FAT METABOLISM 729 



as has been known for long, there is marked disturbance of fat metab- 

 olism. 



It will be observed that there is about 0.7 per cent of total fatty sub- 

 stances in normal blood. The fatty acids (palmitic and oleic) amount to 

 about 0.4 per cent, and are equally distributed between plasma and 

 corpuscles; the lecithin, about 0.3 per cent, being twice as abundant in 

 corpuscles as in plasma, and the cholesterol, 0.2 per cent, about equally 

 distributed. In diabetes all of these substances are seen to be increased 

 in proportion to the severity of the disease, the increase being mostly 

 in the plasma. The increase in cholesterol (confined mainly to the 

 plasma) is particularly interesting, since the substance is unaffected in 

 amount by excessive feeding with fat. 



The Destination of the Fat of the Blood. In general, it may be said 

 that the blood fat is transported to three places: (1) the depots for fat; (2) 

 the liver; and (3) the tissues. The fat present in each of these places 

 differs from that in the others, as is revealed by chemical examination 

 by the methods described on page 719. The depot fat usually yields about 

 95 per cent of its total weight as fatty acid. The tissue fat, on the other 

 hand, yields only about 60 per cent of its total weight as fatty acid. 

 This difference indicates that the fatty acid must be combined in the 

 tissues with a much larger molecule than is the case in the fat of the 

 depots. This large molecule is probably that of lecithin or other phos- 

 pholipin, and the smaller molecule in the depots, that of neutral fat. 

 The liver fat takes an intermediate position between depot fat and tissue 

 fat in its yield of fatty acid. When no active metabolism of fat is go- 

 ing on, the liver fat is like that of the tissues ; but when fat metabolism 

 is active, the liver fat occupies an intermediate position between liver 

 fat and depot fat. 



Another difference among the fats in these three places is with regard 

 to the degree of saturation of the fatty-acid radicles. This, it will be 

 remembered, is indicated by the iodine value; the higher the iodine 

 value, the greater the desaturation of fatty acid. In depot fat this value 

 is relatively low for example, about 30 in the goat and about 65 in man ; 

 depending somewhat on the fat taken in the food, compared with which 

 it is usually a little higher. The fat in the tissues, on the other hand, 

 has a high iodine value, possibly 110 to 130. The iodine value of the 

 fat of the liver is remarkably inconstant, being about the same as that 

 of the tissues when fatty-acid metabolism is not particularly active, but 

 approximating that of the depots when fat mobilization is proceeding. 

 The significance of this fact we shall consider later. 



