Influence of postnatal age, energy intake, and weight gain on energy metabolism in the very low-birth-weight infant.

The relative importance and interrelationship of postnatal age, energy intake, and weight gain on metabolic rate is evaluated in 28 studies in 13 formula-fed very low-birth-weight AGA infants. The relationships between metabolic rate, energy intake, weight gain, and age all follow a similar pattern, increasing in the first two weeks of life and subsequently stabilizing. Significant linear correlations are demonstrated between metabolic rate and both energy intake (r = 0.88, P less than 0.001) and weight gain (r = 0.86, P less than 0.001). For each gram of weight gain, 0.67 kcal (2.8 kj) are expended in addition to the maintenance energy requirement of 51 kcal/kg/day. The increase in metabolic rate in the early postnatal period appears to be a consequence of the energy cost of tissue synthesis. Changes in metabolic rate with postnatal age are modulated by increasing energy intake and weight gain.