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New research from the NYU School of Medicine and the NYU Wagner School of Public Service has found that treating very young infants with antibiotics may predispose them to being overweight in childhood. 

The findings are published in the online August 21, 2012, issue of the International Journal of Obesity.

Researchers studied more than 10,000 children and found that on average, children exposed to antibiotics from birth to 5 months of age weighed more for their height than children who weren't exposed. 

By 38 months of age, exposed children had a 22 percent greater likelihood of being overweight. 

Children exposed from 6 months to 14 months did not have significantly higher body mass than children who did not receive antibiotics in that same time period.

Antibiotic use only appeared to have an effect in very young infants. Although children exposed to antibiotics at 15 to 23 months had somewhat greater BMI (Body Mass Indices) for their age and gender by the age of 7, there was no significant increase in their being overweight or obese.

The researchers caution that the study does not prove that antibiotics in early life causes young children to be overweight. It only shows that a correlation exists and further studies will need to be conducted to explore the issue of a direct causal link.

"We typically consider obesity an epidemic grounded in unhealthy diet and exercise, yet increasingly studies suggest it's more complicated," Leonardo Trasande, lead researcher, said. "Microbes in our intestines may play critical roles in how we absorb calories, and exposure to antibiotics, especially early in life, may kill off healthy bacteria that influence how we absorb nutrients into our bodies, and would otherwise keep us lean."

The researchers analyzed health information on the children during three periods between 1991 and 1992: from birth to 5 months of age; 6 months to 14 months; and, finally from 15 to 23 months. They also examined body mass or weight at five different points of time-6 weeks, 10 months, 20 months, 38 months, and 7 years of age.

This is the first time that a study has analyzed the association between the use of antibiotics and body mass starting in infancy. One previous study had identified a link between antibiotic use in early infancy and obesity at seven years of age, but was unable to examine potential impacts of antibiotic use later in infancy on body weight in childhood.