Children who had infections requiring medical care during infancy subsequently gained slightly more weight and had higher odds of obesity at ages 4 to 6 and 10 to 15, a Southern California study found. The association strengthened as the number of infections increased.

The analysis, published in Emerging Infectious Diseases, included children born from 2008 through 2012 in the Kaiser Permanente Southern California health system. Researchers analyzed weight measurements from 66,870 children at ages 4 to 6 and 46,599 children at ages 10 to 15.

Researchers excluded children born prematurely or with a low birth weight, as well as those without continuous health plan coverage or primary care visits during each of their first 2 years. Children also needed an eligible weight measurement to be included in the corresponding follow-up analysis. Obesity was defined as a body mass index above the 95th percentile for the child’s age and sex.

Children were divided into three groups: those with no infections requiring medical care during their first year, those who had one or more infections but received no antimicrobial medication, and those whose infections were treated with antimicrobial drugs. The analysis accounted for maternal obesity, macrosomia, breastfeeding, delivery method, sex, race and ethnicity, and estimated household income. Analyses of absolute weight also accounted for height.

Among children aged 10 to 15, obesity rates were 20% in those with no infections during infancy, 24% in those with untreated infections, and 27% in those with treated infections. The figures were based on 7,594, 18,468, and 20,537 children, respectively, before researchers adjusted for other factors.

After adjustment, children who had an untreated infection during infancy had slightly higher odds of obesity than those without an infection. Their odds were 9% higher at ages 4 to 6 and 11% higher at ages 10 to 15. On average, they weighed 0.09 kilograms more at the younger ages and 0.46 kilograms more at the later ages.

The link became stronger as the number of infections increased. At ages 10 to 15, children who had more than three infections during infancy had 30% higher odds of obesity and weighed 1.19 kilograms more than those who had no infections. A single infection was not linked to obesity or higher weight.

After accounting for the number of infections and other factors, antimicrobial treatment was not linked to a significant increase in obesity risk at ages 10 to 15. Treated children weighed 0.23 kilograms more than those with untreated infections, but additional treatment courses were not consistently linked to greater weight.

In subgroup analyses, children who received broad-spectrum antimicrobial drugs had 9% higher odds of obesity at both follow-up periods. Treatment before 6 months of age was associated with small weight increases at both follow-up periods and higher odds of obesity at ages 4 to 6, but not at ages 10 to 15. Most infections were viral or associated with viral illnesses, particularly upper respiratory infections, bronchiolitis, and ear infections.

The findings suggest that medically attended infections during infancy are associated with later weight gain and obesity, particularly when infections are frequent. However, the study did not establish that infections cause obesity or support changes in how childhood infections are treated. The researchers estimated that early infections might account for about 2.4% to 10% of obesity in this population, but cautioned that the true share may be lower.

The study had several limitations. It relied on past electronic health records and could not account for differences in when families sought medical care or for the type and severity of infections. Researchers also lacked information about day care attendance, older siblings, diet, and parental stress. Because the study was conducted in Southern California, the findings may not apply to other populations.

“This increased risk for later weight increases is proportional to the number of infections diagnosed and is independent of antimicrobial drug exposure,” the authors concluded.

The Southern California Regional Research Committee funded the study. The article did not include an author conflict-of-interest statement.