Although the current research shows that soy may not protect against heart disease at the level once believed, it's still a healthy source of protein, fiber, minerals and is a great alternative to red meat--the Harvard School of Public Health's Nutrition Source website suggests two to four servings per week to replace red meat.
Claim #3: Soy Decreases Fertility and Testosterone in Men
The effect of soy on men's fertility came into question in a 2008 study that showed a decrease in sperm concentration--but not motility, morphology or ejaculate volume, which indicate sperm quality--in men who ate soy compared to those who did not. (8) The most significant effect was found in men who were overweight or who had a higher concentration of sperm to begin with.
According to the National Infertility Association, the normal range for sperm is between 40 million and 300 million per milliliter, and counts of 20 million are considered healthy if morphology and motility are normal.
In the 2008 study, the reduction on average was 35 million, which still leaves a well-above-average sperm count for those at the top of the range who showed the most significant reduction. An earlier study in which healthy volunteers took a supplement containing 40 mg of isoflavones daily for two months also showed no effect on semen quality. (9)
There have been several studies on soy's effect on testosterone, offering varying conclusions, including two recent studies on purified isoflavones and soy foods in men's diets that found no significant changes in testosterone levels. (10,11)
A recent analysis of 29 trials and 32 treatment groups found no significant effects of soy protein or isoflavone intake on testosterone levels. (12) Based on this latest analysis, there doesn't appear to be any cause for concern for men who enjoy a few servings of soy foods each week.
Defining moderate is tricky, so I'm going to keep it simple and follow what Harvard suggests, which is a few servings a week.
The 2008 study that showed a decrease in sperm concentration was most pronounced in men who had a higher-than-average sperm count to begin with and it showed no decrease in sperm quality. As for soy reducing testosterone in men, there's simply not enough current evidence that shows this to be the case.
Moderation Is the Key
Though the degree to which soy may protect against particular diseases may have come into question lately, soy continues to be an excellent source of protein as part of a balanced diet. Soy is a complete protein, rich in vitamins and minerals, and is a great, low-fat alternative to red meat.
As author Nancy Clark suggests, you should enjoy soy as a quick-and-easy, cook-free protein source that takes on the wonderful taste of native cuisines, such as in Chinese stir-fries or Indian curries. Or, more easily, enjoy soy milk with breakfast cereal or a soy-containing nutrition bar before or after a workout.
Robb Dorf, founder and CEO of PureFit, Inc.--maker of the award-winning line of PureFit Nutrition Bars -- has a Bachelor of Science degree in exercise physiology and more than 10 years of experience in the health and nutrition industry. Visit www.purefit.com for more information about Robb and PureFit Nutrition Bars.
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10. Kumar NB, Krischer JP, Allen K, Riccardi D, Besterman-Dahan K, Salup R, Kang L, Xu P, Pow-Sang J. A Phase II randomized, placebo-controlled clinical trial of purified isoflavones in modulating steroid hormones in men diagnosed with localized prostate cancer. Nutr Cancer. 2007;59(2):163-8.
11. Maskarinec G, Morimoto Y, Hebshi S, Sharma S, Franke AA, Stanczyk FZ. Serum prostate-specific antigen but not testosterone levels decrease in a randomized soy intervention among men. Eur J Clin Nutr. 2006 Dec;60(12):1423-9.
12. 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. J Am Dietetic Assoc. (in press).