Associations of body mass index and Anthropometric indicators of fat mass and fat free mass with all-cause mortality among women in the First and Second National Health and Nutrition Examination Surveys follow-up studies

PURPOSE: This study tests whether fat mass (FM) and fat free mass (FFM) have opposite associations with mortality in a nationally representative sample of females. METHODS: Data on 13,369 female participants from National Health and Nutrition Examination Surveys (NHANES) I and II (aged 25 to 75 years) were analyzed. Mean follow-up time was 16.1 years. Ninety-seven percent of the participants with 3020 deaths were successfully followed. Subscapular and triceps skinfolds thickness were used as a FM indicator (FMI). Upper arm circumference was used as a FFM indicator (FFMI). Cox regression tested the relationships of BMI, FM and FFM with all-cause mortality adjusting for various socio-demographic variables. RESULTS: BMI had a U-shaped relationship with mortality with a nadir of approximately 27 kg/m². When FFMI was added to the model, the relationship between BMI and mortality became more monotonic increasing. FMI showed a significant negative relationship with mortality. CONCLUSIONS: Contrary to expectations, both FFMI and FMI had negative relationships with mortality. These results differ from patterns previously observed in males and may reflect sex differences in fat distribution. Research using superior measures of body fat amount and distribution may resolve these discrepancies.