Food and Energy Intake of Rats Fed Diets Varying in Energy Concentration and Density

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Abstract
Ability to regulate level of energy intake and a validity of including a dietary measure of caloric density (kcal/ml) in predicting voluntary intake of diets were tested in the weanling and growing rat. Weanling rats were found to decrease energy intake significantly from 243 kcal of digestible energy/Wkg0.75 at 33 days of age to 186 kcal/Wkg0.75 at 52 days of age. Increased voluntary consumption was adequate on a basal commercial diet diluted 0, 10, 20 or 30%, with either kaolin or cellulose, to maintain energy intake at periods tested between 28 and 55 days of age. In a second experiment more severe dilutions, 0, 20, 30, 35, 40, 45, 50 and 55% by weight were made with kaolin, cellulose and perlite. These 22 diets were each fed to four rats continually from 30 to 138 days of age. Actual digestible energy (kcal/g) and density (g/ml by water displacement) were determined for each diet. Caloric density as kilocalories/milliliter was found to be a better predictor of digestible energy intake and net body weight gain than digestible energy per unit weight of diet alone (r = 0.95 vs. 0.90). Both measures of nutritive value (kcal/g and kcal/ml) gave highly significant correlation coefficients within diluents, but the regression coefficients between diluents were significantly different using kcal/g and not significantly different using kcal/ml. This is explained on the basis that gastrointestinal capacity becomes a factor limiting intake in relation to both the energy content of the diet (kcal/g) and the space occupied in the gut by that diet (density). The minimum caloric density at which these growing rats could maintain digestible energy intake changed from 3.7 kcal/ml at 36 days of age to between 2.66 and 2.89 kcal/ml at 60 days of age.