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The reproducibility, accuracy, and precision of dual-energy X-ray absorptiometry (DXA) was assessed by scanning 13 piglets (1471-5507 g) in triplicate. In four piglets, fat content was increased with porcine lard around the abdomen; additional measurements were performed on these animals. Reproducibility in DXA measurements from the animals without added fat was 0.09% for body weight, 1.95% for bone mineral content (BMC), and 5.35% for fat content. DXA estimates of body weight, BMC, and fat content were significantly correlated with scale body weight, ash weight, chemical calcium, and chemical fat. Body weight was measured accurately but fat content was overestimated by DXA. Mean BMC estimated by DXA represented 48% of ash weight and 215% of calcium content. The precision of DXA was 0.23% for body weight, 10.99% for ash weight, and 4.44% for calcium content. The precision of DXA for fat content was poor. However, for measurements performed in piglets with > 250 g fat, the precision was 8.85%. Thirty appropriate-forgestational-age term human neonates (birth weight: 3188 +/- 217 g) were scanned once during the first week of life. BMC and fat content were 54 +/- 6 and 470 +/- 92 g, respectively, which corresponded to 26.4 +/- 2.6 g calcium and 427 +/- 82 g fat. These were close to the reference values previously determined by chemical analysis. This study suggests that DXA is accurate and reliable for measurement of calcium and fat contents in human neonates. Further refinements would be beneficial for determining fat content in preterm human infants.

Net calcium absorption was evaluated in 103 low-birth-weight preterm infants by a 72-h balance technique. At birth the infants had a mean (+/- SE) gestational age of 30.9 +/- 0.2 wk and weighed 1.43 +/- 0.03 kg. When tested 3 wk later, their net calcium absorption averaged 58 +/- 1% with an intake of 80 +/- 2 mg Ca.kg body wt-1.d-1. Of the 103 infants, 58 had been fed low-birth-weight formulas supplemented with vitamin D. The remainder received banked human milk, of whom 34 were supplemented with vitamin D and calcium; 11 infants received no supplementation. Calcium absorption in the four subgroups did not differ significantly, with neither vitamin D supplementation nor supplementation with vitamin D and calcium affecting percent absorption significantly. Net calcium absorption was a linear function of intake (40-130 mg Ca.kg body wt-1.d-1) with a zero intercept. Because vitamin D supplementation did not increase net calcium absorption, it is concluded that in preterm low-birth-weight infants calcium absorption proceeds by a nonsaturable route, with the transcellular, vitamin D-regulated mechanism not yet expressed.