Contributions of animal nutrition research to nutritional principles: energetics.

Recognition of the parallels between animal life and flame provided the impetus to view life as combustion. Animal digestion and metabolism experiments revealed principles of nutrient sources of energy, the relation of chemical content to absorbed nutrient value, respiratory quotient, and biological value. Measurements of heat loss by animals revealed dramatic, >30-fold, differences in mass specific biological oxidation, leading to mass exponential descriptions of these metabolic rates, e.g., the Kleiber-Brody Law. More recent animal experiments have explored principles explaining the large mass specific rate variation, leading to principles of visceral organ mass primacy and the importance of ion pumping, proton leak, and membrane lipid composition as drivers of the variation.