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Exposure to antibiotics during the first 2 years of life was significantly associated with an increased body mass index (BMI) at 2 and 12 years of age, according to data from a population-based study of more than 33,000 children in Finland.

Antibiotics are often prescribed to young children and previous research has suggested an association between early exposure to antibiotics and increased weight gain and obesity in children, said Sofia Ainonen, MD, of the University of Oulu, Oulu, Finland, in a presentation at Pediatric Academic Societies (PAS) 2025 Meeting.

“However research on antibiotic exposure in pregnancy and in the perinatal period is more conflicting, and it is unclear whether children are more sensitive to obesity-related effects of early-life antibiotics at specific time points,” Ainonen said in an interview.

Aionen and colleagues conducted a register-based cohort study of 33,095 children delivered vaginally in Finland. Exposure to antibiotics was divided into four time periods: 1 year before pregnancy, during pregnancy, the perinatal period, and the first 2 years of life.

In the study population, 39% of mothers were exposed to antibiotics 1 year before the start of pregnancy, 27% were exposed during pregnancy, 21% of children were exposed in the prenatal period, and 68% of the children were exposed during the first 2 years of life.

The primary outcomes were BMI for age z-score (zBMI) at age 2 years and the cumulative incidence of overweight and obesity at age 12 years. In a regression analysis, only children with antibiotic exposure within the first 2 years of life was linked to a higher BMI compared with children without antibiotic exposure, with a mean increase in zBMI score of 0.067.

In addition, children exposed to antibiotics during their first 2 years were 9% more likely to be overweight and 20% more likely to be obese compared with those with no exposure to antibiotics during the same time period. The association between antibiotics exposure in the first 2 years and increased risk for overweight or obesity persisted at age 12 years, with an adjusted hazard ratio of 1.20 for obesity.

Proceed With Caution When Prescribing Antibiotics

The study’s main limitation was the retrospective design, which prevents conclusions about causality, Ainonen told Medscape Medical News. “A randomized clinical trial of potential intervention to reduce the need for antimicrobial treatment in children could reveal if this effect is causal,” she said.

“We were surprised that exposure especially to antibiotics in the first 2 years of life seemed to have the strongest association to the childhood overweight and obesity, regardless of exposure to other early-life antibiotics, especially perinatal antibiotic exposure,” Ainonen said. This finding was unexpected given previous studies showing an association between perinatal antibiotic exposure and alterations in gut microbiota composition, she noted.

The reason why only exposure to antibiotics in the early years had an effect on obesity remains an open question, said Ainonen. “The growth-promoting effects of antibiotics have been reported, and they are still not well understood,” she said. “There is also a possibility for unmeasured confounding behind this association, such as children from families in obesity-related environments may be more likely to receive antibiotics,” she added.

In clinical practice, the results provide another reason for the cautious prescribing of antibiotics for young children and avoiding unnecessary antibiotics for conditions such as upper respiratory tract infections, Ainonen said.

Interpret Results With Interest and Caution

The current rate of obesity in children and adolescents in the United States remains high, with recent data from the Centers for Disease Control and Prevention indicating a prevalence approaching 20%, said Catherine Haut, DNP, CPNP-AC/PC, in an interview.

“Identifying modifiable causes of obesity in children is very important, as obesity in childhood leads to multisystem acute and chronic health conditions later in life, including early mortality,” said Haut, an associate professor at Thomas Jefferson University and director of Nursing Research and Evidence-Based Practice at Nemours Children’s Health, Wilmington, Delaware, who was not involved in the study.

The findings of the current study were not necessarily unexpected, as previous published research has linked purposeful antibiotic use with weight gain in animals, said Haut. However, “findings have been variable and limit interpretation based on the multitude of factors which contribute to obesity in children, from demographic and social determinants of health to infant overfeeding and genetic disposition,” she noted.

Previous studies have yielded inconsistent results, said Haut. Some data suggest that antibiotic use in children, especially multiple times within the first 2 years of life, is associated with weight gain and obesity, but not consistently, she said. “Antibiotics do alter the flora of the gastrointestinal system which can affect metabolism; however, there are many, many variables associated with obesity in children, so until there are additional data for support, caution is needed in interpreting the results,” she emphasized.

“Antibiotic stewardship is very important in practice and has been promoted to reduce organism resistance, but there are other consequences of using antibiotics when they are not needed,” Haut told Medscape Medical News.

This study received no outside funding. The researchers had no financial conflicts to disclose. Haut had no financial conflicts to disclose.