Is the raised metabolic rate of the small for gestation infant due to his relatively large brain size

Oxygen consumption at different stages of vigilance, and for the whole 24 h, was measured in 13 small-for-gestational age (SGA) and 16 appropriate-for-age (AGA) premature infants at ages of 4-25 days. Brain weight was calculated from head circumference measurements and expressed as percentage of body weight and was found to be significantly higher in SGA infants than AGA ones (14% ± 2.1 (S.D.) in the SGA and 12.3% ± 1.5 (S.D.) in the AGA infants). Metabolic rate (MR) was calculated and expressed as kcal/centimetre head circumference, kcal/g brain wt. and kcal/kg body wt. Whereas MR expressed in kg body wt. was significantly higher in SGA infants than in AGA ones, no difference could be detected in the values when MR was expressed as kcal/cm head circumference, or MR/g brain wt. The difference that exists in the metabolic rates between SGA and AGA infants can be minimised by using head circumference as a reference. Therefore, the apparently high MR found in SGA infants in the neonatal period can be attributed, in a major way, to their relatively large brain size.