Weight gain composition in preterm infants with dual energy X-ray absorptiometry

Whole body composition was investigated using dual energy x-ray absorptiome try in 54 healthy preterm infants, birth weight < 1750 g, who were fed fort ified human milk (n = 20) and preterm formula (n = 34) when full enteral fe eding was attained and then again 3 wk later at around the time of discharg e. Weight gain composition was calculated from the difference between the e arlier and later measurement. The minimal detectable changes in whole body composition over time according to the variance of the population (within g roups of 20 infants) and the minimal detectable changes according to the di etary intervention (between two groups of 20 infants) were determined at 5% significance and 80% power. Whole body composition was similar in the two groups at the initial measurement, but all the measured variables differed at the time of the second measurement. Formula-fed infants showed a greater weight gain (19.9 +/- 3.2 versus 15.9 +/- 2.2 g.kg(-1).d(-1)., p < 0.05), fat mass deposition (5.1 +/- 1.9 versus 3.3 + 1 +/- 1.3 g.kg(-1).d(-1), p < 0.05), bone mineral content gain (289 +/- 99 versus 214 +/- 64 mg.kg(-1.)d (-1), p <0.05), and increase in bone area (1.6 +/- 0.4 versus 1.3 +/- 0.3 c m(2) kg(-1).d(-1), p < 0.05) compared with the fortified human milk group. From these data, a minimal increase from the first measurement of 111 g lea n body mass, 68 g fat mass, and 3.1 g bone mineral content is needed to be detectable in a longitudinal study that includes 20 infants. For significan ce between two groups of 20 infants around the time of discharge, dietary i ntervention needs to achieve minimal differences of 160 g lean body mass, 8 6 g fat mass, and 4.1 g bone mineral content. With respect to weight gain c omposition, the minimal differences required to reach significance are 2.1 g.kg(-1).d(-1) for gain in lean body mass, 1.2 g.kg(-1).d- for gain in fat mass, and 76 mg.kg(-1).d(-1) for gain in bone mineral content. We conclude that dual energy x-ray absorptiometry allows evaluation of the effects of d ietary intervention on whole body and weight gain composition in preterm in fants during the first weeks of life.