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Soya and health: getting the facts straight

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

Soya has been acknowledged by a number of international authorities, including the UK Joint Health Claims Initiative (JHCI) and the US Food and Drug Administration (FDA), as a food that can help to reduce blood cholesterol levels. This beneficial action has been determined from a number of well-controlled clinical studies that, taken together, were able to demonstrate that consuming at least 25g of soya protein a day can lower total and LDL blood cholesterol levels.
However, since the FDA claim was first approved, studies into the effects of soya protein on blood cholesterol have continued and a large number of new research papers have been published. In order to evaluate this more recent data, the Nutrition Committee of the American Heart Association (AHA) undertook a new meta-analysis, publishing their findings on soy protein, isoflavones and cardiovascular health in January this year.(1)
Based on this new analysis, the AHA now affirms that soya foods such as tofu, soya nuts, soya milk and soya burgers are likely to be beneficial to cardiovascular health because of their low saturated fat content and their high polyunsaturated fat, fibre, and vitamin and mineral contents. In addition to their beneficial nutritional profile, the AHA also estimated that soya protein directly lowered LDL cholesterol by about 3%. This decrease is consistent with findings from another recently published meta-analysis,(2) but is somewhat lower than the 5.25% reduction observed in the meta-analysis conducted by Zhan S et al,(3) and is much lower than the 12.9% reduction obtained by Anderson in his original meta-analysis in 1995, which was instrumental in first recognising soya's cholesterol-lowering effects.(4) Since the recent data are indicative of a more modest effect of soya in reducing blood cholesterol levels than was suggested by the earlier research, the rationale for including soya at all in a heart-healthy diet has been questioned.
The fact remains that the AHA still recognises that soya protein consumption has been shown to decrease LDL cholesterol by 2-7%. The impact of this, on a public health basis, can still be significant. It is estimated that cholesterol reductions of this magnitude could reduce coronary heart disease at the population level by as much as 10%.(5,6) Furthermore, the AHA found that cholesterol reductions were independent of baseline cholesterol levels, which means that consuming soya may benefit far more people than was suggested by the earlier analyses.
It has been suggested that soya may have other coronary benefits, aside from its cholesterol-lowering properties. On examining two of these, the AHA found there was insufficient evidence to conclude that soya protein lowers levels of lipoprotein (a) or blood pressure. Unfortunately, the AHA did not consider recently published research which found that soya protein markedly lowered blood pressure in hypertensive patients.(7)
Other possible coronary benefits of soya that have been investigated, although not directly considered by the AHA, include: improving vascular function; inhibiting platelet activation; and increasing antioxidant protection of LDL oxidation.
It may be this combination of heart-protective factors, in association with the beneficial nutritional profile of soya that accounts for the low risk of coronary heart disease seen in some Asian populations who regularly consume soya foods.(8)
 
Soya and fertility
Concerns have been raised about the safety of soya consumption on fertility following media headlines reporting results from animal and test-tube studies. To date there is no conclusive evidence that soya, when eaten as part of a healthy balanced diet, has any effect on human fertility. In fact, its safety has been extensively reviewed by a number of major committees, including the UK Committee on Toxicity,(9) the Expert Committee of the Joint Health Claims Initiative(10) and the US FDA.(11)
The fears relating to soya have been raised because of its phytoestrogen content, specifically the isoflavones genistein and daidzein. Isoflavones have a chemical structure similar to that of human oestrogen but possess a much lower potency (10,000-14,000 times lower than oestrogen). Despite the lower potency, the oestrogen-like similarity has led to the question of whether isoflavones can influence reproduction and fertility.
To date, the studies that have been conducted to address this question have mainly been in animals or on cells in a test tube. Extrapolation of this data to what "might happen" in the human body cannot be done. This is because interpreting results from animal and test-tube studies are problematic:

  • Often an isolated and pure isoflavone compound (eg, genistein) is given in a very high dose. This does not reflect soya in its normal state, nor normal concentrations from a balanced diet.
  • In some animal studies, isoflavones are injected subcutaneously. This is very different from consuming soya orally, as injecting isoflavones bypasses the important processes of digestion and metabolism, which are known to change the isoflavones into a different form.
  • Animals metabolise isoflavones in a very different way from humans.
  • The timing of sexual development differs between animals and humans.
  • Cells in a test tube are not the same as cells that form part of an organ.

