Impacts of infancy rapid weight gain on 5-year childhood overweight development vary by age and sex in China

Objectives: To investigate the critical period for overweight development during early childhood by examining growth trajectory and related sex differences. Methods: Using piecewise linear mixed models and logistic regression, we examined the effect of growth trajectory at different periods on overweight at age 4-5 by sex among 136 971 regularly followed children (mean: 12.2 times) during 2000-2005 in south China. Results: The high-body mass index (BMI) group (>top tertile of BMI Z score at age of 4-5 years) had faster growth rates of BMI, BMI Z score, weight and height than the low-BMI group in the first 3 months of life. Boys were more likely to be overweight [odds ratio (OR) = 2.0, 95% confidence interval: 1.5-2.7] than girls; the male high-BMI group had higher growth rates during the first 3 months than girls with high-BMI, independently of environmental factors. Those fast grown (in the upper tertile of growth rates in BMI and BMI Z score) in periods 0-3 months had relatively higher OR of at risk of overweight at age of 4-5 years than those in other periods. Conclusions: Overweight risk develops during the first 3 months of life. Boys have an earlier peak in growth than girls, which may help explain why overweight is more prevalent in boys in China.