The problem with soy is that it is a hugely hot area of research—more than 10,000 scientific papers have been published within just the last decade alone on soyfoods and health. With that level of investigation, there will always be a few studies here and there that show scary findings. By pouncing on those findings—rather than looking at what the majority of the research shows—critics can build a case against soy that sounds credible, even when it isn’t.

Soyfoods are an essentially unique source of isoflavones—which are a type of phytoestrogen, or plant estrogen. So it’s not surprising that some of the stories turning up on the internet have focused on sperm count and testosterone levels. Let’s take a look at what the research really says in these areas.

Soy and Sperm Count

There was lots of interest in a small pilot epidemiologic study showing that soy consumers had lower sperm concentrations than men who didn’t eat soy. But for the most part, the men who consumed soy experienced an increase in semen volume, so that the concentration of sperm was lower—not the total number of sperm. And there were no implications for fertility, since sperm concentration seemed to decrease only among men who had above average sperm concentration to begin with.

In any event, small epi studies don’t really tell us too much of anything. We can draw real conclusions only from clinical intervention studies. Three such studies have examined the impact of soy intake on sperm and semen, all with reassuring results:

In the first, British men took supplements that contained 40 milligrams of isoflavones (the amount in about 1 ½ servings of traditional soyfoods). There was no effect on sperm count or quality.

In a second study, Italian men were given much higher amounts of isoflavones—equaling four to twelve times what men in Japan typically eat. There was still no effect on sperm.

Finally, Canadian researchers compared sperm production in men consuming soy protein to men consuming milk protein—and again, there were no differences in sperm concentration.


Only two studies—of the many conducted on this topic—found reductions in hormone levels related to soy intake. In one, the isoflavone intake of the subjects was unusually high—about four times what men in Japan typically eat. And the researchers didn’t actually make any comparisons to the control group—that is, the men who didn’t eat soy. So there was no way to know for certain that there was any difference between the two groups.

The other study had only 12 subjects and nearly all of the reduction in testosterone levels were from just two of those subjects (one of whom had unusually high levels of testosterone to begin with).

On the other hand, the research showing that soy has no effect on testosterone levels is much stronger. A recent analysis of 32 studies found that neither soy protein nor soy isoflavones had any effect on testosterone levels. The researchers looked at the data in a number of different ways—they used several different statistical models—and the results were always the same. Furthermore, studies published too late to be included in this analysis showed the same thing—that soy had no effect on testosterone levels.

Looking at the evidence, there is little reason to think that soyfoods aren’t safe for men. And in fact, when it comes to prostate health, there is good reason to think that soyfoods have some real advantages for men. More on that tomorrow.