U.S. women who eat a lot of beef while pregnant give birth to sons who grow up to have low sperm counts, researchers reported Tuesday.
They believe pesticides, hormones or contaminants in cattle feed may be to blame. Chemicals can build up in the fat of animals that eat contaminated feed or grass, and cattle are routinely given hormones to boost their growth.
"In sons of 'high beef consumers' (more than seven beef meals a week), sperm concentration was 24.3 percent lower," the researchers wrote in their report, published in the journal Human Reproduction.
The team at the University of Rochester Medical Center in New York studied data on the partners of 387 pregnant women in five U.S. cities between 2000 and 2005, and on the mothers of the fathers-to-be.
Of the 51 men whose mothers remembered eating the most beef, 18 percent had sperm counts classified by the World Health Organization as sub-fertile.
"The average sperm concentration of the men in our study went down as their mothers' beef intake went up. But this needs to be followed carefully before we can draw any conclusions," said Shanna Swan, who led the team.
Swan said she would like to study infertile men to see if similar findings might hold for them.
"I was really surprised when we found this. It was a really strong association," Swan said.
Swan is perhaps best known for controversial findings that male sperm counts are falling in many regions. She has been doing research to find out if environmental hormones may be to blame.
"We know from rodent studies that even tiny amounts of estrogen in utero (while in the womb) can affect sperm count," Swan said.
Talking to mothers
She and colleagues set up a study of pregnant women and their partners, with funding from the U.S. National Institutes of Health.
"They came in, we got a sperm count, they did an interview," Swan said. Starting in 2000, they also gave questionnaires to the mothers of the men.
"Earlier this year I became more aware of the controversy about growth hormones in beef," she added.
So her team went back through the questionnaires and the data on sperm count and analyzed the data.
Limited, but intriguing finding
She admits the study is limited — this type of study, called a retrospective study, is not as powerful as a study that follows people in real-time.
But she believes that women can remember fairly accurately what they ate while pregnant.
"When you are pregnant you are very aware of what you eat — you are watching your weight and some things make you sick and you need to get enough of x and y so you focus on that," she said.
The mothers of the men were asked only if they ate beef more than once a day or less — something Swan believes they could remember accurately.
Swan now wants to test young men living in the European Union, where hormones have been banned in beef since 1988.
"Given the widespread use of hormones to stimulate animal growth in the United States, the findings of this study that a mother's consumption of beef could be linked with a reduced sperm count in her son is plausible," said Alastair Hay, professor of environmental toxicology at Britain's University of Leeds.
Swan said beef eating was the only real link between the women whose sons had low sperm counts.
"Almost nobody ate a lot of other meat and if they did, they also ate a lot of beef," she said.