Although any results from these types of studies are interesting, conclusions from these need to be interpreted with caution.
What must be remembered is that soya foods have been a mainstay of the diet in Asia for thousands of years, including that of men, pregnant women and children. Oriental populations who traditionally eat a soya- rich diet do not appear to show any signs of reduced fertility.
On the contrary, it is has been suggested that exposure to phytoestrogens early in life may be one of the reasons why these populations have a lower incidence of certain chronic diseases, such as heart disease and cancers, although this needs to be confirmed in clinical studies.
Furthermore, in a study by Mitchell et al the effect of a phytoestrogen food supplement on reproductive health in normal human males was studied.(12) Healthy volunteers took a supplement of 40mg of soya isoflavones daily for two months. Semen samples and blood samples were taken. There were no observable effects on endocrine measurements, testicular volume or semen parameters (sperm count, sperm motility and morphology, sperm concentration, ejaculate volume).

Soya - a healthy food
As highlighted by the AHA, many soya products should be considered beneficial to cardiovascular and overall health because of their high content of polyunsaturated fats, fibre, vitamins and minerals and their low content of saturated fat. Foods such as tofu, soya nuts, soya milk and soya burgers, miso and edamame can replace foods high in animal protein that contain saturated fat and as such can help towards meeting current healthy eating recommendations. In fact, using dietary data from the Third National Health and Nutrition Examination Survey, a large population-based study conducted in the USA from 1988 to 1994, it is possible to calculate the beneficial effects on the nutritional composition of the diet from replacing beef or pork products with plant protein.(13)
Following the analysis of 11,929 subjects' 24-hour dietary recalls, 6,837 reported eating beef or pork products at lunch or dinner. The effect of substitution with one of three different types of veggie burgers (all containing some soya protein) on the nutritional content of the diet was found to be:

  • Reduction in total energy intake by 65kcal/day.
  • Total fat intake reduced (35% of calories to 30.2%).
  • Saturated fat reduced (12.2% of calories to 9.9%).
  • Fibre increased (3.4g/1,000kcal).

Conclusion
Despite the conflicting media attention soya has received, its excellent nutritional profile fits in well with current healthy eating recommendations, and as such it can help towards maintaining a healthy heart.
Soya foods have been enjoyed by many populations, for thousands of years, without any adverse effects on health; in fact in many countries where soya is an important part of the diet, there are lower rates of heart disease and certain cancers such as breast, prostate and colon cancer.
Soya foods' health benefits extend far beyond heart health, and ongoing research is investigating its potential role in the areas of bone health, menopausal symptoms and cancer.
For more information about soya and health, the Soya Protein Association is hosting workshops at the Nursing in Practice Event on Tuesday 23 May 2006 in Manchester and Tuesday 26 September 2006 in London. Alternatively visit www.soyahealth.net

References

  1. Sacks F, et al. Circulation 2006:113(1):1-12.
  2. Balk E, et al. Effects of soy on health outcomes. Rockville, MD: Agency for Healthcare Research and Quality; 2005.
  3. Zhan S, Ho SC. Am J Clin Nutr 2005;81(2):397-408.
  4. Anderson JW, et al. N Engl J Med 1995;333(5):276-82.
  5. Law MR, et al. BMJ 1994;308 (6925):367-72.
  6. Law MR, et al. BMJ 1994;308(6925): 363-6.
  7. He J, et al. An Int Med 2005;143(1):1-9.
  8. Zhang X, et al. J Nutr 2003;133:2874-78.
  9. COT. Phytoestrogens and human health. Available from http://www.food.gov.uk/multimedia/pdfs/fsa030503.pdf#page=1
  10. JHCI. Approved health claim for soya protein and blood cholesterol. Available from http://www.jhci.org.uk/approv/schol2.htm
  11. US Food and Drug Administration. Available from http://www.fda.gov/OHRMS/DOCKETS/98fr/102699a.txt
  12. Mitchell, et al. Clin Science 2001;100:613-8.
  13. Dietrich M, et al. Nutrition Today 2005;40(4):156-64.