Relative Fat Mass as an Estimator of Abdominal Adiposity in Youth Across the BMI Spectrum from Normal Weight to Obesity

Background Relative fat mass (RFM) has been used to estimate whole‐body fat percentage (%BF). However, whether RFM can mirror abdominal adiposity is unknown. Objective In youth, we examined if RFM reflects abdominal adiposity, if there are age/race/sex differences in the relationships between RFM and abdominal adiposity, and if our present‐youth‐cohort‐derived RFM (RFM‐Y) improves the prediction of %BF and abdominal adiposity. Methods In 358 youth (aged 10–19 years; 168 black/198 white) with normal weight and overweight/obesity (without secondary or syndromic obesity), visceral (VAT), subcutaneous (SAT) and total abdominal adipose tissue (TAT) were assessed by MRI/CT and %BF with DXA. RFM was calculated for ages 10–14 years as [74 − (22 × height/waist circumference [WC]) + (5 × sex)] and 15–19 years as [64 − (20 × height/WC) + (12 × sex)]; sex = 0 for boys, 1 for girls. To develop and validate RFM‐Y, linear regression, concordance correlation coefficient, Bland–Altman plots and intraclass correlation coefficient were conducted. Results RFM associated with VAT, SAT and TAT ( r = 0.66, 0.79, 0.79, respectively; all p < 0.001). The slope was significantly greater in (a) younger versus older youth for VAT, SAT and TAT; (b) whites versus blacks for VAT; and (c) girls versus boys for SAT and TAT. RFM‐Y versus traditional RFM showed improved predictive ability for %BF ( R 2 = 0.83 vs. 0.77) and abdominal adiposity ( R 2 ranging from 0.51 to 0.80 vs. 0.37 to 0.63). Conclusions RFM reflects abdominal adiposity besides %BF and could be used in longitudinal/interventional youth trials, obviating the need for expensive imaging to assess changes in total and abdominal adiposity.