This file photo taken on June, 2003, shows soybeans in fields in the northern Argentine province of Santa Fe. (Enrique Marcarian/Files/Reuters)

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.

STRIKING DIFFERENCE

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)