Influence of socioeconomic status on the association between body mass index and cause-specific mortality among older Japanese adults: The AGES Cohort Study

Objective: Many studies have suggested a U-shaped curve for the association between body size and mortality risks, i.e., mortality risks increase in those who are both overweight and underweight. The strength of the associations may vary according to socioeconomic statuses (SES), as they determine levels of access to healthcare and psychosocial stresses. We investigated the modifying effects of SES on the relationship between body mass index (BMI) and mortality. Method: We used prospective cohort data of participants in the Aichi Gerontological Evaluation Study in 2003 (n=. 14,931), who were 65. years or older and physically and cognitively independent at baseline, and residing in eight municipalities in Japan. Data on all-causes mortality and mortality from cancer, cardiovascular disease, and respiratory disease was obtained from municipal government registries. Results: Proportional hazard regression analyses showed that, among men, the associations between overweight (BMI≥25kg/m²) and higher mortality risks by any cause were stronger among lower income groups. Even adjusting for multiple confounding factors, hazard ratios (95% confidence intervals) for mortality by all causes among low income group (household income<1.5 million yen) were 1.96 (1.02-3.73) for overweight compared with BMIs between 23.0 and 24.9, whereas they were 0.94 (0.57-1.38) among men in high income group (income>3 million yen). The modifying effects of income were not marked among women. Conclusion: Household income, which may directly reflect accessibility to healthcare and psychosocial stress among older Japanese men, may be an important modifying factor in the health risks attributable to overweight.