BMI trajectory of 8,155,894 Japanese adults from exhaustive health checkup data: the contributions of age-related changes in height and weight.

This study aimed to clarify the trajectory of BMI alongside age-related changes in height and weight among Japanese adults. Data from annual health checkups between 2015 and 2020 by the Japan Health Insurance Association were retrospectively analyzed. A total of 4,777,891 men and 3,378,003 women (age 35-69 years) were stratified into 14 subgroups based on sex and 5-year age categories. We used linear mixed-effects model to estimate values for each outcome, with six-time points (2015-2020) as the independent variable. Mean BMI changes were positive across all subgroups, indicating a trend of increasing BMI (men, 0.02 to 0.14 kg/m²/year; women, 0.05 to 0.16 kg/m²/year). In younger subgroups, the changed were relatively large, with the weight transitions mirroring those of BMI. However, the mean changes were negative (men, -0.06 kg/year; women, -0.01 kg/year) in the oldest subgroups. Height reduction increased with age across subgroups (men, -0.14 to -0.03 cm/year; women, -0.18 to -0.01 cm/year). In conclusion, BMI tended to increase with age in both sexes across all age groups of Japanese adults. The increase in BMI appeared to be influenced by weight gain in young to middle age, whereas height reduction influenced increased BMI in older age groups.