Body fat percentage is a more accurate predictor of long-term mortality in young adults than body mass index, according to new research based on nationally representative health data.
The findings suggest that clinical reliance on body mass index (BMI) may overlook individuals at elevated risk—particularly those with normal BMI but high body fat.
Investigators analyzed data from over 4,200 U.S. adults aged 20 to 49 who participated in the National Health and Nutrition Examination Survey (NHANES) from 1999 to 2004. Participants were followed for 15 years using National Death Index records to assess the relationship between body composition and mortality.
The study compared three body composition measures—BMI, body fat percentage (BF%), and waist circumference (WC)—to determine which most accurately predicted death from all causes and from heart disease. Both BF% and WC showed significant associations with mortality, while BMI did not retain statistical significance in adjusted models that accounted for age, race, and income level.
Individuals with high body fat—defined as ≥27% for men and ≥44% for women—had a 78% higher risk of dying from any cause over 15 years. Those with large waist measurements had a 59% increased risk. Both were also linked to higher heart disease mortality, with BF% associated with more than triple the risk and WC with approximately fourfold increased risk, compared with individuals in the healthy range.
In contrast, BMI was only associated with heart disease mortality in unadjusted models. Once demographic factors were considered, this relationship was no longer statistically significant, indicating that BMI may not accurately reflect health risk in younger adults, especially those with higher muscle mass or "normal-weight obesity" (normal BMI but high fat).
Body fat percentage was measured using bioelectrical impedance analysis (BIA), a method that estimates fat content by passing a small electrical current through the body. Although older BIA devices were used, BF% still outperformed BMI as a predictor of mortality. Modern BIA tools offer improved accuracy and clinical practicality.
Only about 60% of participants were classified the same way by both BMI and BF%, highlighting that BMI frequently misclassifies individuals as healthy or unhealthy. This includes people with normal weight obesity who may face increased health risks despite having a normal BMI.
Waist circumference was also a strong predictor of mortality, but its clinical use can be challenging due to inconsistent measurement technique and anatonmical variability. In contrast, BF% provides a standardized, reproducible measurement that more directly reflects adiposity.
These findings support the use of body fat percentage in routine health assessments. As BIA technology becomes more accessible, it may help clinicians better identify patients at risk of cardiovascular disease and early mortality—especially those overlooked by BMI alone.
The authors reported no conflicts of interest.
Source: Annals of Family Medicine