4) The portfolio diet was high in soya protein, almonds and cereal fibre as well as plant sterols. This diet was then tested on 34 hyper-lipidaemic patients, comparing it with a low-fat diet (control diet) and with a low-fat diet plus 20mg lovastatin (statin diet). The volunteers followed each regimen for one month, with a break in between each treatment cycle. The low-fat diet lowered LDL cholesterol by 8.5% after a month. Statins lowered LDL by 33%, and the portfolio diet lowered LDL by nearly 30%.

Soya and cardiovascular health benefits
High consumption of soya foods may be one of the possible explanations for the low risk of coronary heart disease in some Asian populations.
A direct relationship between soya food intake and the incidence of coronary heart disease (CHD) was examined among participants in the Shanghai Women's Health Study.(5) Of the 64,915 women included in this two and a half year study, 62 cases of CHD were documented: 43 nonfatal myocardial infarctions (MIs) and 19 CHD deaths. There was a clear inverse relationship between soya protein intake and risk of total CHD. The group of women consuming the highest amount of soya protein (>11.19g per day) had a 75% lower risk of total CHD than those in the group consuming the lowest intake (As well as the beneficial effect on blood cholesterol, other well-documented cardiovascular effects of soya and its isoflavones include lowering blood pressure, improving vascular function, inhibiting platelet activation and increasing antioxidant protection of LDL oxidation.

Soya and the menopause
It is believed that symptoms associated with the menopause, such as hot flushes, result from decreasing oestrogen levels. Phytoestrogens are a group of plant compounds that are structurally similar to the female hormone oestrogen, and as such may exert weak oestrogenic-like effects. Soya is recognised as the major source of dietary phytoestrogens, specifically the isoflavones genistein and daidzein.
Extensive research has shown that 18-20% of Asian women who traditionally consume soya foods suffer from hot flushes, whereas in western countries the figure is 80%.(6) These observations have fuelled an interest in the possible role of soya consumption in decreasing hot flushes.
The evidence to date to support this is inconsistent - some studies show a benefit and others show no effect. One of the problems with these studies is a profound placebo effect. However, in a review of 19 clinical trials it has been suggested that soya/isoflavones may achieve a modest reduction and appear to be more effective in women who have more frequent hot flushes. In fact, hot flush frequency decreased by about 5% (above placebo or control effects) for every additional hot flush per day in women who were experiencing five or more hot flushes a day.(7)
What does this mean in clinical practice? Basically the more severely a woman is affected by hot flushes, the more likely she is to gain relief from soya/isoflavones.
In line with this, the North American Menopause Society also recommends that women consider trying isoflavones for the relief of their symptoms.(8) With regards to dosage, it has been suggested that women start at an initial dose of 50mg of isoflavones per day (typically equates to 25g soya protein), with an upper limit of approximately 100mg/day.(6)

Soya and cancer
In many countries where soya is an important part of the diet, there is a lower rate of certain cancers such as breast and prostate cancer. Ongoing research in this area suggests that soya may protect against certain cancers, and the Food and Drug Administration (FDA) is in the process of reviewing a petition for a health claim, focusing on 58 studies, that suggests the consumption of soya protein-based foods may reduce the risk of these cancers.

Practical advice
The average soya protein intake in Asian populations is approximately 7-11g of soya protein a day, with a corresponding isoflavone intake of 30-50mg. In contrast, intake in Western countries is negligible.
In 2004, 650 new soya products were launched. It is this huge variety now available, along with their improved taste that is making it easier than ever to include soya protein into our diets. Table 1 outlines the most common sources of soya protein available.(9)

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Conclusion

  • The inclusion of soya into the diet can help towards meeting healthy eating recommendations.
  • 25g of soya protein a day has a proven benefit in reducing cholesterol levels.
  • Soya protein appears to have other health benefits, which are the focus of many ongoing clinical trials.
  • Health professionals have a vital role in advising patients on the benefits of soya foods, as well as providing practical recommendations on how soya can effectively be included into the diet.(10)

For more information about soya and health, the Soya Protein Association is hosting a workshop at the Nursing in Practice Event in London on 27 September at 11:30am-1:00pm. All are welcome. Alternatively visit www.soyahealth.net

References

  1. Anderson JW, et al. N Engl J Med 1995; 333:276-82.
  2. Zhan S, Ho SC. Am J Clin Nutr 2005;8:397-408.
  3. Generic health claim for soya protein and blood cholesterol. JHCI  Available at URL: http://www.jhci.co.uk
  4. Jenkins DA, et al. Am J Clin Nutr 2005;81:380-7.
  5. Zhang X, et al.  J Nutr 2003; 133:2874-8.
  6. Sturdee DW, et al. Curr Opin Obstet Gynecol 1997;7:190-6.
  7. Messina M, Hughes C. J Med Food 2003;6(1):1-11.
  8. North American Menopause Society. Menopause 2004;11(1):11-33.
  9. Various ­manufacturers' product information. NB These levels may vary between products.
  10. Maskarinec G, et al. J Am Diet Assoc 2003; 103:861-6.