Body Mass Index vs Body Fat Percentage as a Predictor of Mortality in Adults Aged 20-49 Years.

Purpose: Body mass index (BMI) is the current standard body composition measurement. We examined BMI vs body fat percentage (BF%) for 15-year mortality risk among adults aged 20-49 years.

Methods: In this nationally representative cohort of US adults in the National Health and Nutrition Examination Survey, Cox proportional hazard regression was computed for 15-year mortality risk. Healthy and overweight/obese BMI were 18.5-24.9 and ≥25 kg/m², respectively. The unhealthy BF% and waist circumference (WC) thresholds were ≥27% and ≥44%, and >40 and >35 inches, for men and women, respectively. The 15-year mortality risk for BMI, BF%, and WC was computed.

Results: Body fat percentage and WC are both significantly associated with 15-year, allcause mortality (BF%: unadjusted hazard ratio [HR] 2.01; 95% CI, 1.45-2.78; P < .001; adjusted HR 1.78; CI, 1.28-2.47; P < .001; WC: unadjusted HR 1.94; CI, 1.38-2.72; P < .001; adjusted HR 1.59; CI, 1.12-2.26; P = .009) and heart disease mortality (BF%: unadjusted HR 4.20; CI, 1.94-9.11; P < .001; adjusted HR 3.62; CI, 1.55-8.45; P = .003; WC: unadjusted HR 4.75; CI, 2.45-9.21; P < .001; adjusted HR 4.01; CI, 1.94-8.27; P < .001). Body mass index has no statistically significant relationship with all-cause mortality. Body mass index has a significant association with heart disease mortality in the unadjusted model but not in the adjusted model.

Conclusion: Body fat percentage is a better predictor of 15-year mortality risk factor in young adults than BMI. A move to using BF% rather than BMI may change how we measure body composition for risk stratification.