The Facts About Soy
Chester J. Zelasko, Ph.D. | May 11, 2004

Enough is enough. There have been a number of widely forwarded e-mails warning of the hazards of soy and its by-products; there are also a few websites dedicated to the same topic. As a result we've gotten numerous questions about whether soy is safe to eat. This Newsletter will not address every issue raised by these e-mails and websites--there are simply too many for one Letter--but here are answers to a few of the important ones.

"Soy Has Estrogenic Effects on Men"
Soy is thought to be beneficial for men to reduce the risk of developing heart disease and prostate cancer. The correspondence implies that the regular use of soy products will decrease the effects of testosterone in the male body resulting in a feminization of males. The questions raised appear to be valid--soy isoflavones do exert an estrogenic effect in women. That's why soy may be beneficial for menopausal women to help with symptoms such as hot flashes.

But there is no evidence that soy can be feminizing to men in any way. Researchers examined the effect of replacing meat as a source of protein in the diet of British men with protein from soy (1). There was no measurable difference in hormone levels while on the soy-protein diet compared with a typical diet using meat. In another study, soymilk intake was unrelated to sex hormone levels in an analysis of almost 700 men (2). While the research completed to date is certainly not exhaustive, there is nothing to support the innuendo that soy makes a man less of a man.

"Soy Protein Can't Build Muscle"
This e-mail criticism of soy suggests that soy is not a complete source of protein, a topic also debated on muscle-building websites. Interestingly, most of the websites also sell whey protein, so there seems to be an inherent bias in their criticism of soy. Soy has lower amounts of the sulfur-containing amino acids, but it is still a complete protein--especially as soy protein concentrate.

Research has demonstrated that regular use of soy protein can help sustain and increase muscle mass. In a randomized cross-over design, Romanian Olympic athletes increased muscle mass when using a soy protein concentrate compared to a normal diet with no negative effects on performance (3). In a 12-week weight-training study, there were no differences in the increase in muscle mass between a group who used lean beef versus those who used soy protein concentrate. The weight training made the difference, not the source of protein.

"Soy Is Not Approved"
The e-mail states that soy has not been approved for human consumption by the U.S. Food and Drug Administration as "Generally Regarded As Safe" or GRAS for short. What they don't tell you is that it's not necessary to be approved as GRAS in the ways soy is currently being used. For example, one form of soy used frequently is soy protein isolate. While not approved as GRAS, it is on the FDA's list of "Everything Added to Food in the United States" (EAFUS) and has not been challenged. In effect, it's what they're not telling you that's really important. Why not? Then they wouldn't have a scary story to tell.

Is everything about soy great? There's no way to answer that at this time. Legitimate questions about the use of soy in infant formulas are currently being investigated. But soy has been used as a protein substitute for lactose-intolerant infants for over 50 years, so we would expect to see more problems than have been reported if the problems with soy were significant.

The motives for the scare tactics in e-mails and websites are not always obvious and it's beyond our purpose to question them in this forum. What we can do is inform you about the current state of research so you can make an informed decision. Use soy or not based on your preference for it and what science says--not based on the scare tactics of people with nothing better to do.


  1. Habito RC. Effects of replacing meat with soybean in the diet on sex hormone concentrations in healthy adult males. Br J Nutr. 2000;84(4):557-63.

  2. Allen NE, et al. Soymilk intake in relation to serum sex hormone levels in British men. Nutr Cancer. 2001;41(1-2):41-6.

  3. Dragan I, et al. Studies regarding the efficiency of Supro isolated soy protein in Olympic athletes. Rev Roum Physiol. 1992 Jul-Dec;29(3-4):63-70.

  4. Haub MD, et al. Effect of protein source on resistive-training-induced changes in body composition and muscle size in older men. Am J Clin Nutr. 2002;76(3):511-7.
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