Dual-energy X-ray absorptiometry: fat estimation errors due to variation in soft tissue hydration

Abstract

Dual-energy X-ray absorptiometry (DXA) is rapidly gaining acceptance as a reference method for analyzing body composition. An important and unresolved concern is whether and to what extent variation in soft tissue hydration causes errors in DXA fat estimates. The present study aim was to develop and validate a DXA physical hydration model and then to apply this model by simulating errors arising from hypothetical overhydration states. The DXA physical hydration model was developed by first linking biological substance elemental content with photon attenuation. The validated physical model was next extended to describe photon attenuation changes anticipated when predefined amounts of two known composition components are mixed, as would occur when overhydration develops. Two overhydration models were developed in the last phase of study, formulated on validated physical models, and error was simulated for fluid surfeit states. Results indicate that systematic errors in DXA percent fat arise with added fluids when fractional masses are varied as a percentage of combined fluid + soft tissue mass. Three independent determinants of error magnitude were established: elemental content of overhydration fluid, fraction of combined fluid + soft tissue as overhydration fluid, and initial soft tissue composition. Small but systematic and predictable errors in DXA soft tissue composition analysis thus can arise with fluid balance changes.

Validation of dual energy X-ray absorptiometry (DXA) by comparison with chemical analysis of dogs and cats

“…Other studies point to an important role for tissue hydration in the error in estimates of fat content by DXA. 23,30 In particular, Fusch et al 23 found that tissue hydration declined as a function of increasing body mass in their experimental subjects, and this decrease in hydration was linked to an increasing underestimate of the fat content of the animals. Although Going et al 31 manipulated hydration status and found no effect on fat content by DXA, in a sample of 19 adults, the extent of their manipulations (less than 2%) was small relative to the individual variability in tissue hydration, and in the range where our own in vitro manipulations detected no effect (Figure 3b).…”

Validation of DXA Body Composition Estimates in Obese Men and Women

“…FFM hydration was not significantly correlated (R 2 = 0.157: P = 0.16) with 4C/DXA %BF differences in our obese subjects. van der Ploeg et al regarded the lack of association between variation in FFM hydration and %BF error as understandable given the theoretical calculations of Pietrobelli et al (20). The latter determined that hydration changes of 1-5% would only lead to small DXA errors (<1%).…”

Prediction of DXA-determined whole body fat from skinfolds: importance of including skinfolds from the thigh and calf in young, healthy men and women

“…DXA has been validated against various multicompartment models in young (Prior et al, 1997;Clasey et al, 1999), old (Clasey et al, 1999) and a wide age range of healthy sedentary individuals (Gallagher et al, 2000). Nevertheless, the validity of the method has remained subject to question, particularly with regard to concerns over tissue thickness and hydration levels (Laskey et al, 1992;Jebb et al, 1995;Pietrobelli et al, 1996Pietrobelli et al, , 1998Wang et al, 1998;van der Ploeg et al, 2003), which will vary between individuals and groups of subjects.…”