Kids who receive several rounds of antibiotics before age two may be at an increased risk of being overweight by age five, suggests a new study.

The research shows a link between antibiotics and obesity, but it can’t say whether the antibiotics are "at fault," said Dr. L. Charles Bailey, the study’s lead author from the Children’s Hospital of Philadelphia.

“That said, we’re certainly interested in the possibility that antibiotics are actually contributing to the risk of obesity,” Bailey told Reuters Health by email.

“One of the more interesting hypotheses is that your body’s management of its weight involves interacting with the bacteria that live in your intestines, and affect the way you digest food,” he added.

Antibiotics kill some of the bacteria in the stomach and could leave the body in a state where it is more likely to become obese, Bailey said. The drugs could also change which foods taste good or change your activity pattern due to other side effects.

For the new study, published in JAMA Pediatrics, Bailey and his coauthors used data from health records collected between 2001 and 2013 from 64,580 children. They had data on doctor visits during the children's first five years of life.

Almost 70 percent of the kids were exposed to antibiotics before age two with an average of about two prescriptions per child.

By age two, 23 percent of the kids were overweight or obese for their age, which rose to 30 percent at age three and 33 percent at age four.

The more times the children had been exposed to antibiotics at a younger age, the more likely they were to be obese later on. Obesity was specifically linked to broad-spectrum antibiotics, like amoxicillin or tetracycline, not narrow-spectrum antibiotics like azithromycin. (Broad-spectrum means the antibiotic acts against a broad range of disease-causing bacteria.)

Kids who had been exposed to antibiotics at least four times before age two were 11 percent more likely to be obese later than kids who had not been exposed to any antibiotics.

That’s a small change in risk for individual kids, but since obesity is so common and has so many causes it is important to track down as many of these causes as possible, Bailey said.

Other than the antibiotics, the cause of the increased risk of obesity may also be the disease that the child got the medication to treat, Bailey noted.

Additionally, he said that having other family members who are obese, whether or not an infant is breastfed, and an infant's diet and activity are also connected to later obesity risk.

“Infancy is a vulnerable period for many things,” said Dr. Stephen Cook, an associate professor of pediatrics and community health at the University of Rochester Medical Center.

“This is a recent cohort, it’s large and worth noticing,” he told Reuters Health by phone. He added that it’s nearly impossible to directly test this theory with human children, but studies in animals have found that antibiotic use leads to weight gain.

Cook, who is also a member of the executive committee of the American Academy of Pediatrics' Section on Obesity, was not part of the new study.

Pediatricians are focused now on making sure they only prescribe antibiotics when the child really needs them, and on picking the right drug with the least effect on other bacteria in the body, he said.

Children get more fevers and similar illnesses as infants and toddlers, Bailey said, and it’s the most common age for ear infections, which have traditionally been a reason to use antibiotics.

But recent evidence indicates that antibiotics may not offer much benefit for ear infections among children (see Reuters story of November 17, 2010 here: reut.rs/YCeNnu).

Doctors are more likely to give very young infants antibiotics because they may get serious infections right after birth, before their immune systems have a chance to adapt to the world around them, Bailey said.

“We’d like to see the use of antibiotics go down significantly, but we also recognize that there are not just medical but also social and economic factors that are part of decisions about antibiotic use,” he said.

Still, he said, these results offer another reason for parents not to insist on antibiotics for their sick child if the doctor advises against them.

“There may be other reasons that are bigger considerations for most parents, such as reducing the chance that their child will acquire antibiotic-resistant bacteria,” Bailey said. “But possibly avoiding a risk factor for obesity is another reason to use antibiotics, like most medicines, when you really need them, but to avoid them when you don’t.”