Objective: The frequently observed U-shaped relationship between body mass index (BMI; kg/m(2)) and mortality rate may be due to the opposing effects of fat mass (FM) and fat-free mass (FFM) components of BMI on mortality rate. The purpose is to test the hypothesis stated above.
Design: Longitudinal prospective cohort studies. The mortality follow-up of the first and second National Health and Nutrition Examination Surveys (NHANES I and NHANES II).
Subjects: A total of 10 169 male subjects aged 25-75 who participated in NHANES I and II were selected for analyses. Follow-up continued until 1992. The mean follow-up time was 14.6 y for NHANES I and 12.9 y for NHANES II. Ninety-eight percent of the participants were successfully followed representing a total of 3722 deaths.
Measurements: Subscapular and triceps skinfolds thickness were used as FM indicators, whereas upper arm circumference was used as a FFM indicator. The Cox proportional hazards model tested the relationships of BMI, FM and FFM with all-cause mortality adjusting for age, smoking status, race and education levels.
Results: BMI had a U-shaped relationship with mortality, with a nadir of approximately 27 kg/m(2). However, when indicators of FM and FFM were added to the model, the relationship between BMI and mortality became more nearly monotonic increasing. Moreover, the relationship between FM indicator and mortality was monotonic increasing and the relationship between FFM indicator and mortality was monotonic decreasing.
Conclusion: These results support the hypothesis that the apparently deleterious effects of marked thinness may be due to low FFM and that, over the observed range of the data, marked leanness (as opposed to thinness) has beneficial effects.