Specific metabolic rates of major organs and tissues across adulthood. evaluation by mechanistic model of resting energy expenditure

Background: The specific resting metabolic rates ([K.sub.i]; in kcal x [kg.sup.-1] x [d.sup.-1]) of major organs and tissues in adults were suggested by Elia (in Energy metabolism: tissue determinants and cellular corollaries. New York, NY: Raven Press, 1992) to be as follows: 200 for liver, 240 for brain, 440 for heart and kidneys, 13 for skeletal muscle, 4.5 for adipose tissue, and 12 for residual organs and tissues. However, Ella's [K.sub.i] values have never been fully evaluated. Objectives: The objectives of the present study were to evaluate the applicability of Elia's [K.sub.i] values across adulthood and to explore the potential influence of age on the [K.sub.i] values. Design: A new approach was developed to evaluate the [K.sub.i] values of major organs and tissues on the basis of a mechanistic model: REE = [summation]([K.sub.i] x [T.sub.i]), where REE is whole-body resting energy expenditure measured by indirect calorimetry, and [T.sub.i] is the mass of individual organs and tissues measured by magnetic resonance imaging. With measured REE and [T.sub.i], marginal 95% CIs for [K.sub.i] values were calculated by stepwise univariate regression analysis. An existing database of nonobese, healthy adults [n = 131; body mass index (in kg/[m.sup.2]) 50 y (n = 37). Results: Elia's [K.sub.i] values were within the range of 95% CIs in the young and middle-age groups. However, Ella's [K.sub.i] values were outside the fight boundaries of 95% CIs in the >50-y group, which indicated that Ella's study overestimated [K.sub.i] values by 3% in this group. Age-adjusted [K.sub.i] values for adults aged >50 y were 194 for liver, 233 for brain, 426 for heart and kidneys, 12.6 for skeletal muscle, 4.4 for adipose tissue, and 11.6 for residuals. Conclusion: The general applicability of Elia's [K.sub.i] values was validated across adulthood, although age adjustment is appropriate for specific applications. Am J Clin Nutr 2010;92:1369-77. doi: 10.3945/ajcn.2010.29885.