Temperature/Energy Regulation

The preterm neonate’s energy expenditure can be minimized by keeping the infant in a thermoneutral environment. The thermoneutral environment is a range of ambient temperatures within which the metabolic rate of the infant is minimal and the infant can maintain a normal body temperature without any regulatory changes in metabolic heat production or evaporative heat loss. The thermoneutral range varies with gestational age. In an unclothed resting adult, the lower limit of the thermoneutral range is 78.8°F to 82.4°F (26°C to 28°C) in an environment of 50% relative humidity and still air. Under similar conditions, the lower limit of the thermoneutral range is 89.6°F (32°C) or higher in a naked term neonate and 95°F (35°C) or higher in a naked preterm neonate.

Diet-induced thermogenesis, also known as specific dynamic action, thermic effect of food, or postprandial thermogenesis, is the increase in metabolic rate that follows food intake. It represents the energy consumption necessary for digestion, absorption, and assimilation of nutrients. The magnitude of increase in energy expenditure following the ingestion of nutrients is estimated to vary between 4% and 30% in both term and preterm neonates. Diet-induced thermogenesis is lower with continuous than with intermittent enteral feeding.

Metabolic cost of growth represents the energy required for the formation of new tissue, and it varies with the composition of the synthesized tissue. The cost of depositing absorbed dietary fat into adipose tissue is much less than that of synthesizing new protein. The overall metabolic cost of growth in neonates is estimated at approximately 4.4 kcal/g of weight gain.

Energy expenditure increases with physical activity, but because neonates sleep 80% to 90% of the time, physical activity is a small component of their energy expenditure compared with that of adults. It is estimated that physical activity contributes to only about 10% of the total energy expenditure in preterm neonates.

Resting metabolic rate increases steadily from birth in both term and preterm neonates. The resting metabolic rate in term neonates is estimated at approximately 40 kcal/kg per day at 3 days of postnatal age, increasing to approximately 60 kcal/kg per day at 3 months of postnatal age. A similar but smaller increase in resting metabolic rate is observed in preterm neonates.

References:
Bruck K. Neonatal thermal regulation. In: Polin RA, Fox WW, eds. Fetal and Neonatal Physiology. 2nd ed. Philadelphia, Pa: WB Saunders Co; 1998:676-702
Leitch CA, Denne SC. Energy expenditure in the extremely low-birth weight infant. Clin Perinatol. 2000;27:181-195