Despite conflicting views on whether or not our guts are already colonized by hundreds of bacteria at birth (see here versus here), there is no argument about the role they play on our lives, affecting everything from immunity to metabolic function. Getting the right bacteria into babies’ guts is important for their future health, and it’s within the first two years that this is crucial — this time frame is when many conditions, like allergies and asthma, can be prevented. Whereas breastfeeding seems to be one way to get a healthy gut microbiota, new research from the Children’s Hospital of Philadelphia shows that taking some antibiotics is not.

The research found that kids younger than 2, who were given broad-spectrum antibiotics, the kind that kills all types of bacteria, were more likely to be obese by age 5. It makes sense; their digestive tracts are sensitive during their first two years, and with a weak immune system, they develop infections in the ear or sinuses at a higher frequency. The antibiotics the doctor prescribes them with then goes on to kill some gut bacteria, causing imbalances in the variety of bacteria, and altering metabolic function.

For the study, the researchers looked at health records from over 64,000 children who went to a primary care physician within their first 24 months of life, following up with them every year until they turned 5. They found that 41 percent of the children had been exposed to the broad-spectrum antibiotics at least once, compared to 62 percent who’d been exposed to narrow-spectrum antibiotics, the kind that target certain families of bacteria.

When the children turned 5, they found that those who were treated four or more times with any kind of antibiotic were 11 percent more likely to be obese. Those treated with broad-spectrum antibiotics were even more likely, with a 16 percent higher risk. In their study, the researchers wrote that “repeated use of antibiotics may have an impact on intestinal flora that alters” the way energy is metabolized. They also suggest that physiological, environmental, and socioeconomic factors may also contribute to a child’s obesity.

The findings support another study from last year, which found that children exposed to antibiotics had higher body mass indexes at age 7. Roughly one in five kids under 5 will be overweight or obese before they begin kindergarten, according to an Institute of Medicine report. Although it’s a cause for concern, parents, and eventually the kids themselves, may be able to take steps to reduce their weight anyway. Studies have shown that gut bacteria changes with a person’s diet, so it could be possible that altering a kid’s diet may help the right bacteria grow.

Source: Bailey LC, Forrest C, Zhang P, Richards T, Livshits A, DeRusso P. Association of Antibiotics in Infancy With Early Childhood Obesity. JAMA Pediatrics. 2014.