Lower metabolic rate in individuals heterozygous for either a frameshift or a functional missense MC4R variant

Humans with functional variants in the melanocortin 4 receptor (MC4R) are obese, hyperphagic, and hyperinsulinemic but have been reported to have no difference in energy expenditure.

We investigated the association of two MC4R variants, Arg165Gln (R165Q) and A insertion at nucleotide 100 (NT100), with adiposity in 3,074 full-heritage Pima Indians, a subset of whom had metabolic measures including 24-h energy expenditure (n = 252) and resting metabolic rate (RMR) (n = 364).

Among the 3,074 subjects, 43 were heterozygous for R165Q and 14 for NT100 (frequency = 0.007 and 0.002). Mean (+/- SD) BMI was higher among subjects with R165Q (39.3 +/- 8.6 kg/m(2)) or NT100 (41.2 +/- 7.8) than subjects without either variant (37.1 +/- 8.4) (P = 0.04 and 0.02, adjusted for age, sex, and birth year and accounting for family membership). The 24-h energy expenditure (four with NT100; three with R165Q) or RMR (six with NT100; two with R165Q) was lower in heterozygous subjects but only met statistical significance when heterozygous subjects were combined and compared with subjects without either variant: least-squares means, 2,163 kcal/24 h (95% CI 2,035-2,291) vs. 2,307 kcal/24 h (2,285-2,328), P = 0.03 for 24-h energy expenditure, and 1,617 kcal/24 h (1,499-1,734) vs. 1,754 kcal/24 h (1,736-1,772), P = 0.02 for RMR; adjusted for age, sex, fat-free mass, and fat mass). For RMR, this difference persisted, even after accounting for family membership.

Pima Indians heterozygous for R165Q or NT100 in MC4R have higher BMIs and lower energy expenditure (by approximately 140 kcal/day), indicating that lower energy expenditure was a component of the increased adiposity.