The researchers say residues of hormones given to beef to promote growth may be a factor, but that's not certain.
The beef industry disputes that theory.
"Nothing from this study changes the fact that during pregnancy, naturally nutrient-rich beef is a vital part of a healthy, wholesome diet for a mother and her child," Mary K. Young, MS, RD, tells WebMD in an emailed statement.
Young is the executive director of nutrition for the National Cattlemen's Beef Association (NCBA).
About the Study
The researchers included Shanna Swan, PhD, director of the Center for Reproductive Epidemiology at the University of Rochester Medical Center in Rochester, N.Y.
Their study appears in Human Reproduction.
Swan and colleagues studied 387 men (born between 1949 and 1983) whose partners were pregnant. The men provided semen samples and completed surveys about their diet and other lifestyle habits.
The men were also instructed to ask their mothers what they had eaten while pregnant with them decades earlier.
The moms reported eating four weekly servings of beef during pregnancy, on average.
Fifty-one moms reported eating beef more than seven times per week during pregnancy. The average sperm concentration for their adult sons was 24% lower than men whose moms ate beef less often during pregnancy, the study shows.
In addition, about 17% of men whose moms ate beef more than seven times weekly during pregnancy had sperm concentrations in the "subfertile" range, the researchers note.
However, all of the men fathered children without medical treatment, according to the study.
The study shows no links between other foods eaten during pregnancy and the men's sperm count. In addition, the men's beef intake as adults wasn't associated with sperm count.
Hormones have long been administered to U.S. beef cattle to promote cattle growth, the researchers note.
Traces of such hormones may be more likely to affect fetuses than adults, Swan's team suggests.
However, the researchers note that they don't have data on hormone residue in the beef the moms ate during pregnancy.
The FDA, which oversees food safety in the U.S., has information on its web site about the use of steroid hormones to promote growth in feed animals.
"Certain steroid hormones have been approved for use at very low concentrations to increase the weight gain and/or improve feed efficiency in beef cattle," states the FDA.
"Residue levels of these hormones in food have been demonstrated to be safe, as they are well below any level that would have a known effect on humans," the FDA says.
Beef Industry Responds
"As a mother and registered dietitian, I can tell you that I am very confident in the safety of beef," Young tells WebMD.
"It would be unwise to change dietary patterns based on a study of women trying to recall what they ate at least 20 years ago, especially when iron is such an important nutrient during pregnancy," Young says.
Iron is found in beef and other foods. "One in five women, and half of all pregnant women, are iron deficient," Young says.
"Research shows beef actually plays a positive role in men’s reproductive health, especially the formation of new sperm and maintenance of sperm motility [movement]," Young says.
Young notes that observational studies -- such as the one done by Swan's team -- don't prove cause and effect.
Swan's team agrees that its study doesn't prove that beef intake during pregnancy affects men's sperm. But the researchers say the topic deserves further study.
The mothers' memories of what they ate during pregnancy "is undoubtedly subject to error," write Swan and colleagues. They call for other studies to check the findings.
Meanwhile, Swan's study doesn't include any dietary recommendations for pregnant women.