The purpose of this study is to assess the association between body mass index (BMI) and mortality among nursing home residents in Japan.

A one-year prospective cohort study was conducted with 8,510 elderly individuals across 140 nursing homes. Baseline measurements included age, sex, height, weight, BMI, activities of daily living (ADL) (Barthel Index), and degree of dementia. Information regarding dates of discharge and mortality were also obtained to calculate person-years. Cox’s proportional hazards model was used to estimate hazard ratios.

Mean age and BMI were 84.3 [standard deviation (SD) 8.1] years and 20.6 (SD 3.8) kg/m², respectively. Hazard ratios of mortality adjusted for sex, age, ADL, degree of dementia, and type of home were 2.4 [95 % confidence interval (CI): 1.9–3.1] for the 1st quintile of BMI (<17.3 kg/m²), 1.7 (95 % CI: 1.3–2.3) for the 2nd quintile (17.3–19.2 kg/m²), 1.5 (95 % CI: 1.2–2.0) for the 3rd quintile (19.3–21.1 kg/m²), and 1.2 (95 % CI: 0.9–1.6) for the 4th quintile (21.2–23.5 kg/m²) (P for trend <0.001), compared with the reference 5th quintile (23.6≤ kg/m²).

There was a clear inverse dose-dependent relationship between BMI and mortality. Future studies should be conducted to determine the effects of nutritional intervention on mortality in institutionalized elderly adults.