Objective: Three critical periods have been suggested for the development of obesity during childhood: fetal, ages 4-6 y, and adolescence. The prevalence of obesity in elementary school children is increasing in Japan, and the present study examines whether this rising prevalence occurs during the elementary school period (age 6-11 y) or is occurring prior to entry into elementary school.
Design: Repeated cross-sectional sampling of cohorts of children for the prevalence of obesity.
Setting and participants: The data from 81 264 first grade and 87 849 seventh grade children (94 and 87% of the total populations, respectively) between 1989 and 2002 in Kagoshima City were analyzed. Data were also obtained from nationwide surveys published by the Ministry of Japan between 1989 and 2001.
Measurements: Obesity was defined by the body mass index for an age- and sex-specific 95th percentile cutoff point in Japanese children. Trends in obesity and odds ratios of the prevalence of obesity were also determined.
Results: The period 1989-2001/2 showed significant increases in the prevalence of obesity for both genders, in both first and seventh grades, and in both Kagoshima City and nationwide. The odds ratios for the prevalence of obesity of 12-y-old children calculated against the prevalence of obesity within the same cohort at 6 y old revealed that a significant risk for development of obesity during the elementary school years applied only to boys from around 1993 onward in Kagoshima City and applied throughout the study period in nationwide Japan.
Conclusion: Obesity prevalence increases for boys during elementary school years but does not significantly increase for girls. A rising trend for becoming obese before starting elementary school was present for both boys and girls over the period 1989-2001. Educational programs to improve nutrition and physical activity, especially for boys, are becoming increasingly necessary.