Larger mass of high-metabolic-rate organs does not explain higher resting energy expenditure in children.
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.
Hsu
Amy
A
Institute of Human Nutrition, College of Physicians and Surgeons, Columbia University, New York, NY, USA.
Heshka
Stanley
S
Janumala
Isaac
I
Song
Mi-Yeon
MY
Horlick
Mary
M
Krasnow
Norman
N
Gallagher
Dympna
D
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*