Soy has garnered attention as a healthy source of protein, and has been linked to a multitude of health benefits, including protection from breast cancer, prostate cancer, menopausal symptoms and heart disease. But some studies have suggested that soy isoflavones could affect male testosterone levels, due to their similarity in chemical structure to estrogen, which means they bind to estrogen receptors and can exert estrogen-like effects.
The meta-analysis of data was carried out by researchers at the University of Minnesota who wrote: “Because of the increasing popularity of soy foods and the availability of isoflavone supplements, there is an important public health need to understand the impact of soy isoflavones on reproductive hormone levels in men.”
Low levels of testosterone in men can lead to depressed mood, loss of muscle mass, weight gain, erectile dysfunction and osteoporosis.
‘No significant effects’
Although two of the studies examined did assert that testosterone levels were lowered as a result of soy consumption, the authors wrote: “No significant effects of soy protein or isoflavone intake on T [testosterone] or SHBG [sex hormone-binding globulin] were detected…There was no significant pooled effect of isoflavone consumption on the hormonal profile, T and SHBG.”
Contradictory studies
Furthermore, the authors criticized the studies that reported lowered testosterone levels for their methodology.
Referring to one study that showed such an effect in response to isolated soy protein, the authors wrote: “There were only 12 subjects in this study, it did not include a control group, and it did not describe the method used to assess hormones.”
Another study found a response to 120mg a day of isoflavones from soy flour, in which the researchers reported that serum testosterone decreased by five percent over six weeks.
Reviewing this study, the authors wrote: “Baseline and final T values from the control group fed wheat flour were not presented…Without these data it is not possible to know whether the change in T levels that occurred in the soy flour group was significantly different from the change in the wheat flour group.”
They also noted that for all of the studies, soy protein and isoflavone intake “greatly exceeded” typical Japanese dietary intake.
They concluded: “These results suggest that consumption of soy foods or isoflavone supplements would not result in the adverse effects associated with lower [testosterone] levels.”
Source: Published online ahead of print
doi:10.1016/j.fertnstert.2009.04.038
“Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis”
Authors: J. M. Hamilton-Reeves, G. Vazquez, S. J. Duval,
W. R. Phipps, M. S. Kurzer, and M. J. Messina.