"We typically consider obesity an epidemic grounded in unhealthy diet and exercise, yet increasingly studies suggest it's more complicated," said co-author Leonardo Trasande of the New York University School of Medicine.
"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 study adds to a growing body of research warning of the potential dangers of antibiotics, especially for children.
Preliminary studies have linked changes in the trillions of microbial cells in our bodies to obesity, inflammatory bowel disease, asthma and other conditions. However, direct causal proof has not yet been found.
This was the first study analyzing the relationship between antibiotic use and body mass starting in infancy.
Image: Cute baby, via Shutterstock