title: Larger mass of high-metabolic-rate organs does not explain higher resting energy expenditure in children.
abstract: BACKGROUND
  Children have a high resting energy expenditure (REE) relative to their body weight. The decline in REE during growth may be due to changes in body composition or to changes in the metabolic rate of individual organs and tissues.
  
  
  OBJECTIVE
  The goals were to quantify body-composition components in children at the organ-tissue level in vivo and to determine whether the observed masses 1) account for the elevated REE in children and 2) account, when combined with specific organ-tissue metabolic constants, for children's REE.
  
  
  METHODS
  This was a cross-sectional evaluation of 15 children (aged 9.3 +/- 1.7 y) and 13 young adults (aged 26.0 +/- 1.8 y) with body mass indexes (in kg/m(2)) < 30. Magnetic resonance imaging-derived in vivo measures of brain, liver, kidney, heart, skeletal muscle, and adipose tissue were acquired. REE was measured by indirect calorimetry (REE(m)). Previously published organ-tissue metabolic rate constants were used to calculate whole-body REE (REE(c)).
  
  
  RESULTS
  The proportion of adipose-tissue-free mass as liver (3.7 +/- 0.5% compared with 3.1 +/- 0.5%; P < 0.01) and brain (6.2 +/- 1.2% compared with 3.3 +/- 0.9%; P < 0.001) was significantly greater in children than in young adults. The addition of brain and liver mass significantly improved the model but did not eliminate the role of age. REE(c) with published metabolic coefficients underestimated REE(m) (REE(c) = 3869 +/- 615 kJ/d; REE(m) = 5119 +/- 769 kJ/d; P < 0.001) in children.
  
  
  CONCLUSIONS
  The decline in REE during growth is likely due to both a decrease in the proportion of some of the more metabolically active organs and tissues and changes in the metabolic rate of individual organs and tissues.
authors.affiliation: Institute of Human Nutrition, College of Physicians and Surgeons, Columbia University, New York, NY, USA.
authors.forename: Amy
authors.initials: A
authors.lastname: Hsu
authors.forename: Stanley
authors.initials: S
authors.lastname: Heshka
authors.forename: Isaac
authors.initials: I
authors.lastname: Janumala
authors.forename: Mi-Yeon
authors.initials: MY
authors.lastname: Song
authors.forename: Mary
authors.initials: M
authors.lastname: Horlick
authors.forename: Norman
authors.initials: N
authors.lastname: Krasnow
authors.forename: Dympna
authors.initials: D
authors.lastname: Gallagher
mesh_terms: D000293:Adolescent; D000328:Adult; D001823:Body Composition; D001921:Brain / Q000033:anatomy & histology* / Q000378:metabolism; D002648:Child; D003430:Cross-Sectional Studies; D004734:Energy Metabolism / Q000502:physiology*; D006801:Humans; D008099:Liver / Q000033:anatomy & histology* / Q000378:metabolism; D008279:Magnetic Resonance Imaging; D009929:Organ Size; D012146:Rest / Q000502:physiology*