Associations of body mass index categories with all-cause mortality across metabolic status in 3518 Japanese adults: a 10- and 22-year prospective cohort follow-up study

This study aimed to evaluate whether metabolic status modifies the association between body mass index (BMI) and all-cause mortality in a Japanese general population. We analyzed data from 3518 community-dwelling adults (1554 men and 1964 women; mean age: 64 ± 13 years) enrolled in the Nomura cohort studies (baseline surveys in 2002 or 2014). Mortality status was confirmed using Japan’s Basic Resident Register. Participants were classified into phenotypes based on BMI categories and the number of MetS components. Cox proportional hazards models with age as the time scale were used to estimate adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) for all-cause mortality. Over a median follow-up period of 17.3 years, 34.0% of participants died. Mortality risks varied according to BMI category and metabolic status. Among participants with 0–1 and 2 MetS components, underweight individuals consistently exhibited the highest incidence rates, the lowest cumulative survival, and significantly increased HRs for all-cause mortality (1.71; 95% CI, 1.25–2.34 and 2.26; 95% CI, 1.47–3.48, respectively). In contrast, among individuals with 3–4 MetS components, mortality risk was highest in those with obesity (2.07; 95% CI, 1.18–3.64), followed by those with underweight. A significant interaction was observed between BMI and metabolic status ( p = 0.030). Across BMI categories, participants with greater numbers of MetS components generally exhibited higher incidence rates of all-cause mortality. The association between BMI and all-cause mortality differed according to metabolic status. These findings suggest that metabolic status may influence the relationship between BMI and mortality risk in Japanese adults.