In fact, some clinical studies have been made on men who consume isoflavones have reported a drop in testosterone levels. However, many other studies have not found any effects whatsoever. Because it is necessary for us to consider the total evidence before reaching conclusions about a possible clinical effect, a systematic review of studies evaluate the effects of isoflavone-rich products on total and free testosterone levels in men was undertaken.
Regardless of statistical model, no significant effects of soy protein or isoflavone intake were observed on total testosterone levels in men. The results of this show that neither soyfoods nor isoflavone supplements derived from soy and red clover affect either free or total testosterone levels. However it should be noted, that many of the studies included in the full model of the meta-analysis were of poor quality; for example, many were not placebo-controlled and many were relatively short-term, with the average study duration being 10 weeks. On the other hand, isoflavone exposure was relatively quite high compare to typical Asian and Japanese intake. Overall, the results are reassuring that soy does not lead to any adverse or feminizing effects such as reducing testosterone levels in men.