FATS AS ESSENTIAL DIETARY COMPONENTS 841 



due to the fact that the animals receiving the fat did not experience a loss 

 of energy entailed by the transformation of carbohydrate to fat. 



(6) Experiments in Which Paired Feeding Tests Were Employed. In 

 most of the tests in which variabilities in caloric intake and palatabihty 

 of the food were eliminated by the use of isocaloric diets, groA\i,h tests in 

 general still fa-\^ored the high-fat regimen. Thus, in the experiments of 

 Forbes et al.,^^ which were carried out ■udth pair-fed rats on isocaloric 

 diets containing 2, 5, 10, or 30% of fat, which was mostly lard, the gains 

 in weight, the digestibility of the fat, and the retention of nitrogen in- 

 creased proportionally as the amount of fat in the diet was augmented. 

 In another series of tests carried on by Forbes and co-workers^^ with 2, 10, 

 and 30% of fat, and with increasing proportions of vitamins as compared 

 with the amount given in the earher series, it was proved that the greater 

 efficiency of the diets having the highest fat content was manifested by 

 significant gains of fat energy and by a decrease in the total heat produc- 

 tion. In a later report from this laboratory 2° it was found that the fat 

 content of the diets had little effect upon nitrogen utilization in the case 

 of adult rats. With a supermaintenance food intake, there was a shght 

 increase in urinaiy nitrogen and a decrease in nitrogen retention in pro- 

 portion to the increasing fat content of the diets. However, as the fat 

 in the diets increased there were slight decreases in the metabolizable 

 energy, and larger decreases in heat production, in the order of the in- 

 creasing fat content. The heat increments therefore decreased with in- 

 creasing fat. The decreasing energy expenditure entailed by utilization 

 of the isocaloric diets, in the order of their increasing fat content, was 

 due mainly to decreasing heat derived from the catabolism of carbohy- 

 drates.^^ No fat was catabolized from a 2% fat diet, while considerable 

 heat was produced from fat in the 30% diet (for a further discussion of 

 this point see the following section on associative dynamic action) . French, 

 Black and Swift" did report that, when the protein intake of growing rats 

 was decreased from 22% to 7% of the diet, the superior energy utilization 

 of high-fat diets was still maintained. The only discordant result in this 



'8 E. B. Forbes, R. W. Swift, R. F. Elliott, and W. H. James, /. Xidiilion, 31, 203-212 

 (1946). 



'9 E. B. Forbes, R. W. Swift, W. H. James, J. W. Bratzler, and A. Black, /. Nutrition, 

 5^,387-396(1946). 



2« E. B. Forbes, R. W. Swift, E. J. Thacker, V. F. Smith, and C. E. French, J. Nutri- 

 tion, 32, 397-403 (1946). 



21 E. B. Forbes, R. W. Swift, R. F. EUiott, and W. H. James, J. Nutrition, 31, 213-227 

 (1946). 



" C. E. French, A. Black, and R. \\ . Swift, J. Nutrition, 35, 83-88 (1948). 



