Early-Childhood Changes in BMI z-Score and Serum IGF-1 Levels at Age 6 Years: The Japan Environment and Children Study

Body mass index (BMI) is associated with insulin-like growth factor-1 (IGF-1) concentrations in children, but longitudinal evidence regarding BMI changes during early childhood and IGF-1 concentrations at age 6 years remains limited. We examined associations between early-childhood BMI z-score changes and serum IGF-1 concentrations at age 6 years in 2,757 children from the Japan Environment and Children’s Study. BMI z-scores were assessed at ages 2, 4, and 6 years, and serum IGF-1 was measured at age 6 years. Associations of overall BMI z-score change from ages 2 to 6 years and interval-specific changes from ages 2 to 4 and 4 to 6 years with IGF-1 were examined using multivariable linear regression. Longitudinal associations of repeated BMI z-scores with thyroid-stimulating hormone (TSH), free thyroxine (FT4), and 25-hydroxyvitamin D [25(OH)D] were evaluated using linear mixed-effects models. Overall BMI z-score change from ages 2 to 6 years was positively associated with IGF-1 (B = 18.14 ng/mL; 95% confidence interval [CI], 16.34–19.95; P < 0.001). When interval-specific changes were simultaneously modeled, changes from ages 2 to 4 years (B = 11.43; 95% CI, 8.82–14.15) and 4 to 6 years (B = 21.24; 95% CI, 18.94–23.54) were both associated with IGF-1 (both P < .001). Associations with TSH and FT4 were weaker and inconsistent, while the age-dependent association with 25(OH)D was attenuated after adjustment for season of blood collection. These findings suggest that changes in body size during early childhood are associated with variation in serum IGF-1 concentrations at school age.