Body fat percentage outperforms BMI in predicting early mortality

Background and goal: Although body mass index (BMI) is widely used in clinics as the standard measure of body composition, it can potentially misclassify muscular individuals as overweight and miss cases of "normal-weight obesity," masking serious metabolic and heart disease risks. This study examined BMI vs body fat percentage for 15-year mortality risk among adults aged 20-49 years.

Study approach: Researchers from the University of Florida analyzed data from 4,252 participants in the 1999-2004 National Health and Nutrition Examination Survey (NHANES), a nationally representative sample of non-institutionalized U.S. adults ages 20 to 49. All participants had complete technician measured body composition data, including height, weight and waist circumference. Body-fat percentage was assessed by bioelectrical impedance analysis. Records were linked to the National Death Index through 2019. 

Main results:

• Adults with a high body-fat percentage (27% or more in men; 44% or more in women) were 1.78 times more likely to die from any cause than individuals in the healthy body fat range (HR 1.78; 95% CI, 1.28 to 2.47). Adults with a high body-fat percentage were also 3.62 times more likely to die from heart disease (HR 3.62; 95% CI, 1.55 to 8.45).

• Overweight/obese BMI (25 kg/m² or higher) was not associated with a statistically significant higher risk of death from any cause, compared with adults in the healthy BMI range (HR 1.25, 95% CI 0.85 to 1.84).

• Adults with a high waist circumference (more than 40 inches in men; more than 35 inches in women) were 1.59 times more likely to die from any cause than individuals in the healthy range (HR 1.59; 95% CI, 1.12 to 2.26). Adults with a high waist circumference were also 4.01 times more likely to die from heart disease (HR 4.01; 95% CI, 1.94 to 8.27).

Why it matters: The study findings support reexamining how body composition is measured in clinical settings. Historically, it has been difficult to measure body fat percentage in primary care outpatient settings. New bioelectrical impedance devices have the potential to change this.