Background: Adiposity is a key risk factor for asthma and impaired pulmonary function. Objectives: We aimed to identify the critical period of life course adiposity for asthma in childhood and young adulthood, and to determine whether associations of adiposity and asthma vary across ages. Methods: Birth weight and body mass index (BMI) from birth to 17 years of age were assessed in 6130 children from the Taiwan Children Health Study. Logistic regression for asthma outcome and linear regression for pulmonary function outcome were used to investigate associations of adiposity with asthma. Seventeen BMI-related single-nucleotide polymorphisms were used to obtain genetic instrumental variables for adiposity to perform Mendelian randomization (MR) analysis. Results: Using both regression model and MR analyses, we confirmed that the critical period of adiposity in predicting childhood asthma would be before age 6 years. Further, we discovered that the sensitive period of adiposity gain related to young adulthood asthma was the prepubertal stage. Risks of asthma at age 17 per unit increase in z-score of the BMI increased from 0.94 (95% CI: 0.79–1.11) at birth, and became greater than 1.00 between age 11 and 12, then increased to 1.08 (95% CI: 0.95–1.22) at age 17. The associations of life course BMI with asthma and pulmonary function impairment at age 12 and with asthma at age 17 increased with age. The aforementioned association was most prominent among central obesity indicators. Conclusions: To prevent asthma in childhood and young adulthood, we should aim at promoting healthy growth at the toddler period and prepubertal stage.
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