Background & Aims Childhood obesity is increasing and is associated with adult obesity. Antibiotics have been used to promote weight gain in livestock for several decades. Antibiotics are commonly prescribed for children, but it is not clear how exposure to antibiotics early in life affects risk for obesity. We performed a population-based cohort study to assess the association between antibiotic exposure before age 2 years and obesity at age 4 years. Methods We performed a retrospective cohort study of 21,714 children in The Health Improvement Network—a population-representative dataset of >10 million individuals derived from electronic medical records from 1995 through 2013 in the United Kingdom. Eligible subjects were registered within 3 months of birth with complete follow-up and height and weight were recorded within 12 months of their 4th birthday. Antibiotic exposure was assessed before age 2 years, and classified based on anti-anaerobic activity. The primary outcome was obesity at age 4 years. We performed logistic regression analyses, adjusting for maternal and sibling obesity, maternal diabetes, mode of delivery, socioeconomic status, year and country of birth, and urban dwelling. Results In the cohort, 1306 of the children (6.4%) were obese at 4 years of age. Antibiotic exposure was associated with an increased risk of obesity at 4 years (odds ratio [OR] = 1.21; 95% confidence interval [CI]: 1.07–1.38). ORs increased with repeated exposures: for 1–2 prescriptions, OR = 1.07 (95% CI, 0.91–1.23); for 3–5 prescriptions, OR = 1.41 (95% CI, 1.20–1.65); and for 6 or more prescriptions, OR = 1.47 (95% CI, 1.19–1.82). Antifungal agents were not associated with obesity (OR = 0.81; 95% CI, 0.59–1.11). Conclusions Administration of 3 or more courses of antibiotics before children reach an age of 2 years is associated with an increased risk of early childhood obesity.
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