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Adults

□ Liver □ Brain □ Heart ■ Kidneys □ SM □ Residual

Figure 4 Proportional contribution of each organ/tissue to Adipose Tissue Free Mass (ATFM). Liver (i i). brain (□), heart (□), kidneys (h), skeletal muscle mass (i i), residual mass (□), * p <0.01 and ** p <0.001 for children vs. adults. Reproduced with permission from Hsu A, Heshka S, Janumala I, Song MY, Horlick M, Krasnow N, and Gallagher D (2003) Larger mass of high-metabolic-rate organs does not explain higher resting energy expenditure in children. American Journal of Clinical Nutrition 77: 1506-11.

high metabolic rate organs, specifically liver and brain, is greater in children compared to young adults (Figure 4). Findings thus far have shown that after accounting for this disproportion, the specific organ/tissue metabolic constants available in the literature (Table 3) are not adequate to account for REE in children. These results therefore imply that the decline in REE per kilogram body weight (or per kilogram FFM) during the growth years is likely due to both changes in body composition and changes in the metabolic rate of individual organs/tissues. When this approach was applied to young adults (31.2 ± 7.2 years), REEc and REEm were highly correlated, with no significant differences between them. When this approach was applied to persons over 70 years, both older men and women had significantly lower REEm compared to REEc, and the magnitude of the differences were 13% and 9.5%, respectively, for men and women. These findings suggest that even after adjustment for age-related organ and tissue atrophy in the elderly, whole body REE by indirect calorimetry continues to be lower than expected. The latter suggests that the metabolic rate constants used (Table 3) for specific organs and tissues may not be appropriate in the elderly.

At the individual or clinic level, the measurement of REE by indirect calorimetry is frequently unavailable. An alternate approach has been to estimate REE

based on body weight, height, age, and sex. Many studies have examined the association between these basic and easily acquired measures and REE. A small number of studies have included FFM in their REE prediction equations. Table 4 lists published equations for the prediction of REE in healthy individuals.

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