FATS AS ESSENTIAL DIETARY COMPONENTS 855 



(7) The Effect of Dietary Fat upon Nitrogen Retention 



The al)ility of a foodstuff to spare protein has always been considered 

 to be an important index of nutritional \'akie. For a number of years it 

 has been generally assumed and accepted that nitrogen equilibrium was 

 more readily established when carbohydrate was given simultaneously 

 Avith protein foods than when fat served as the auxiliary foodstuff. In 

 the classical experiments of Graham Lusk made over sixty years ago^* 

 it was reported that men on a constant protein intake exhibited a reduced 

 urinary nitrogen when carbohydrate was ingested concomitantly, but that 

 no such decrease in nitrogen elimination was observed when fat was the 

 additional foodstuff consumed. 



However, it has recently been demonstrated by a different type of ex- 

 perimental approach that fat actually does play a commanding role in 

 protein metabolism when one is concerned solely with the endogenous pro- 

 tein metabolism. Thus Willman and collaborators^^ were able to demon- 

 strate that the low level of "wear and tear quota" or the so-called nitro- 

 gen minimum (which is the lowest 1c\t1 of protein catabolism obtained 

 after a prolonged period on a protein-free diet) could be maintained only 

 when generous amounts of fat were present in the diet. In experiments 

 on rats no differences were noted in the level of urinary nitrogen when the 

 protein-free diets were fed at 100% or at 75% of the twenty-four-hour 

 caloric requirement, irrespective of whether the diet contained 20% of 

 fat or was fat-free. On the other hand, when only 50% of the required 

 calories were given, the increase in nitrogen excretion over the basal 

 level, namely the level when 100% of required calories were fed, was 

 only 24 mg. when the animals received the protein-free diet containing 

 20% of fat, as contrasted with an increase of 311 mg. for the group of 

 rats which received the protein-free diet which was also deficient in fat. 

 "When the caloric intake was reduced to 25% of the recjuired level, the 

 variations between the fat-containing and fat-free diets were exaggerated 

 to an even greater extent. Thus the increase in nitrogen excretion over 

 the basal was 254 mg. for the animals receiving the fat diet as contrasted 

 with the mean of 569 mg. for those on the fat-free regimen. Similar 

 though less striking results were noted when methionine was added to the 

 protein-free diet.^^ Apparently the same situation exists in the case of 



" G. Lusk, Z. Biol, 27 (n.s.O), 459-481 (1890). 



6* W. Willmnn, M. Brush, H. Clark, and P. Swanson, Federation Proc, 6, 423-424 

 (1947); also personal communication from P. Swanson. 



«« D. Schwimmer and T. H. McGavack, New York Stale J. Med., 4S, 1797-1799 

 (1948). 



