Ultra Dual-Energy X-ray Absorptiometry (DXA): Accurate and Precise Quantitative Multi-compartment Body Composition

Correlation between total body water from deuterated water, versus total body water estimated from BIA. body water is directly reported by the BIA device.

The standard 4-component model combines 4 measures (weight, DXA bone mineral mass, body volume by ADP, and body water using deuterium) into one equation to estimate body fat. The proposed 4C-DXA method estimates pixel by pixel body fat in the DXA image by combining pixel level measures. These measures are the low- and high-energy X-ray absorption of the tissue visualized in the pixel, and the pixel thickness measured using 3D optical tissue thickness. When no bone is present, these three measures provide three estimates of tissue protein, fat, and water. When bone is present, the same measures used to estimate bone, fat, and lean mass where lean is the sum of water and protein masses. The protein/water ratio is estimated from nearest neighbor pixels that contain no bone. This results in a description of the fat, water, protein, and mineral masses of all pixels. Summing the pixels provides total fat. The correlation of the standard to the proposed estimates of body fat is the outcome.

Using the mesh points that make up the image data of 3D images, one can estimate body percent fat by treating all mesh points as variables and describing their shape variance using principal component analysis (PCA). The result of the PCA method is that it creates variables that describe the variance of all the mesh points. The PCA variables can then be used to estimate percent body fat. In this secondary outcome, we will compare the estimate of percent body fat using PCA variables to the percent body fat of the standard 4-compartment model.

In this secondary analysis we compare using correlation statistics the total body fat mass from the standard 4C model to that derived using a simplified whole body 4C model. In the simplified model, BIA-estimated body water is substituted for deuterium-dilution, and a derivation of body volume using DXA for ADP body volume. DXA can estimate body volume by first determining the total fat, lean, and bone reported for a participant, and then using the previously derived physical densities of fat, lean, and bone masses, estimate the total body volume. The singular comparison for this aim is the correlation of standard 4C model body fat to the simplified 4C model body fat.