Scientists say a recent study suggests infants treated with antibiotics in the first 6 months of their life may predispose them to be overweight by the age of three.

The study was based on a group of 11,532 children born in England during 1991 and 1992. They are part of the long-term Avon Longitudinal Study of Parents and Children that is tracking their health and development.

Studying the data, researchers at the New York University School of Medicine and the NYU Wagner School of Public Service found that infants who took antibiotics sometime between birth and 5 months of age weighed more relative to their height than children who hadn't taken the drugs.

By the time those same children were aged between 10 and 20 months, the researchers found they had small increases in Body Mass Index -- even when such factors as diet, physical activity and parental weight were taken into account.

The difference was more marked by the time the kids reached 38 months of age, with the exposed kids demonstrating a 22 per cent greater likelihood of being overweight.

But children administered their first antibiotics between the age of 6 and 14 months old did not have a significantly higher BMI than children given no antibiotics at all.

According to the lead researcher at the NYU School of Medicine, associate professor of pediatrics and environmental medicine Leonardo Trasande, the findings are another reason to think obesity is caused by more than just diet and inactivity.

“The obesity epidemic is still largely a product of diet and exercise,” Trasande told CTV News. “But increasingly, evidence suggests that there are other factors, that how we ingest food, digest food, can lead to caloric weight gain.”

Trasande noted in a statement that exposure to antibiotics, especially early in life, “may kill off healthy bacteria that influence how we absorb nutrients in our bodies and would otherwise keep us lean.”

He noted that the findings appear to show that the earlier the exposure to antibiotics, the more permanent the effect on microbes in the intestine.

This is the first study to focus on the possible link between antibiotic use and BMI from infancy, but the researchers caution it does not prove a direct causal link between the two.

Instead, they say the indication of a correlation should be taken as cause for further research.

"For many years now, farmers have known that antibiotics are great at producing heavier cows for market," study co-author Jan Blustein, a professor of population health and medicine at NYU Wagner, said in a statement.

"While we need more research to confirm our findings, this carefully conducted study suggests that antibiotics influence weight gain in humans, and especially children too."

Trasande said antibiotics are important, but said doctors should prescribe them “carefully and judiciously,” to avoid excessive use among patients.

Dr. Katherine Morrison, an obesity researcher at McMaster University in Hamilton, Ont., agreed, saying children are often given antibiotics for ailments they are not meant to treat.

“It’s not unusual for doctors to have parents in the office asking for antibiotics to treat something that may be a viral infection,” Morrison told CTV. “So we all have to be aware, there may be risks and you have to use it carefully.”

Results of the study are published online in the latest edition of the International Journal of Obesity.

With a report from CTV’s medical specialist Avis Favaro and producer Elizabeth St. Philip