The partition of maintenance energy expenditure and the pattern of substrate utilization in intrauterine malnourisheed newborn infants before and during recovery.

Total heat production and its major components, as well as the pattern of substrate utilization were measured by indirect calorimetry during a period of several hours in 15 small-for-gestational-age infants between the 2nd and 15th postnatal day (non-growing period) and between 16th and 39th day (growing period). 1. Mean total heat production of the non-growing infants did not exceed 50 Cal/kg/24 hrs. Mean basal metabolism in this period was not higher than that reported for premature infants of similar body size and postnatal age. It is concluded that the caloric input necessary to meet the cost of maintenance was less than that calculated from the components of energy metabolism. 2. Total energy expenditure was significantly higher in the growing period, and its partitioning showed that all the three components (basal-, resting-, and activity metabolism) were contributing to the increment. 3. The increase in total heat production, as well as that of the components, was closely related to the difference in age and body weight at the first and second examination. The individual increment in heat production during growth was determined chiefly by postnatal age and weight gain. 4. Carbohydrate utilization was the dominant component of the total energy expended by the non-growing infants. During recovery, a further rise occurred in carbohydrate oxidation, while there was a moderate decrease in the participation of fat and protein.