802 VII. LIPID DISTRIBUTION IN SPECIFIC TISSUES 



the arterial and venous blood, respectively. Both of these groups of in- 

 vestigators suggest that fat itself is the source of the milk fat. 



In the more recent studies in which a comparison of the lipid content of 

 the arteriovenous system of the mammary gland has been followed, it has 

 been shown that the gland absorbs blood neutral fat and glycerides, but 

 probably no other fraction, unless it is the cholesterol ester fraction. 643-545 

 However, all workers realize that the blood fat cannot directly serve as a 

 source of milk fat without a considerable modification in its glyceride struc- 

 ture. Not only is enough fat made available by the blood supplying the 

 udder to account for the milk fat, but also an additional supply is removed 

 from the blood to be oxidized and so to supply the energy requirements of 

 the mammary gland. 546 It is suggested that, by this process, 547 the short- 

 chain acids are formed which are present in milk fat but which do not occur 

 in the blood lipids. However, the later results of Shaw and collabo- 

 rators 544 - 547 failed to confirm the fact that the mammary gland removed fat 

 from the blood in excess of that excreted in the milk. 



There has been considerable corroborative evidence that milk fat orig- 

 inates from the neutral fat fraction of blood, in addition to data obtained by 

 a study of arteriovenous differences in the lactating udder. 



Hilditch 548 developed a theory to account for the formation of the typical 

 glyceride structure in ruminant milk, particularly the oleoglycerides, by 

 chemical transformation of the neutral fat fraction of blood. It is suggested 

 that the oleic acid residues in glyceride combination are broken down in 

 the udder by a series of oxidations and reductions to give rise to the short- 

 chain acids and to the A 9 - 10 -unsaturated acids of the C10-C16 series which 

 appear to be characteristic of milk fat. The proportion of totally saturated 

 glycerides occurring with certain proportions of saturated fatty acids are 

 characteristic of milk fats, as well as of mammalian depot fats, but are not 

 found in other natural fats. This hypothesis was summarized and ex- 

 tended by Achaya and Hilditch, 54M in 1950. Although the hypothesis of 

 short-chain acids arising from the long-chain acids is attractive, it is not 

 supported by the results of Appel et al. b3b When odd-numbered long-chain 

 fatty acids were fed to a lactating sheep, no odd-chain acids below Cn were 

 found in the milk. Moreover, the synthesis of the short-chain acids from 

 acetate, as described below, would seem to offer an entirely logical explana- 

 tion for the formation of the short-chain acids. 



544 J. C. Shaw and W. E. Petersen, J. Dairy Sci., 23, 538-539, 1045-1056 (1940). 



545 L. Voris, G. Ellis, and L. A. Maynard, J. Biol. Chem., 183, 491-498 (1940). 



546 J. C. Shaw and W. E. Petersen, Am. J. Physiol., 128, 183 P (1938). 



647 J. C. Shaw and C. B. Knodt, /. Biol. Chem., 138, 287-292 (1941). 



648 T. P. Hilditch, Analyst, 62, 250-259 (1937). 



649 K. T. Achaya and T. P. Hilditch, Proc. Roy. Soc, B137, 187-211 (1950). 



