Among the questions recently raised concerning soy protein and isoflavones is their effect on testicular cells. The testes is the primary site of testosterone synthesis in the body, and anything that could damage cells in the testes would have an adverse effect on testosterone synthesis in men. Some studies have found that genistein, a major soy isoflavone, may promote a process in testicular cells called apoptosis, which involves a cascade of intracellular signaling that results in cellular death. In short, during apoptosis, cells commit suicide.
A closer look at those studies, however, reveals aspects of isoflavones that may not apply to nutritional use of soy products. The studies involved isolated mouse testicular cells that were exposed to large doses of genistein for 48 to 72 hours. The likelihood of such a scenario in humans is remote, especially since isoflavones aren’t very well absorbed, and you’d have to eat a heck of a lot of soy to even approach the level of genistein used in the mouse-cell studies.
That isolated-cell, or in vitro, study is similar to the kind used to ‘prove’ the cancer-causing effects of saccharin years ago, which involved exposing isolated mouse cells to huge doses of saccharin, leading to cellular changes indicating cancer promotion. Later testing in humans revealed no such effect, since no human being took in the level of saccharin used in the mouse study.
Interestingly, a 1988 study featured 20 healthy men engaged in an intensive exercise program who took in either soy or whey protein sources. The men eating the soy’but not the whey’showed an increased plasma antioxidant level. Higher blood antioxidant levels offer protective effects during exercise and may increase exercise recovery.
So what should you do about soy protein? Existing studies show that in smaller doses, such as no more than 25 grams daily, soy protein offers several beneficial health effects with no apparent adverse hormonal effects. That means you should limit soy protein intake to that amount to play it safe. As little as six to eight grams of soy protein daily appears to confer protective cardiovascular effects, according to studies involving Japanese adults. Those effects include decreasing the oxidation of lipids in the blood, relaxing coronary arteries through a natural calcium-channel blocking effect and an anti-inflammatory effect in blood vessels.