Objective: Sleeping metabolic rate (SMR) in 12-week-old infants was adjusted for body size and body composition and then compared between the sexes and between breast-fed and formula-fed infants.
Design: A cohort of 50 infants was studied at 12 weeks. SMR was measured by indirect calorimetry, and body size and body composition by anthropometry and a stable isotope technique. The effects of body size and body composition on SMR were removed to permit comparison between groups.
Results: Mass independent SMR differed between the diet groups when expressed in terms of body weight, but this difference could be explained by differences in fat free mass. The SMR of male infants was 386 +/- 31 kcal/d, and that of female infants was 341 +/- 31 kcal/d: P < 0.0001. The sexes differed whether body weight or fat-free mass was considered. Multiple regression analysis showed that the effect of sex was stronger than that of diet.
Conclusions: There is a relationship between SMR and infant sex and diet even after body size has been taken into account. The relationship could be explained by body composition for diet but not for sex. These differences are equivalent to differences in heat production and may have significance for infant health.