Pregnant women who consume meat daily could be more likely to have sons with lower sperm counts than mothers on low-beef diets, suggests a new study.
While the development of sperm occurs in steps throughout a guy's life from the pre-natal months to adulthood, a critical stage of development occurs in the womb.
"The average sperm concentration of the men in our study went down as their mothers' beef intake went up," said researcher Shanna Swan, director of the Center for Reproductive Epidemiology at the University of Rochester Medical Center.
"I don't think this is cause for alarm or immediate action at all. This is the first study of its kind," Swan added.
The scientists have yet to pin down what causes the effect. They suggest, in an article detailing the research in the March 28 issue of the journal Human Reproduction, that growth hormones and other chemicals in beef could play a role.
Swan and her colleagues surveyed 387 mother-son pairs in five U.S. cities between 1999 and 2005. The participants included men born between 1949 and 1983, a time when grocers had yet to offer beef without chemical additives. Mothers who reported eating an average of one or more red-meat meals a day were considered "high beef consumers."
Among the 51 men whose mothers were the highest beef eaters, almost 18 percent had sperm counts classified as "sub-fertile" by the World Health Organization. Just 5 percent of men whose mothers ate less beef had sub-fertile counts, below 20 million per milliliter, with overall sperm concentrations that were 24 percent higher than the high-beef group.
Even so, all participants were able to conceive a child without medical assistance.
The mother's intake of other meats, such as pork and chicken, as well as the men's beef consumption during their own lives had no effect on sperm quantity.
Several factors could explain the findings, including pesticides and other contaminants in cattle feed or particular lifestyles during pregnancy linked higher-beef diets.
To tease out the cause, the scientists have proposed a study of young men born in Europe after 1988, when hormones were banned from commercial beef. The scientists hope to compare results with a parallel study in the U.S. where six hormones, including two types of estrogen, are legal and commonly used in cattle.
In terms of how women should heed the results, Swan suggests a conservative approach. "Women will have to make a choice as they do whenever the science is new or uncertain as to whether they want to modify their behavior," Swan told LiveScience.
If they wanted, women could choose beef not treated with hormones, often available in health food stores and some supermarkets.