Objective: To describe differences in the 22 y mortality risk associated with body mass index (BMI), body fat or fat-free 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 fifth to 1.4 (95% confidence interval 1.11-1.99) in men in the highest fifth of percentage fat mass. For BMI the lowest risk was observed for men belonging to the middle fifth of BMI. When the relative risk was set at 1.00 for subjects belonging to the middle fifth of BMI the risk associated with the low BMI fifth was 1.3 (95% confidence interval 0.94-1.68) and that with the highest fifth was 1. 5 (95% confidence 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. International Journal of Obesity (2000)24, 33-37