SOURCES OF FAT IN ANIMAL BODY 545 



On the other hand, fatty acids with more than two carbons apparently 

 cannot be incorporated into the long-chain fatty acids. Rittenberg, 

 Schoenheimer, and Evans 146 were unable to demonstrate the presence of 

 any deuterium in the long-chain fatty acids of rats after deuteriobutyric 

 acid or deuteriocaproic acid had been fed. Morehouse 147 likewise proved 

 that all the deuterium present in the rat after the feeding of deuteriotri- 

 butyrin was present in the unmetabolized tributyrin temporarily present. 

 No deuterium was demonstrable in the storage fat. 



Although these two sets of observations may appear to refute each other, 

 it is possible that the acids themselves cannot be used for further synthesis 

 of the long-chain acids, but only their aldehydes. Although butyric acid 

 can be converted to acetic acid, 143 the amount of such conversion may be 

 too small to be demonstrated when the total fatty tissues are examined. 

 For the most recent hypothesis on the chemical mechanism of fat synthe- 

 sis, the reader is referred to Volume III, Chapter II. 



d. The Nature of the Fat Synthesized from Carbohydrate. As has been 

 discussed earlier, the fats laid down after a high-carbohydrate diet are hard 

 fats composed largely of palmitic, stearic, palmitoleic, and oleic acids. 

 Although we now know that it is possible to synthesize the monoethenoid 

 acids from the saturated acids, there is conclusive proof that diethenoid or 

 polyethenoid acids cannot be produced de novo in the animal body. 



Longenecker 51 reported that the storage fat of rats contained only 

 approximately 5% of hexadecenoic (palmitoleic) acid when they received a 

 normal diet. On a high-carbohydrate regimen, as well as on a diet com- 

 posed almost exclusively of protein, the concentration of palmitoleic acid 

 was increased from 4 to 16%. 



Schoenheimer and Rittenberg 40 demonstrated that monoethenoid acids 

 can originate directly from the corresponding saturated acids in vivo. 

 Presumably stearic acid is the parent substance for oleic acid. 



From both a qualitative and a quantitative standpoint, carbohydrate is a 

 less satisfactory source of body lipids than is fat itself. The experiments of 

 Williams and co-workers 148 demonstrated that the amount of fat laid down, 

 as well as the essential lipids in this fat, were distinctly inferior in rats which 

 were sacrificed after receiving 3000 Calories contained in the particular diet 

 under investigation, when a high-carbohydrate diet was fed as compared 

 with a high-fat regimen. The animals were twenty-two days old when the 



146 D. Rittenberg, R. Schoenheimer, and E. A. Evans, Jr., /. Biol. Chem., 120, 503- 

 510 (1937). 



147 M. G. Morehouse, J. Biol. Chem., 155, 33-38 (1944). 



148 H. H. Williams, H. Galbraith, M. Kaucher, and I. G. Macy, /. Biol. Chem., 161, 

 463-474 (1945). 



