Title of article:

Differential associations of body mass index and adiposity with all-cause mortality among men in the first and second National Health and Nutrition Examination Surveys (NHANES I and NHANES II) follow-up studies

Authors: Allison DB, Zhu SK, Plankey M, FaithMS, Heo M.
Journal: Int J Obes Relat Metab Disord, Mar 2002;26(3):410-6

Abstract

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.

Comments and Key points

A key point: "The estimated nadir of the BMI curve was at 27.3 kg/m²." This means, the lowest mortality occurs at this BMI level.

Wait a minute? Is that right? Apparently so, and a closer inspection reveals more. Other studies have also found elevated mortality with low BMI^1,2, and lowest mortality around the median BMI levels.

The article showed a graphic like the one below, but I have relabled fat mass indicator as "fat" and fat-free mass indicator as "muscle" ( to make the graph easier to understand).

The graph shows that health hazard ( risk of death ) increases as fat increases, but risk diminishes dramatically as muscle mass increases. And body mass index has "U-shaped" curve that has its lowest risk at 27.3 kg/m².

As you can see, increasing your muscle mass (by fitness, exercise, weight-lifting, etc) is the most effective way to become healthier. Dieting to reduce fat isn't as effective.

References

1. Kalmijn S, Curb JD, Rodriguez BL, Yano K, Abbott RD. The association of body weight and anthropometry with mortality in elderly men: the Honolulu Heart Program. Int J Obes Relat Metab Disord 1999 Apr;23(4):395-402
2. Heitmann BL, Erikson H, Ellsinger BM, Mikkelsen KL, Larsson B. Mortality associated with body fat, fat-free mass and body mass index among 60-year-old swedish men-a 22-year follow-up. The study of men born in 1913. Int J Obes Relat Metab Disord 2000 Jan;24(1):33-7

Review & comments by Steven B. Halls, MD, Edited on 23-June, 2008, Copyright.
Back to Bibliography of BMI articles.