The whole body DXA exams in NHANES were acquired according to the procedures recommended by the manufacturer on a QDR 4500A fan beam densitometer (Hologic, Inc., Bedford, MA). All subjects changed into paper gowns and were asked to remove all jewelry and other personal effects that could interfere with the DXA exam. The DXA exams were reviewed and analyzed by the University of California, San Francisco Department of Radiology Bone Density Group using industry standard techniques. Analysis of all exams was performed using Hologic Discovery software version 12.1 in its default configuration. Exams that contained artifacts which could affect the accuracy of the DXA results, such as prosthetic devices, implants or other extraneous objects had the regional and global DXA results for these exams set to missing in the dataset. The precision of the DXA instrument used in the NHANES study has been reported on elsewhere [5] (link), [6] (link), [7] (link).
Body composition measurements are technology and calibration dependent and hence results provided by different instruments vary widely. The DXA instruments used in the NHANES survey employed the calibration proposed by Schoeller et al. [8] (link), whereby DXA lean mass results were calibrated to lean mass measured in 7 independent studies utilizing total body water (4 studies), hydrodensitometry (1 study), and four compartment measures (2 studies). The seven independent studies involved a total of 1195 subjects (602 male, 593 female). The BMD and BMC results were calibrated by the DXA manufacturer and maintained by an internal reference system that periodically measures bone and soft tissue equivalent reference standards during the patient measurement.
The NHANES data sets contained whole body DXA measurements of bone mineral content (BMC, g), areal bone mineral density (BMD, g/cm2), fat mass (g) and lean mass including BMC (g) and percent fat, calculated as (fat mass divided by total mass) ×100 along with demographic information for each subject. The above measurements were also available for a number of pre-defined anatomical regions, including the head, arms, legs, trunk, pelvic regions, sub-total whole body (excluding only the head) and whole body. From these whole body measures the following derivative values were calculated: FMI (fat mass/height2), lean mass/height2, appendicular lean mass/height2. For adults, only total body reference values and the above derivative reference values were generated. For children, (subjects less than 20 years of age), total body and sub-total body reference values and selected derivative reference values were generated.
There is increasing realization that fat distribution may be as important as total fat mass, so two indices of fat mass distribution, %fat of the trunk divided by %fat of the legs and fat mass of the trunk divided by fat mass of the limbs (fat mass of arms plus legs) were included in this analysis for adults. These indices may have a role in defining metabolic syndrome or lipodystorphy [9] (link), [10] (link).
Body composition measurements are technology and calibration dependent and hence results provided by different instruments vary widely. The DXA instruments used in the NHANES survey employed the calibration proposed by Schoeller et al. [8] (link), whereby DXA lean mass results were calibrated to lean mass measured in 7 independent studies utilizing total body water (4 studies), hydrodensitometry (1 study), and four compartment measures (2 studies). The seven independent studies involved a total of 1195 subjects (602 male, 593 female). The BMD and BMC results were calibrated by the DXA manufacturer and maintained by an internal reference system that periodically measures bone and soft tissue equivalent reference standards during the patient measurement.
The NHANES data sets contained whole body DXA measurements of bone mineral content (BMC, g), areal bone mineral density (BMD, g/cm2), fat mass (g) and lean mass including BMC (g) and percent fat, calculated as (fat mass divided by total mass) ×100 along with demographic information for each subject. The above measurements were also available for a number of pre-defined anatomical regions, including the head, arms, legs, trunk, pelvic regions, sub-total whole body (excluding only the head) and whole body. From these whole body measures the following derivative values were calculated: FMI (fat mass/height2), lean mass/height2, appendicular lean mass/height2. For adults, only total body reference values and the above derivative reference values were generated. For children, (subjects less than 20 years of age), total body and sub-total body reference values and selected derivative reference values were generated.
There is increasing realization that fat distribution may be as important as total fat mass, so two indices of fat mass distribution, %fat of the trunk divided by %fat of the legs and fat mass of the trunk divided by fat mass of the limbs (fat mass of arms plus legs) were included in this analysis for adults. These indices may have a role in defining metabolic syndrome or lipodystorphy [9] (link), [10] (link).
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