Determinants of arm muscle and fat accretion during the first postnatal month in preterm newborn infants

We measured arm muscle and fat areas in 22 preterm appropriate for gestational age infants at birth (mean ± 1 SD birth weight: 1,640 ± 484 g; gestational age: 31 ± 2 weeks). Birth arm muscle and fat areas correlated significantly with gestational age (arm muscle: R = 0.86; p < 0.001; arm fat: r = 0.75; p < 0.001) and with birth weight. Deviations of birth weights from gestational age means (birth weight z-scores) were related more to variations in arm muscle area (r = 0.69; p < 0.001) rather than arm fat area (r = 0.44; p = 0.04). Sixteen infants were followed over 4 weeks. They were most physiologically unstable (mean Physiologic Stability Index score = 5.3 ± 3.5) during the first postnatal week when they also all lost weight. Their mean arm muscle area decreased significantly during the first week by >10%, whereas the mean arm fat area remained unchanged. First week arm muscle losses were directly correlated with the lack of protein intake (r, 0.52; p < 0.05). The regression equation predicted a protein intake of 4.06 g/kg/day (95% con1, accompanied by a significant reduction in physiologic instability (PSI score = 1.9 ± 1.9; p < 0.001). Over the subsequent 3 weeks, arm muscle area increased more (138 ± 58 mm²) than arm fat area (59 ± 36 mm²; p < 0.001) and weekly rates of arm muscle and fat accretion became more dependent on energy intake and postnatal age, rather than protein intake or postconceptional age. Infants fed high-fat, low-protein own mother’s milk (n = 7) did not have more fat accretion or less muscle accretion than infants fed significantly lower fat and higher protein content formulas (n = 9). If extrapolatable to total body stores, this study demonstrates that the major determinants of muscle and fat accretion in preterm infants are different at the time of birth, during periods of physiologic instability, and during the early period of postnatal growth.