Relative fat mass as an estimator of body fat percentage in Chilean adults

The Relative Fat Mass (RFM) is an alternative index to body mass index (BMI) for estimating whole body fat percentage (BF%). Our aims were to determine the accuracy of the RFM for 1) identifying individuals with elevated BF% and, 2) estimating the BF% compared to Dual-energy X-ray absorptiometry (DXA) in a Chilean adult population.

Body composition was assessed by DXA in 270 healthy participants (125 women/145 men). Anthropometric measurements were assessed to calculate RFM and BMI. Receiver operating characteristic (ROC) curves were obtained to assess the sensitivity and specificity of both, RFM and BMI. Bland-Altman analysis between BF% measured by DXA vs. predicted BF% derived from RFM was performed to assess validity. Pearson´s correlation coefficients to analyze the association between BMI, RFM and DXA were also calculated.

For RFM, the cut-off for elevated BF% was ≥22.7% for men and ≥32.4% for women and for BMI was ≥24.4 kg/m² for men and ≥24.1 kg/m² for women. The area under the ROC curve between RFM and BMI was not significantly different in men (0.970 vs. 0.959; p = 0.420) and women (0.946 vs. 0.942, p = 0.750). The Bland-Altman analysis showed that the estimation bias is more pronounced in men than in women.

RFM is an accurate tool for identifying individuals with elevated BF%, although it was not as accurate as DXA for estimating the BF%. RFM may be an alternative method useful in primary care to select individuals for lifestyle counseling and in research to select patients for epidemiological studies.