OBJECTIVE: To describe differences in the 22 y mortality risk associated with body mass index (BMI), body fat or fatfree mass, in order to examine if the differential health consequences of fat and fat-free mass may be responsible for elevated mortality rates at both high and low BMI. DESIGN: Prospective cohort study, a 22 y follow-up. SETTING: General community. The study of men born in 1913, Gothenburg. SUBJECTS: 787 men aged 60 y. MAIN OUTCOME MEASURES: Number and time of total deaths from 1973 to 1995. RESULTS: The risk of dying was a linear function of percentage fat and fat-free mass, and increased from a relative risk of 1.00 in men belonging to the lowest ®fth to 1.4 (95% con®dence interval 1.11 ± 1.99) in men in the highest ®fth of percentage fat mass. For BMI the lowest risk was observed for men belonging to the middle ®fth of BMI. When the relative risk was set at 1.00 for subjects belonging to the middle ®fth of BMI the risk associated with the low BMI ®fth was 1.3 (95% con®dence interval 0.94 ± 1.68) and that with the highest ®fth was 1.5 (95% con®dence interval 1.09 ± 1.96). Analyses including both body fat and fat-free mass showed that total mortality was a linear increasing function of high fat and low fat-free mass. CONCLUSION: The apparent U-shaped association between BMI and total mortality may be the result of compound risk functions from body fat and fat-free mass.

No abstract