Young farm animals given antibiotics gain weight quickly. Now a new study suggests that the same thing may happen to human infants treated with antibiotics.

Researchers writing online last week in The International Journal of Obesity studied 11,532 English children born at normal weight in 1991 and 1992. Controlling for birth weight, parental body mass index, breast-feeding and other factors, they found that infants given antibiotics within six months of birth were 22 percent more likely than those given none to be overweight at age 3. Among those given antibiotics between 6 and 14 months, there was no link to body mass in childhood, but exposure from 15 to 23 months was linked to higher body mass index at age 7.

Antibiotics change the composition of the microbiome, the trillions of microbes that inhabit the body, and this, researchers say, may help explain the result. The exact mechanism is unknown.

"This study does not suggest that antibiotics are bad for you," said the lead author, Dr. Leonardo Trasande, an associate professor of pediatrics at New York University. "But our findings add to concerns about the inappropriate use of antibiotics."

There may be more to the obesity problem than just diet and exercise, he said. "Increasingly, studies suggest the need for a perspective that incorporates environmental exposures as well."
