LIPID METABOLISM 



I l8 3 



when such drastic changes are made in the diet, 

 variations in individual responses are great (113). 



Essential Fatty Acid (EFA) "Deficiency" 



The fact that serum lipids can be lowered, when 

 dietary fats rich in polyunsaturated fatty acids are 

 fed, has stimulated considerable interest in the bio- 

 chemistry of the essential fatty acids (linoleate, 

 arachidonate, etc.), their role in nutrition, and, in 

 particular, their possible role in the metabolism of 

 cholesterol. Excellent reviews and discussions of these 

 subjects are available (1, 51, 106, 146, 189). Holman 

 (106) has suggested that the term essential fatty acid 

 (EFA) include "only those substances which are 

 active both for growth and for maintenance of dermal 

 integrity, limiting the term to linoleic and arachidonic 

 acids and to such other acids as may be derived 

 metabolically from them." As has been pointed out 

 by Aaes-Jorgensen (1), this definition leaves out lino- 

 lenic acid and C22 polyenoic acids from brain phos- 

 phatides which have been shown by Thomasson (50, 

 198) to be active only as growth factors. 



Despite numerous studies since 1929, when Burr 

 & Burr (40) first recognized EFA deficiencv in 

 young rats, the EFA requirement for human adults 

 has not been determined. In fact, EFA deficiency in 

 adult man has not been demonstrated. In 1958, the 

 Food and Nutrition Board of the National Research 

 Council (65) suggested that one per cent of calories 

 should be the minimum daily EFA allowance for 

 humans. In any reasonable variation of the American 

 diet, this quantity is certainlv present. In the human 

 infant, however, Hansen and associates (90) have 

 shown that linoleic acid is definitely required in 

 amounts as little as 1.3 per cent of daily dietary 

 calories to prevent or cure certain dermatoses. In- 

 fants fed low fat diets (EFA-deficient) exhibited low 

 serum values for dienoic and tetraenoic fatty acids, 

 and high serum values for trienoic acids. The reverse 

 serum picture was obtained following addition of 

 linoleic acid to the diet. 



On the basis of evidence now available it seems 

 unlikely that Sinclair's (190) hypothesis attributing 

 "nutritional" hypercholesteremia and atherosclerosis 

 to essential fatty acid deficiency is correct. Patients 

 with clinically manifest atherosclerosis and elevated 

 levels of serum total cholesterol do not necessarily 

 exhibit a lack of linoleic acid in their serum or depots 

 (103, 112). Moreover, Ahrens and associates (5) have 

 shown that formula diets, containing as their source 

 of fatty acids the nonessential polyethenoid fatty 



acids predominating in certain fish oils, lower serum 

 cholesterol as effectively as formula diets containing 

 oils exceedinglv rich in linoleic acid. 



Chain Length, Unsatnration, and Melting Point 



Serum lipid responses to dietary fats have been 

 correlated with certain characteristics other than 

 essential fatty acid content or iodine value. Two 

 major variables affecting the physical and biochemical 

 properties of fatty acids are degree of unsaturation 

 and chain length (table 1). For example, linoleic acid 

 (2 double bonds) and stearic acid (no double bonds) 

 have the same chain length, and yet the melting 

 point of linoleic acid is — 1 1 C, whereas that of stearic 

 acid is 69.4 C. Stearic acid (C w ) and capric acid 

 (C10) are both fully saturated acids; however, the 

 melting point of the shorter chained capric acid 

 is 31.5 C. 



Since the melting point of a fat (as well as other 

 characteristics) depends upon the component fatty- 

 acids, it is possible to lower the melting point of a tri- 

 glyceride either by increasing the unsaturation or by 

 reducing the chain length of its fatty acids. Accord- 

 ingly, many of the physical characteristics of a fully- 

 saturated fat containing shorter chain saturated fatty 

 acids may resemble those of a highly unsaturated fat 

 containing predominantly long-chain monounsatu- 

 rated or polyunsaturated fatty acids. 



As previously mentioned, when subjects are fed 

 diets containing as their fat source solid fats such as 

 butter and mutton tallow, their cholesterol levels tend 



table 1 . Classification of Fatty Acids According to Chain 

 Length and Degree of Unsaturation 



