Association between complementary food introduction before age 4 months and body mass index at age 5–7 years: A retrospective population-based longitudinal cohort study

Background: The optimal time of starting complementary foods (CFs) in infants remains a subject of debate. This population-based longitudinal cohort study evaluated the association between early CF introduction and body mass index (BMI) in children aged 5–7 years. Methods: The present study included 917,707 children born in Korea during 2008–2009. Initial timing of CF introduction was obtained by questionnaires administered between 4 and 6 months and 9 and 12 months of age. The cohort consisted of 154,565 eligible individuals who properly completed the screening programme, including structured questionnaires, anthropometric measurements and physical examinations. To balance baseline characteristics, children were subjected to propensity score matching based on 95 covariates, including indicators of baseline health such as perinatal condition, birth weight, economic status, clinical disease and drug exposure. Exposure was defined as introduction to CF at age < 4 months, and outcomes were overweight (BMI z-score > 85th percentile) and obesity (BMI z-score > 95th percentile) at ages 5–7 years. Results: Of the 154,565 eligible children in the observed cohort, 10,499 (6.8%) were introduced to CF at age < 4 months and 144,066 (93.2%) at age ≥ 4 months. Propensity score matching yielded 9680 children introduced to CF at age < 4 months and 35,396 at age ≥ 4 months. The risk for being overweight or obese at age 5–7 years was slightly higher among those who started CF at age < 4 months than at age ≥4 months (adjusted relative risk = 1.06; 95% confidence interval = 1.02–1.09). A similar but stronger association was observed for being obese at age 5–7 years (adjusted relative risk = 1.12; 95% confidence interval = 1.05–1.19). Conclusions: Early CF introduction before age 4 months was associated with increased BMI at age 5–7 years.