Efficiency of utilization of various sources of energy for 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 fat, however, was a more efficient source of energy. Compared to sucrose with 3.94 kcal/g (1 cal = 4.184 J), the average potency of dietary fat was 11.1 kcal/g, or approximately 124% of the expected value of 9 kcal/g. Fat supplements increased the deposition of body fat even when total energy intake was severely limited. The Atwater value of 9 kcal per g of fat is not appropriate under these conditions and probably not under other conditions.