836 XIV. NUTRITIONAL VALUE OF FATS 



Fat is therefore the foodstuff to be employed in greatest proportion when 

 it is desirable to reduce the bulk of the food to the minimum. 



Since the classical experiments of Burr and Burr,^ first published in 

 1929, there has been positive evidence of the specific nutritional value of 

 fats themselves. As a result of this discovery it is now known that certain 

 polyunsaturated fatty acids are required in the diet, since they cannot be 

 manufactured in the body. The primary polyunsaturated fatty acids 

 undoubtedly are linoleic and arachidonic acids, although linolenic acid and 

 certain other related compounds can also serve in the same capacity, 

 probably because of the ability of the animal body to convert them to one 

 of the primary types. These "essential" acids are known to be required 

 for growth and for the maintenance of normal skin conditions by a wide 

 variety of animals, and by man. More recently it has been shown that 

 the essential fatty acids mediate not only growth but also certain diverse 

 functions such as protection from x-irradiation injury, maintenance of 

 capillary resistance in the skin capillaries, and the normal transport and 

 metabolism of cholesterol. Essential fatty acids are also required for 

 pregnancy and lactation. Thus, the extent to which the essential fatty 

 acids contribute to the metabolic processes of the body must be considered 

 as indicating the indispensability of fats in nutrition. 



In addition to the importance of fat in serving as a source of the essen- 

 tial fatty acids there is also some evidence that fat per se may likewise 

 be a required foodstuff, since certain physiologic functions are carried out 

 better when it is present in the diet. Thus fats exert a protein-sparing 

 action, increase work capacity, extend the length of survival during fast- 

 ing, and protect the subject from stress factors such as occur in hypothyroid- 

 ism. It is not believed that all of these beneficial effects of fat are to be 

 attributed solely to the essential fatty acids. 



In addition to the several functions already listed for fats and lipids, 

 they are essential for the normal functioning of the tissues and organs, 

 as well as of the body as a whole. In this capacity, they are secondary 

 only to protein. Thus, in the master tissue of the body, namely the brain, 

 as well as in the spinal cord and associated nervous system, phospholipids, 

 cholesterol, and galactolipids form a large part of the essential portion of 

 the functioning tissue. The lipids are part of the structural elements, 

 and these cells could not function without them. Moreover, all cell 

 membranes throughout the body apparently contain lecithin, a phospho- 

 lipid which is derived from fat. One of the effects of prolonged fat defi- 

 ciency in rats is the failure of normal kidney function; this is in all proba- 

 bility to be ascribed to the failure of the body to build up normal cell 



3 G. O. Burr and M. M. Burr, /. BM. Chem., 82, 345-367 (1929). 



