Hamilton-Reeves JM et al. 1 conducted a meta-analysis to determine whether isoflavones exert estrogen-like effects in men by lowering the concentration of biologically available testosterone. The effects of soy protein or isoflavone on testosterone, SHBG, free testosterone and androgen index (FAI) have been studied in men.
PubMed and CAB Abstracts databases were searched until July 1, 2008, using controlled vocabulary specific for databases such as soy, isoflavones, genistein, phytoestrogens, androgen, testosterone and SHBG. Peer-reviewed studies, published in English, were selected when adult men consumed soy food, isolated soy protein or isoflavone extracts (from soy or red clover) and 2 testosterone, SHBG, free testosterone, or calculated FAI were assessed. The data were extracted by two independent reviewers.
Fifteen placebo-controlled studies with baseline and endpoints were analyzed. In addition, 32 reports with 36 treatment groups were evaluated in simpler models to determine results.
No significant effect of soy protein or isoflavone intake on testosterone, SHBG, free testosterone or FAI was detected, regardless of the statistical model.
The results of this meta-analysis suggest that neither soy foods nor isoflavone supplements alter the levels of bioavailable testosterone in men.
The results obtained show that soy protein is not harmful to testosterone levels in men. Well, probably the result of the experiment also depends on the dose of soy protein, soy products or isolated isoflavones themselves.
However, it is worth remembering that according to the DIAAS scale, soy protein is inferior to WPC and WPI.