THE METABOLISM OF FAT 557 



But it is not necessary that each OH group in the alcohol should be 

 replaced by the same fatty acid, and when this does not occur we have 

 hetero-acid fats. For instance, one can be replaced by steeiric acid, 

 and the remaining two by palmitic acid, yielding a fat called ' stearo- 

 dipalmitin.' Conversely, one OH may be replaced by palmitic and two 

 by stearic acid, forming palmito-distearin. Similarly, a dioleo-stearin 

 (glycerin combined with two molecules of oleic and one of stearic acid), 

 and an oleo-distearin (glycerin combined with two molecules of stearic 

 and one of oleic acid) are known. Such compounds have been isolated 

 from the fat of animals, and also formed synthetically. Again, each 

 of the OH groups in the alcohol can be replaced by a different fatty acid. 



It is obvious, then and this is the point to which these chemical 

 details are intended to lead up that the number of different fats 

 which the animal organism has at its disposal for concocting those 

 varied mixtures designated as body fat is very great, and that there 

 is room for a considerable degree of specificity in the fat stores of 

 different animals, and it may be in the fat contained in different 

 organs of the same animal, even if this specificity is not as marked 

 as in the case of the proteins. It may be added, in connection with 

 the composition of the body fat, that small quantities of free fatty 

 acids and of glycerin may be present ; but there is reason to believe 

 that these are simply the surplus of raw materials which is about to 

 be synthetized to neutral fat, or the surplus of decomposition 

 products of the neutral fat which have not yet left the fat depots 

 to take their place in the metabolism of the tissues. 



The discussion of the metabolism of fat involves a study (i) of 

 the transformations and migrations of the food fat before it begins 

 to be utilized ; (2) of the possible production of fat from other con- 

 stituents of the food ; (3) of the processes and the stages by which 

 fat, whatever its origin, undergoes katabolism to its end products. 

 The fat of the food, passing along the thoracic duct into the blood- 

 stream, is soon removed from the circulation, for normal blood 

 contains only traces, except during digestion. Where does it go ? 

 What is its fate ? 



Transformation and Migration of the Food Fat. The presence 

 of adipose tissue in the body might suggest a ready answer 

 to these questions. The fat-cells of adipose tissue are ordinary 

 fixed connective-tissue cells which have become filled with fat, 

 the protoplasm being reduced to a narrow ring, in which the 

 nucleus is set like a stone. It would, at first thought, seem natural 

 to suppose that the fat of the food is rapidly separated by these 

 cells from the blood, and slowly given up again as the needs of 'the 

 organism require, just as carbo-hydrate is stored in the liver for 

 gradual use. And it has been found that a lean dog, fed with a 

 diet containing much fat and little protein, puts on more fat, as 

 estimated by direct analysis, or keeps back more carbon, as esti- 

 mated by measurements of the respiratory exchange, than can be 

 accounted for on the supposition that even the whole of the carbon 

 of the broken-down protein corresponding to the excreted nitrogen 



