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.
The relative efficiency of dietary sucrose, protein sources, and fats in depositing body protein and fat (total energy) was directly estimated in young rats by feeding graded levels of each as supplements to a fixed amount of a basal diet that was presumably adequate in all essential nutrients except for energy. Under these conditions, the net gain in total body energy was a linear function of the amount of supplement added and the data fulfill the criteria of a valid slope-ratio bioassay. The available energy measured by this technique for sucrose and protein were similar, as would be expected. Dietary Rice et al. (9) and that the available energy from dietary fat under these conditions compared to sucrose is substantially larger than predicted from the Atwater values. METHODSThe design of the four experiments reported was similar to that proposed by Rice et al. (9). The three basal diets used (Table 1) were formulated to provide all of the essential nutrients when fed in limiting amounts, so the added energy supplement would be the only variable to affect the rates of weight gain and change in body composition. The weanling male rats of the Charles River strain (Charles River Breeding Laboratories) used in the four experiments received daily 5 g of one of the basal diets to which a fixed amount of the energy supplement under study was added.In each experiment, groups of animals received graded levels of sucrose. The response of these animals was compared to those that received fat (Spry, Lever Brothers), casein (General Biochemicals), or gluten (General Biochemicals). The amounts of the various supplements provided in the four different experiments are shown in the tables.After the weanling rats were received in the laboratory, they were fed a laboratory rat chow for 3 days and 5 g of the basal diet for the next 3 days. They were then divided into groups of similar mean weight. Each group in experiments 1 and 2 contained five animals; there were four animals per group in experiments 3 and 4. In each experiment, one group of animals was killed for analysis of body composition. The remaining animals received the appropriate diet for 21 days and were then killed for analysis. § The animals were housed in raised individual cages, kept at a constant temperature and humidity, and weighed two or three times weekly. Papers were placed under the cages so that any spilled food could be weighed and an equivalent amount added to the diet in the next few days. Fortunately, spillage was insignificant.When the animals were killed, the stomach and cecum were removed and discarded to avoid residual food, and the
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