Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis.
OBJECTIVE
To determine whether isoflavones exert estrogen-like effects in men by lowering bioavailable T through evaluation of the effects of soy protein or isoflavone intake on T, sex hormone-binding globulin (SHBG), free T, and free androgen index (FAI) in men.


METHODS
PubMed and CAB Abstracts databases were searched through July 1, 2008, with use of controlled vocabulary specific to the databases, such as soy, isoflavones, genistein, phytoestrogens, red clover, androgen, testosterone, and SHBG. Peer-reviewed studies published in English were selected if [1] adult men consumed soy foods, isolated soy protein, or isoflavone extracts (from soy or red clover) and [2] circulating T, SHBG, free T, or calculated FAI was assessed. Data were extracted by two independent reviewers. Isoflavone exposure was abstracted directly from studies.


METHODS
Fifteen placebo-controlled treatment groups with baseline and ending measures were analyzed. In addition, 32 reports involving 36 treatment groups were assessed in simpler models to ascertain the results.


RESULTS
No significant effects of soy protein or isoflavone intake on T, SHBG, free T, or FAI were detected regardless of statistical model.


CONCLUSIONS
The results of this meta-analysis suggest that neither soy foods nor isoflavone supplements alter measures of bioavailable T concentrations in men.
Hamilton-Reeves
Jill M
JM
Department of Family, Consumer, and Nutrition Science, St. Catherine University, St. Paul, Minnesota 55105, USA. jhreeves@stkate.edu
Vazquez
Gabriela
G
Duval
Sue J
SJ
Phipps
William R
WR
Kurzer
Mindy S
MS
Messina
Mark J
MJ
D000328:Adult; D000465:Algorithms; D002986:Clinical Trials as Topic / Q000706:statistics & numerical data; D042341:Gonadal Hormones / Q000378:metabolism*; D006801:Humans; D007529:Isoflavones / Q000494:pharmacology*; D008297:Male; D012098:Reproduction / Q000187:drug effects; D012737:Sex Factors; D030262:Soybean Proteins / Q000494:pharmacology*; D016896:Treatment Outcome
D042341:Gonadal Hormones; D007529:Isoflavones; D030262:Soybean Proteins