Body-size dependence of resting energy expenditure can be attributed to
nonenergetic homogeneity of fat-free mass.
Heymsfield, Steven B., Dympna Gallagher, Donald P. Kotler, Zimian Wang, David B.
Allison, and Stanley Heshka.
New York Obesity Research Center, St. Luke's-Roosevelt Hospital, Institute of
Human Nutrition, Columbia University, College of Physicians and Surgeons, New York,
New York 10025
APStracts 8:0205E, 2001.
An enduring enigma is why the ratio of resting energy expenditure (REE) to
metabolically active tissue mass, expressed as the REE/fat-free mass (FFM) ratio, is
greater in magnitude in subjects with a small FFM than it is in subjects with a large FFM.
This study tested the hypothesis that a higher REE/FFM ratio in subjects with a small
body mass and FFM can be explained by a larger proportion of FFM as high-metabolic-
rate tissues compared with that observed in heavier subjects. REE was measured by
indirect calorimetry, FFM by dual-energy X-ray absorptiometry (DEXA), and
tissue/organ contributions to FFM by whole body magnetic resonance imaging (MRI) in
healthy adults. Four tissue heat-producing contributions to FFM were evaluated, low-
metabolic-rate fat-free adipose tissue (18.8 kJ/kg), skeletal muscle (54.4 kJ/kg), and bone
(9.6 kJ/kg); and high-metabolic-rate residual mass (225.9 kJ/kg). Initial evaluations in
130 men and 159 women provided strong support for two key, developed models, one
linking DEXA FFM with MRI FFM estimates and the other linking REE predicted from
the four MRI-derived components with measured REE. There was an inverse association
observed between measured REE/FFM and FFM (r2 = 0.17, P < 0.001). Allometric
models revealed a similar pattern of tissue change relative to body mass across males and
females with greater proportional increases in fat-free adipose tissue and skeletal muscle
than in FFM and a smaller proportional increase in residual mass than in FFM. When
examined as a function of FFM, positive slopes were observed for skeletal muscle/FFM
and pooled low-metabolic-rate components, and a negative slope for residual mass. Our
linked REE-body composition models and associations strongly support the hypothesis
that FFM varies systematically in the proportion of thermogenic components as a
function of body mass and FFM. These observations have important implications for the
interpretation of between-individual differences in REE expressed relative to
metabolically active tissue mass.
Received 9 March 2001; accepted in final form 17 August 2001
APS Manuscript Number E112-1.
Article publication pending Am J Physiol Endocrinol Metab
ISSN 1080-4757 Copyright 2001 The American Physiological Society.
Published in APStracts on 22 October 2001