Sex differences in resting energy expenditure and their relation to insulin resistance in children (EarlyBird 13)

Background: Insulin resistance is believed to be the process underlying type 2 diabetes and premature cardiovascular disease. We have established that a relation between body mass and insulin resistance calculated by homeostasis model assessment (HOMA-IR) exists by 5 y of age in contemporary UK children. Resting energy expenditure (REE) is variable among individuals and is one of many factors controlling body mass. Objective: The objective was to investigate the relations between REE, body mass, and HOMA-IR in young children. Design: EarlyBird is a nonintervention prospective cohort study of 307 healthy 5-y-olds that asks the question: Which children develop insulin resistance and why? REE by indirect calorimetry and HOMA-IR were measured in addition to total body mass, fat-free mass (FFM) by bioimpedance, body mass index (BMI; in kg/m2), and skinfold thickness when the mean age of the cohort was 5.9 ± 0.2 y. Results: Whereas the BMI of the boys was lower than that of the girls (mean ± SD: boys, 15.9 ± 1.9; girls, 16.5 ± 1.9; P = 0.03), their REE was higher by 6% (mean ± SD: 4724 ± 615 compared with 4469 ± 531 kJ/d; P = 0.002). This difference persisted after adjustment for FFM and other anthropometric variables (P = 0.04). In boys, there was a weak, although significant, inverse correlation between REE and HOMA-IR, independent of fat mass and FFM (boys: r = -0.21, P = 0.03; girls: r = 0.12, P = 0.34). Conclusion: There is a sex difference in REE at 6 y of age that cannot be explained by body composition. The difference appears to be intrinsic, and its contribution to sex differences in adiposity and HOMA-IR in children merits further exploration.