Organ-Tissue Level Model of Resting Energy Expenditure Across Mammals: New Insights into Kleiber's Law.

Kleiber's law describes the quantitative association between whole-body resting energy expenditure (REE, in kcal/d) and body mass (M, in kg) across mature mammals as REE = 70.0 × M^0.75. The basis of this empirical function is uncertain. Objectives. The study objective was to establish an organ-tissue level REE model across mammals and to explore the body composition and physiologic basis of Kleiber's law. Design. We evaluated the hypothesis that REE in mature mammals can be predicted by a combination of two variables: the mass of individual organs/tissues and their corresponding specific resting metabolic rates. Data on the mass of organs with highmetabolic rate (i.e., liver, brain, heart, and kidneys) for 111 species ranging in bodymass from0.0075 (shrew) to 6650 kg (elephant) were obtained froma literature review. Results. REE_p predicted by the organtissue levelmodel was correlated with bodymass (correlation r = 0.9975) and resulted in the function REE_p = 66.33×M^0.754, with a coefficient and scaling exponent, respectively, close to 70.0 and 0.75 (P > 0.05) as observed by Kleiber. There were no differences between REE_p and REEk calculated by Kleiber's law; REE_p was correlated (r = 0.9994) with REEk. The mass-specific REE_p, that is, (REE/M)_p, was correlated with body mass (r = 0.9779) with a scaling exponent -0.246, close to -0.25 as observed with Kleiber's law. Conclusion. Our findings provide new insights into the organ/tissue energetic components of Kleiber's law. The observed large rise in REE and lowering of REE/M from shrew to elephant can be explained by corresponding changes in organ/tissue mass and associated specific metabolic rate. [ABSTRACT FROM AUTHOR]