Soy has no known effect on testosterone levels in men.
This myth has come about because soy contains natural phytoestrogens, known as isoflavones, which resemble estrogen chemically. Before proper research was conducted, some people thought that they would affect testosterone in the same way that estrogen does. But they don’t because they are not estrogen. In fact, one study suggests that soy isoflavones have just 1/10,000 the potency of ‘real’ estrogen.1
Another study involved seven healthy young men who ate large amounts of soy beans every day for a week. At the end, they showed no changes in estrogen or total and free testosterone at all, but the study did find an increase in brain activity, specifically an improvement in spatial cognitive performance.2
A meta-analysis of all published research relating to soy and testosterone was undertaken in 2010. It looked at 15 placebo-controlled studies and 32 reports on 36 treatment groups and concluded unequivocally: ‘Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men’.3
And there is more good news: phytoestrogens can have powerfully beneficial effects. In Asia, where consumption of soy products is greater than in the US, prostate cancers are less common in men. In fact, consumption of soy is associated with a 20-30 percent reduced risk of prostate cancer.4 Incredibly, research has shown that consuming soy can actually help slow down the rate of prostate cancer if it has already taken hold.5
Correspondingly, research shows that women who consume more soy suffer less breast and uterine cancers. There have been lots of studies done on this issue and one found that women who have just one cup of soy milk per day (or half a cup of tofu) have 30 percent less risk of developing breast cancer compared with women who have little or no soy.6 The sooner we start the better, as breast tissue forms in adolescence, but it is also never too late. The Women’s Healthy Eating and Living Study found that women who had previously been diagnosed with breast cancer would do well to include soy products in their diet as those who consumed the most soy products cut their risk of cancer recurrence or mortality in half.7
Soy is a good guy. It definitely doesn’t lower testosterone and it does protect against cancer.
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