Epidemiology and Population Health

Defining cutoffs to diagnose obesity using the relative fat mass (RFM): Association with mortality in NHANES 1999–2014

Abstract

Background/objectives

We have recently proposed and validated a simple and accurate method to estimate whole-body fat percentage in adults, the relative fat mass (RFM), derived from the ratio of height to waist circumference. We aimed to identify RFM cutoffs to diagnose obesity based on the association between RFM and all-cause mortality.

Subjects/methods

We used data from adult 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 measured whole-body fat percentage by dual energy X-ray absorptiometry. 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, 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% for women and 30% for men to diagnose obesity and identify individuals at higher risk of death.

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Fig. 1
Fig. 2: Discriminative accuracy of RFM for obesity.
Fig. 3: Adjusted hazard ratio (HR) for mortality by RFM category among women and men.

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Acknowledgements

We thank the Centers for Disease Control and Prevention (CDC) and the National Center for Health Statistics (NCHS) for providing access to the NHANES datasets. This study was self-funded. The author’s responsibilities were as follows: OOW designed the research, conducted the research, performed the statistical analysis, and wrote the paper. RNB contributed with the design of the study and revised the final draft. OOW takes full responsibility for the work as a whole.

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Correspondence to Orison O. Woolcott.

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Woolcott, O.O., Bergman, R.N. Defining cutoffs to diagnose obesity using the relative fat mass (RFM): Association with mortality in NHANES 1999–2014. Int J Obes (2020). https://doi.org/10.1038/s41366-019-0516-8

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