Sex-age-specific association of body mass index with all-cause mortality among 12.8 million Korean adults: a prospective cohort study

Background: Despite differences in body shape and adiposity characteristics according to sex and age, a single range of healthy weight [body mass index (BMI, kg/m(2)) of 18.5–24.9) regardless of sex and age has been recommended. The aim of the study is to examine whether the association between BMI and all-cause mortality varies by sex and age, and, if relevant, to estimate sex-age-specific optimal BMIs associated with a minimal risk of death. Methods: A total of 12 832 637 Korean adults aged 18–99 years who participated in health examinations during 2001–04 were followed up until 2013. Hazard ratios of death in sex-age groups were calculated using Cox regression models after adjustment for age, smoking status and known pre-existing illness. Results: During follow-up, 456 175 men and 241 208 women died. Among men, the age-specific optimal BMI was 23.0–25.9 (kg/m(2)) at 18–34 years, 24.0–27.9 at 45–54 year, and 25.0–28.9 at 65–74 years. Among women, it was 15.5–24.9 at 18–34 years, 21.0–26.9 at 45–54 years and 24.0–28.9 at 65–74 years. Patterns of sex-age-specific association generally did not differ between never-smokers with no known illness and all participants. Progressively increased risks above and below sex-age-specific optimums were observed (reverse J-curve). Smoking had a limited impact on the observed associations. Conclusions: Women had a lower optimal BMI than men, especially at younger ages. The optimal BMI increased with age. Change in optimal BMI with age, however, was more profound in women than in men. Sex-age-specific optimums were generally higher than the current normal weight (BMI of 18.5–24.9), except in women below 50 years. Sex-age-specific guidelines related to body weight may be needed to guide people for better health.