[Adaptation of energy metabolism in under-fed ruminants. Quantification in the whole animal and body tissues].

Ruminants respond to underfeeding by a progressive adaptation until a new equilibrium between dietary supply and requirements is reached. The resulting drop in maintenance energy requirements originates from a decrease in basal metabolism that depends on the level and duration of undernutrition. These modifications are not totally accounted for by changes in live weight but might be explained by differential changes in individual tissue weight and metabolism. Tissues contribute to whole animal expenditure in a disproportionate manner relative to their weight. Thus digestive tract, liver and muscle masses contribute 5-11, 1-2.5 and 35-50% to live weight but about 16-29, 17-31 and about 16% to whole animal energy expenditure, respectively, because of differences in tissue metabolic activity. Underfeeding results in a decrease in the contribution of viscera to energy expenditure, whereas results on muscle mass are scarce. These changes in contribution are due to changes in tissue mass, nutrient supply (blood flow) and/or in metabolic activity.