Were the real cowboys as macho as Clint Eastwood?

Well, a new research suggests that pregnant women who consume beef daily could be more likely to deliver sons with lower sperm counts than others.

Sperm development takes place in stages throughout a guy's life from the pre-natal months to adulthood, but a critical moment for this process occurs in the uterus. "The average sperm concentration of the men in our study went down as their mothers' beef intake went up. I don't think this is cause for alarm or immediate action at all. This is the first study of its kind," said researcher Shanna Swan, director of the Center for Reproductive Epidemiology at the University of Rochester Medical Center.

The precise cause is yet unknown, but growth hormones and other chemicals in beef are amongst the suspects, besides pesticides and other contaminants in cattle forage or particular lifestyles associated to higher-beef diets.

Swan's team monitored 387 mother-son pairs in five U.S. locations between 1999 and 2005. The team also investigated men born between 1949 and 1983, when additives were forbidden from cattle's food. Mothers with one or more red-meat meals daily were assessed as "high beef consumers."

18 % of the 51 men whose mothers were classified as the highest beef eaters presented "sub-fertile" sperm counts by the standards of the World Health Organization. Only 5 % of men with mothers assessed as eating less beef were found to have sub-fertile counts, below 20 million per milliliter, with overall sperm concentrations that were 24 % higher than the group of high-beef intake.

But all participants could conceive a child without medical assistance.

The consumption of pork or chicken during pregnancy did not influence sperm count, nor the men's beef eating during their lifetime.

The team wants to compare these results with a research on young men born in Europe after 1988, when hormones were banned from beef, while in the U.S., six hormones, including two estrogen variants, are legal and commonly used in cattle. "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," said Swan.

Beef not treated with hormones would be an alternative.