Some recent postings suggest that people avoid soy (tofu and soymilk) because of its effects on hormone levels. While confusion around this topic is certainly understandable, I can assure you as someone who has studied this area professionally for the past 20 years, when one considers the clinical (human intervention) data, there is no reason to be concerned about soyfoods. Confusion arises only when one considers the results of animal studies. However, not only do these studies have little relevance to humans—in part because rodents metabolize soy differently than humans1—there is no reason to rely upon them for insight because of the wealth of clinical data available on soyfoods.
With respect to the effects of soy phytoestrogens (isoflavones) on hormone levels, a recently published meta-analysis that included 32 clinical studies showed that neither isoflavone-rich soy protein nor isoflavones affect reproductive hormone levels in men, including both total and free testosterone and total and free dihydrotestosterone.2 Similarly, a meta-analysis that included 47 clinical studies showed neither soy nor isoflavones had meaningful effects on reproductive hormone levels in pre- or postmenopausal women. 3 Also, a recent comprehensive review found soy does not exert feminizing effects in men; this review included 9 studies that showed soy does not affect estrogen levels.4 It is noteworthy that many of the studies included in the two meta-analyses and one review cited above involved interventions in which soy and isoflavone exposure far exceed that typical for Asia.
As to the fermented vs. non-fermented soyfood issue, both foods are good choices. Both types contain equivalent amounts of isoflavones, so if isoflavones are a concern, one type of food doesn’t hold an advantage over the other. Arguments that fermented soyfoods are superior to non-fermented ones are often based on claims that fermentation inactivates or destroys protease inhibitors (which inhibit protein digestion) and phytic acid (which inhibits mineral absorption), compounds which are naturally present in soybeans and many plant foods. However, the digestibility of soy protein from non-fermented soyfoods approaches 100%, so there is little room for improvement.5-9 Furthermore, despite the presence of phytic acid and oxalate, current research indicates that daily soy consumption does not impact mineral absoroption over the long term.10
Finally, there is a considerable amount of evidence indicating that soyfoods provide health benefits independent of their nutrient content. For example, there are both animal and epidemiologic data indicating soy consumption during childhood and/or adolescence reduces breast cancer risk. In addition, there is evidence indicating soy may be useful for both the prevention and treatment of prostate cancer, and for several reasons, soyfoods are very heart-healthy. One can have a healthy diet without ever eating soyfoods but it would be unfortunate for people to avoid these nutritious foods because of a misunderstanding about their health effects.
Dr. Messina is the co-owner of Nutrition Matters, Inc., a nutrition consulting company, an adjunct associate professor at Loma Linda University, and the Executive Director of the Soy Nutrition Institute. Full bio >>
1. Guha N, Kwan ML, Quesenberry CP, Jr., Weltzien EK, Castillo AL, Caan BJ. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study. Breast Cancer Res. Treat. 2009, 118, 395-405.
2. Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil. Steril. 2010, 94, 997-1007.
3. Hooper L, Ryder JJ, Kurzer MS, Lampe JW, Messina MJ, Phipps WR, Cassidy A. Effects of soy protein and isoflavones on circulating hormone concentrations in pre- and post-menopausal women: a systematic review and meta-analysis. Hum. Reprod. Update. 2009, 15, 423-40.
4. Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil. Steril. 2010, 93, 2095-104.
5. Gilani GS, Sepehr E. Protein digestibility and quality in products containing antinutritional factors are adversely affected by old age in rats. J. Nutr. 2003, 133, 220-5.
6. Chen ML, Lin T, Chen JS. Digestion and utilization of vegetable proteins in human subjects. Chin. J. Physiol. 1972, 21, 79-88.
7. Mariotti F, Mahe S, Benamouzig R, Luengo C, Dare S, Gaudichon C, Tome D. Nutritional value of [15N]-soy protein isolate assessed from ileal digestibility and postprandial protein utilization in humans. J. Nutr. 1999, 129, 1992-7.
8. Mariotti F, Pueyo ME, Tome D, Mahe S. The bioavailability and postprandial utilisation of sweet lupin (Lupinus albus)-flour protein is similar to that of purified soyabean protein in human subjects: a study using intrinsically 15N-labelled proteins. Br. J. Nutr. 2002, 87, 315-23.
9. World Health Organization. Protein and amino acid requirements in human nutrition. World Technical Series 935, Report of a Joint WHO/FAO/UNU Expert Consultation, United Nations University, 2007, Geneva.
10. Messina M, Watanabe S, Setchell KD. Report on the 8th International Symposium on the Role of Soy in Health Promotion and Chronic Disease Prevention and Treatment. J Nutr. 2009 Apr;139:796S-802S.