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ARS Home » Plains Area » Houston, Texas » Children's Nutrition Research Center » Research » Publications at this Location » Publication #106829


item Ellis, Kenneth

Submitted to: Annals of the New York Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/1999
Publication Date: 5/1/2000
Citation: N/A

Interpretive Summary: Dual-energy X-ray absorptiometry (DXA) is a type of state-of-the-art technology that is used to assess a human being's body composition. DXA instruments offer adult and pediatric mode settings whose use is indicated by the size of the body. The instrument's guidelines suggest that one mode or the other be used based on the subject's weight. The pediatric mode is deemed appropriate for infants who weigh less than 10 kg, while the adult mode is supposed to be used for subjects greater than 15 kg. This leaves a 5-kg weight gap, in which a baby purportedly may be measured using either mode. We investigated this 5-kg gap by building three phantoms based on infant statistics, using fluid-filled plastic bags shaped to match a baby's body weighing 10, 12.5 and 15 kg. We inserted plastic bags inside the phantoms to represent bones; these bags were shaped like bones, and contained appropriate amounts of calcium for that age and weight. We then scanned the phantoms, using both the pediatric and adult mode, and obtaine estimates of bone mineral content and density, lean tissue mass and fat mass. The pediatric mode provided more accurate estimates of bone mineral content and density. However, total body mass was measured accurately by both modes. Further tests are needed to evaluate the estimates for the fat and lean tissue, but preliminary results indicate that the pediatric mode may overestimate the degree of fatness for children at these body weights. This information is extremely useful to researchers and doctors who deal with body composition aspects of health, especially the growing problem of obesity in children.

Technical Abstract: OBJECTIVE: To evaluate the accuracy of the Hologic QDR-2000 DXA for measuring infants weighing between 10-15 kg, using anthropomorphic phantoms of known chemical composition. METHODS: Phantoms were constructed, based on infant morphology, using sealed, fluid-filled, plastic bags shaped into cylinders to simulate various regions of an infant's body. Bones were simulated by known amounts of hydroxyapatite sealed in thin plastic cylinders of appropriate size. Fluid composition was of tissue-equivalent element concentrations, representing soft tissue. The assembled phantoms closely approximated the shape of a human infant. The phantoms were measured using both the pediatric scan mode (V5.71p) and adult scan mode (V5.71), yielding estimates of bone mineral content (BMC), fat, and lean tissue mass (LTM). We analyzed the accuracy of body composition results from both scan modes. RESULTS: The pediatric mode most accurately measured BMC at these body sizes; the results were within approximately 4% of expected values. The adult mode underestimated BMC by about 25%. However, total body mass was measured fairly accurately by both modes; the adult mode averaged about 1.5% less than predicted, while the pediatric mode overestimated total mass by about 2.5%. The pediatric mode also attributed more of the soft-tissue compartment to fat than did the adult mode, resulting in fat values that were elevated by about a factor of 2. CONCLUSION: The pediatric mode more accurately measured BMC than did the adult mode. Additional tests must be run to better evaluate the soft- tissue results, using phantoms with varying amounts of fat and lean composition. Preliminary findings indicate that the pediatric mode may overestimate the degree of true fatness for children at these body weights.