First incidence and associated factors of overweight and obesity from preschool to primary school: longitudinal analysis of a national cohort in Japan.

BACKGROUND/OBJECTIVES: To assess longitudinal changes in the first incidence of overweight and obesity and associated factors from preschool to primary school age in Japan.

SUBJECTS/METHODS: We obtained individual-level data from the "Longitudinal Survey of Newborns in the 21st Century" on children born in Japan in 2001. This analysis included 15,427 children (7951 boys and 7476 girls) with complete anthropometric data reported annually from age 42 months to 12 years. We followed the criteria of the International Obesity Task Force to define overweight and obesity. We assessed annual and cumulative incidence proportions and estimated odds ratios for the first incidence at preschool age (42-66 months) and primary school age (7-12 years), using multivariable logistic regression.

RESULTS: The annual incidence of overweight and obesity combined was 3.8% at age 54 months (3.5% in boys, 4.2% in girls), and 1.2% at age 12 years (1.6% in boys, 0.7% in girls). The cumulative incidence was 19.7% at age 12 years (21.9% in boys, 17.3% in girls). Odds ratios (95% confidence intervals) of incidence for girls compared with boys were 1.28 (1.12-1.46) at preschool age and 0.62 (0.55-0.68) at primary school age. Other associated factors at preschool age included birth month, birthweight, gestational length, region and municipality of residence, living with grandparents, and behavioral factors (bedtime hours, television viewing time, and skipping breakfast). These associations persisted at primary school age except birthweight and gestational length and associated factors covered maternal education, living without siblings, and school travel mode.

CONCLUSIONS: The first incidence decreases from preschool to primary school age and follows different age trajectories for boys and girls in a high-income population with low levels of childhood overweight and obesity. Policy-makers should consider such age-based changes in incidence and associated factors to target appropriate groups for primary prevention.