Efficiency and utilization of various energy sources for growth.

The efficiency of energy utilization by animals during growth is influenced by a number of factors including composition of the diet and sex and strain of the animal. Net energy efficiency is "the proportion of total energy intake [that] is not lost in the stool, urine, or as diet-induced heat losses (specific dynamic action) and, thus, is available for obligate energy demands (basal metabolic rate), for physical work, and for storage (or growth)". In studies reporting an increased efficiency in the utilization of energy from fat than from equal amounts of energy from carbohydrate, this increased efficiency is indicated by less energy lost by the animal as heat and more energy retained in the carcass of the animal as fat. Because the energy yield from nutrients and the efficiency of energy utilization appear to differ under various conditions, energy intakes using the Atwater values cannot be assumed to be physiologically equivalent in providing energy to the animal during growth.