OBJECTIVE: To investigate whether antibiotic exposure during the first year of life is associated with increased childhood body mass index (BMI). DESIGN: Secondary analysis from a multi-centre, multi-country, cross-sectional study (The International Study of Asthma and Allergies in Childhood Phase Three). SUBJECTS: A total of 74 946 children from 31 centres in 18 countries contributed data on antibiotic use in the first 12 months of life and current BMI. METHODS: Parents/guardians of children aged 5-8 years completed questionnaires that included questions about their children's current height and weight, and whether in the child's first 12 months of life, they had received any antibiotics, paracetamol, were breastfed or the mother/female guardian smoked cigarettes, and whether the child had wheezed in the past 12 months. A general linear mixed model was used to determine the association of antibiotic exposure with BMI, adjusting for age, sex, centre, BMI measurement type (self-reported or measured), maternal smoking, breastfeeding, paracetamol use, gross national income and current wheeze. RESULTS: There was a significant interaction between sex and early-life antibiotic exposure. Early-life antibiotic exposure was associated with increased childhood BMI in boys (+0.107 kg m(-2), P<0.0001), but not in girls (-0.008 kg m(-2), P=0.75) after controlling for age, centre and BMI measurement type. The association remained in boys (+0.104 kg m(-2), P<0.0007), after adjustment for maternal smoking, breastfeeding, paracetamol use and current wheeze. There was no interaction between age, maternal smoking, breastfeeding, paracetamol use, gross national income and current wheeze in the association between early antibiotic exposure and BMI. CONCLUSIONS: Exposure to antibiotics during the first 12 months of life is associated with a small increase in BMI in boys aged 5-8 years in this large international cross-sectional survey. By inference this provides additional support for the importance of gut microbiota in modulating the risk of obesity, with a sex-specific effect.
