Effect of Ratio of Basal Diet Fat to Test Fat on the True Metabolizable Energy of the Test Fat

Experiments were conducted to evaluate the influence of basal dietary fat on the true metabolizable energy (TME) of test fats differing in composition and physical properties. Two samples of blended fats and one sample of tallow were tested using a simplified corn-soy basal diet with all of the fat extracted and with the extracted fat added back in increasing increments up to 10% of the diet. The test fats were added at levels of 5 to 15%, resulting in ratios of basal fat to test fat of 0:15 to 10:5. The TME of tallow and one of the blends was improved as the ratio of basal fat:test fat was increased from 1:4.5 up to 7.5:7.5, at which point there were no significant differences in TME among the fats. Even at a more narrow ratio (5:10), the TME of the tallow was equal to or higher than that of either blend. Fatty acid absorption was determined using the fat-free diet to determine endogenous fat excretion. Absorption of 18:0 and, to a lesser extent, 16:0 was improved as the ratio of basal fat:test fat was increased from 0:15 to 7.5:7.5. The TME of the tallow was the same when fed with extracted (fat-free) basal as with the unextracted basal suggesting involvement of two separate mechanisms.