Longitudinal study of body mass index and percentage of overweight in Japanese children grouped by maturity

Childhood obesity is a known risk factor for adult diseases, making its evaluation highly important. However, the evaluation is complex because there is no gold standard method. Body mass index (BMI) and percentage of overweight (POW) are widely used in Japan. However, they have the following limitations: it is difficult to set cutoffs for BMI because it dynamically varies in childhood, and POW has not been studied extensively, especially regarding its difference during maturity. Therefore, our study analyzed BMI/POW in Japanese children grouped by maturity. We used longitudinal school check-up data collected from elementary and junior high schools in 20 municipalities. We made percentile curves of BMI/POW and calculated the percentage of participants considered overweight/obese by sex, age, and maturity. Maximum increment age (MIA) was calculated using the graphical fitting method. We included 35,461 subjects aged 15 in 2018. Early-maturing children had higher BMI. The difference among maturity groups decreased by shifting the percentile curves by differences in MIA. Therefore, the use of BMI might lead to the overestimation of overweight/obesity in early-maturing children and underestimation in late-maturing children. The POW percentile curves were “N”-shaped around the MIA, indicating the inappropriate evaluation during this period. The percentile curves of children categorized as overweight/obese were also “N”-shaped, confirming that MIA affects the evaluation of childhood obesity. The possibility of overestimation/underestimation needs verification with the data of accurate age, pubertal changes, and adult diseases. In conclusion, it is difficult to evaluate childhood obesity only with height and weight.