700 METABOLISM 



Another difference among the fats in these three places is ^\ itli regard 

 in the degree of saturation of the fatty-acid radicles. This, it Avill 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 ;S0 in the goal 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, 

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

 fat of the liver is remarkably inconstant, being about Ihc 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. 



The Depot Fat. — The places in the animal body where depot fat is 

 deposited in greatest amount are the subcutaneous and retroperitoneal 

 tissues. These fat depots may sometimes become of enormous size, as 

 in the ease of the famous dog of Pfliiger, of whose total body weight 

 40 per cent was due to fat. Bloor suggests that there may really be two 

 different types of fat storage: one of a purely temporary character, 

 which readily takes up and liberates the fat, but which is of limited 

 capacity and possibly under the control of some quickly acting regulat- 

 ing mechanism, like that of the glycogenic function of the liver; and 

 another of a more permanent nature, into which the fat is slowly taken 

 up, but the capacity of which is very much greater. 



Two questions present themselves concerning this depot fat: (1) where 

 does it come from, and (2) what becomes of it? Regarding the source 

 of the depot fat, there is no doubt that it comes partly from the fat and 

 partly from the carbohydrate of the food; in other Avords, it is either 

 taken ready-made with the food or manufactured in the organism. That 

 some of it comes from the fat of food is now a Avell-established fact, the 

 evidence for Avhich need not detain us long. The best-knoAvn experiment 

 consists in first of all starving an animal until his stores of fat are 

 nearly exhausted and then feeding him with some "ear-marked" fat — 

 that is, with some fat having a characteristic property Avhich it will 

 not lose during absorption. It will be found that the depot fat thereby 

 deposited presents many of the qualities of the fed fat. The "ear- 

 marking" of the fal may be secured by using fats of different melting 

 points, such as mutton fat. which has a high M.P., or olive oil, which has 

 a low M.P. On feeding a previously starved dog with mutton fat, the 

 M.I', of the depot fat approaches that of mutton fat — he becomes a 

 dog in sheep's clothing; whereas when olive oil is fed, the subcutaneous 

 fat becomes oily. Or again we may "ear-mark" the fat by combining it 

 with bromine, when the deposited fat will likeAvise be brominized fat. 



