Background

We have recently proposed and validated a simple and accurate method to estimate whole-body fat percentage in adults, the RFM (Relative Fat Mass), calculated as follows: 64 − (20 X height/waist circumference) + (12 X sex); where sex equals 0 for men and 1 for women. Here we identified RFM cutoffs to diagnose obesity based on the association between RFM and all-cause mortality.

Methods

We used data from participants (≥20 years of age, n=43,793) of the National Health and Nutrition Examination Survey (NHANES) 1999−2014 linked with death certificate records from the National Death Index. Optimal RFM cutoffs were determined using receiver operating characteristic analysis (the Youden’s index and the Euclidean minimum distance to the coordinate (0,1)).

Results

Final dataset for analyses comprised 31,008 adults. During a median follow-up of 8.3 years (IQR, 7.6−13.7), 2,517 deaths occurred. Youden and Euclidean optimal cutoffs of baseline RFM for all-cause mortality were 40.8% and 41.6% for women, and 30.9% and 28.9% for men, respectively. Similar cutoffs were obtained using DXA-measured whole-body fat percentage. Adjusting for age, BMI category, ethnicity, education level, and smoking status, the hazard ratio for mortality using Cox proportional hazard regression was 1.41 (95% CI, 1.02−1.95) among women who had an RFM of 40.0%−44.9% compared with women who had an RFM <35% (P=0.035). Among men, the hazard ratio was 1.57 (95% CI, 1.07−2.30) among those with an RFM of 30.0%−34.9% compared with men who had an RFM <25% (P=0.020). Similar adjusted hazard ratios for same RFM categories were obtained in our validation population (NHANES III 1988−1994, n=12,650, median follow-up: 23.3 years): 1.42 (95% CI, 1.01−2.00) among women (P=0.043) and 1.50 (95% CI, 1.07−2.10) among men (P=0.021).

Conclusions

We suggest rounded RFM cutoffs of 40% body fat for women and 30% body fat for men to diagnose obesity and identify individuals at higher risk of all-cause mortality.