CHICAGO (Reuters) - Eating a half serving a day of soy-based foods could be enough to significantly lower a man's sperm count, U.S. researchers said on Wednesday.
The study is the largest in humans to look at the relationship between semen quality and a plant form of the female sex hormone estrogen known as phytoestrogen, which is plentiful in soy-rich foods.
"What we found was men that consume the highest amounts of soy foods in this study had a lower sperm concentration compared to those who did not consume soy foods," said Dr. Jorge Chavarro of the Harvard School of Public Health in Boston, whose study appears in the journal Human Reproduction.
"It was a relatively large difference," Chavarro said in a telephone interview.
Chavarro said studies in animals have linked high consumption of plant-derived estrogens known as isoflavones with infertility, but so far there has been little evidence of their effect in humans.
"We wanted to know if it would affect sperm production and could serve as a marker for the effects on the reproductive system," Chavarro said.
Chavarro's team analyzed the intake of 15 soy-based foods in 99 men who went to a fertility clinic between 2000 and 2006.
They were asked how much and how often in the prior three months they had eaten soy-rich foods including: tofu, tempeh, tofu or soy sausages, bacon, burgers and mince, soy milk, cheese, yogurt and ice cream, and other soy products such drinks, powders and energy bars.
Because different foods have different levels of isoflavones in them, the researchers set a standard for serving sizes of particular foods. Then they divided the men into groups according to soy consumption levels. Men in the highest group on average ate half a serving per day.
"In terms of their isoflavone content that is comparable to having one cup of soy milk or one serving of tofu, tempeh or soy burgers every other day," Chavarro said.
The difference was striking. Men in the highest intake category had 41 million sperm per milliliter less than men who ate no soy foods. A normal sperm count ranges from 80 million and 120 million per milliliter, and a sperm count of 20 million per milliliter or below is considered low.
"It suggests soy foods could have some deleterious effect on the reproductive system and especially on sperm production," Chavarro said.
The researchers found the association between soy foods and lower sperm count was stronger in overweight men, which might suggest hormones are playing a role.
"Men who are overweight or obese tend to have higher levels of androgen-produced estrogen. They are converting a male hormone into a female hormone in their fat. The more body fat you have, the more estrogen you produce in your fat," Chavarro said.
Chavarro said the study was not sufficient to suggest that soy intake would have health implications such as inducing infertility. Much bigger studies would be needed to answer that question, he said.
Editing by Will Dunham and Xavier Briand