Oversimplification of the Relationship between Ultrasound and Skinfold Measurements of Subcutaneous Fat Thickness

In the March 2021 issue of Medicine & Science in Sports & Exercise, two separate studies (1,2) from different research laboratories both used ultrasound to estimate body fat percentage (%BF). Both Chandler et al. (1) and Tinsley et al. (2) reported using a modification of the Jackson–Pollock (3) seven-site skinfold formula to convert their ultrasound fat thicknesses into estimates of body density and subsequently converted body density to %BF with the Siri (4) formula. Close inspection of their methods reveals that the only “modification” to the Jackson–Pollock equation was a doubling of the ultrasound fat thicknesses. Tinsley et al. provided a rationale for this by stating “values were doubled to reflect the values obtained by skinfold (i.e., a double layer of subcutaneous tissue)” (p. 661). Neither Chandler et al. (1) nor Tinsley et al. (2) provided any citation or corroborating evidence that an ultrasound fat thickness measurement is exactly half that of a skinfold measurement at a given site. I contend that this assumption is erroneous.