More soy, less sperm? New study in the journal Human Reproduction fuels debate but results remain inconclusive

In the wake of the recent study linking soy to reduced sperm concentration in men, newspaper and Internet headlines proclaimed the health food a cause of male infertility. It's true that the study was the largest so far to examine soy's effects on sperm, but the findings don't mean male fans of tofu burgers and soy milk need to abandon the foods just yet. The research on soy and sperm is in its preliminary stages -- and so far, highly conflicting.

"I would not counsel patients about eating or avoiding soy based on the results of this study," said Andrew La Barbera, scientific director of the American Society for Reproductive Medicine.

The 99 men in the study had visited the Massachusetts General Hospital Fertility Center between 2000 and 2006, where they answered questions about the types and amounts of soy foods they had eaten in the last three months. Tofu, tempeh, soy cheese, miso soup, edamame and protein shakes were some of the items on the list.

In the final analysis, men who reported eating the most soy products had an average of 35 million to 41 million fewer sperm per milliliter of ejaculate than those who reported eating no soy. (Men who eschewed soy had an average of 82 million sperm per milliliter; according to the World Health Organization, anything above 20 million sperm per milliliter is normal.) The effect was particularly marked in overweight or obese men, who made up 72% of those in the study.

The researchers, who published their findings online in the journal Human Reproduction, had fairly good reason to think soy foods might harm sperm. Soy and soy foods are rich in isoflavones, plant chemicals that mimic the activity of the female sex hormone estrogen. When lab rats are exposed to plant estrogens (known as phytoestrogens) in utero or early in life, they have smaller testicles and lower testosterone levels than their phytoestrogen-free peers.

But phytoestrogen exposure doesn't always have such detrimental effects. In rabbits, consuming plant estrogens has been shown to improve sperm quality and boost sex drive.

And a 2001 study of 14 men concluded that phytoestrogen intake had no effect on men's reproductive organs or semen. Similarly, a 2006 study of 58 men found that phytoestrogen consumption increased sperm count and improved sperm motility.