Can Soy Protein Hurt Testosterone in Men?

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Testosterone is a hormone found primarily in men that is produced in the testes. It is responsible for muscle mass and strength, as well as libido. Soy protein and its main compounds, isoflavones, have been called testosterone's worst enemy, since they raise estrogen, a hormone that lowers testosterone in the body. Research shows mixed results in terms of soy’s ability to hurt testosterone levels.

Decreases Testosterone

Researchers from the University Hospital of Wales in the United Kingdom investigated the impact of soy on testosterone levels in healthy men. For six weeks, subjects were randomly assigned a diet with or without soya flour that contained 120 mg of soy isoflavones. Testosterone levels were measured before and after treatment. At the end of the study, published in the January 2003 issue of the “European Journal of Clinical Nutrition,” scientists found that the soya flour group experienced decreases in testosterone levels compared to the group without soya flour.

Reduces Testosterone

In research reported in the January 2007 issue of the journal “Cancer Epidemiology, Biomarkers and Prevention,” scientists from the University of Medicine and Dentistry of New Jersey/Robert Wood Johnson Medical School studied the effects of soy protein supplementation on testosterone levels in healthy men. They found that subjects consuming 56 g of soy protein for 28 days had reduced testosterone levels.

Soy Consumption and Prostate Cancer Risk

Scientists from the University of Hawaii examined the relationship between soy consumption and prostate cancer risk in healthy men. In the report, published in the December 2006 issue of the “European Journal of Clinical Nutrition,” researchers found that participants following a high soy diet for three months experienced no differences in testosterone levels compared to those following a low soy diet.

No Effects on Testosterone Levels

Researchers from the National Defense Medical College in Japan investigated the effects of sex hormones in healthy men. For four weeks, subjects received 20 g of soy protein or a control. Scientists reported in the August 2001 issue of the “Journal of Nutritional Science and Vitaminology” that there were no differences in testosterone levels between the two groups.