TUESDAY, March 27, 2007 (HealthDay News) -- Pregnant women who eat beef seven or more times a week may be producing sons with low sperm counts.
The exact reason for the association isn't clear. But hormones, pesticides or other chemicals in beef might affect the development of the testes of the still developing fetus, speculated the authors of a study in the March 28 issue of Human Reproduction.
But expectant mothers and others should weigh the findings judiciously against other evidence, the researchers added.
"We're not saying that people should stop eating beef, and it's particularly important in pregnancy that women get enough protein," said study lead author Shanna Swan, associate chairwoman for research and professor of obstetrics and gynecology at the University of Rochester (N.Y.) School of Medicine and Dentistry.
"Women have to eat protein, although they don't necessarily have to eat meat," she said. "If women want to take action, they could try hormone-free beef or organic beef, although it's not proven, or reduce their consumption of beef or find some other protein."
Dr. George R. Attia, associate professor and director of the Division of Reproductive Endocrinology & Infertility at the University of Miami Miller School of Medicine, added: "It's very hard to draw conclusions from a single study like this. They used historical data, so the mom had to remember what she was eating at the time. That also makes it difficult to get a conclusion."
Six different anabolic hormones are used in cattle in the United States and Canada to stimulate growth. Three are natural hormones -- estradiol, progesterone and testosterone -- and three are synthetic hormones -- zeranol (an estrogen), trenbolone acetate (a steroid with androgen effects) and melengestrol acetate (a progestin), the study authors said.
The use of these hormones has been banned in Europe since 1988. In the United States, use is regulated through measurable levels of the hormones present in muscle, fat, liver, kidney and other organs found in meat products.
According to background information in the study, developing fetuses and pre-pubescent children are most sensitive to exposure to sex steroids, so meat consumption by pregnant women and young children needs to be watched.
For this study, the first to look at beef consumption and semen quality, researchers analyzed semen samples and questionnaires from 387 male partners of pregnant women. The men, born between 1949 and 1983, had reported (with the mothers' input, if possible) on their own mothers' diet during pregnancy.
Sperm concentration was inversely related to how much beef the mother had consumed each week. Sons of women who ate more than seven beef meals a week had sperm concentrations 24.3 percent lower. And the proportion of men with low sperm concentrations was three times higher (17.7 percent vs. 5.7 percent) among sons of women who consumed more than seven meals of beef a week, compared with men whose mothers ate less beef while pregnant.
Sperm concentration was not related to a mother's consumption of other meat, including veal and pork, as well as fish and chicken, the researchers said.
Although none of the men in the study was infertile, 18 percent of those whose mothers ate the most beef had sperm counts classified as "sub-fertile" by the World Health Organization, the researchers said.
"While they (men whose mothers ate high quantities of beef) were fertile, they may have taken a longer time to conceive or, if we had asked them a year before, they might have been having trouble conceiving," Swan said. "They were twice as likely to have visited a doctor because they thought there were problems, so it's not to say there's no effect on fertility."
The study authors don't yet know if anabolic hormones in beef can explain the findings. Most American beef consumed while these women were pregnant was fortified with these hormones, however. Beef also contains residues of pesticides and other industrial chemicals, the study authors said.
Next, Swan and her colleagues hope to repeat this study with men born in Europe after 1988, when such hormones were banned.
"If we did the same study, and there was an association, it couldn't be due to the hormones, because there aren't any," Swan said. "But if we do not see an association, that would actually point to the hormones. That's our plan."
For more on infertility, visit the American Society of Reproductive Medicine.