Background: There are few cross-sectional and longitudinal studies on identification of the age of onset of obesity. The purpose of the present study was therefore to investigate 30 years of cross-sectional and longitudinal changes in the prevalence of obesity from 1978 to 2007 in Japanese children and adolescents between 5 and 17 years of age, using population-based samples.
Methods: Subject data were obtained from the Annual Reports of the School Health Survey published by the Ministry of Education, Culture, Sports, Science and Technology, Japan. Obesity was defined as a body mass index (BMI) at or above the 95th percentile for age and gender based on the reference years from 1979 to 1981 in Japan. The BMI was calculated as weight in kg/(height in m)2.
Results: Cross-sectional analysis of 5-, 8-, 11-, 14-, and 17-year-olds showed that the prevalence of obesity has gradually decreased since the early 2000s, with the highest prevalence in the late 1990s to early 2000s, except for in 17 year-old boys. Longitudinal studies showed that the critical periods for developing obesity were in late infancy (between 5 and 6 years of age) and in the high school period in boys, and mainly in late infancy in girls.
Conclusions: Intervention to prevent obesity should be focused on late infancy in both genders and male adolescents in Japan.