LIPIDS PRESENT IN MILK FAT 801 



b. Blood Lipids as a Source of Milk Fat. In addition to the indirect 

 evidence of the relationship of food fat to milk fat as described in the pre- 

 vious section, data of a more direct nature are available which demonstrate 

 the effect of blood lipids on milk fat production. The blood lipids may, of 

 course, be traced not only to food fats but also to those originating in the 

 liver or derived from body fat. It was reported by Leroy et al. b36 that a 

 close relationship exists between the total fatty acids of the blood and the 

 butterfat content of the milk. Maynard and associates 537 likewise pointed 

 out that a close correlation exists during the lactation cycle between the 

 level of blood lipids and the amount of milk produced. Thus, blood lipids 

 rise to a high level following parturition, and remain high during the period 

 of greatest milk production ; the level of blood lipids gradually declines as 

 milk production diminishes, reaching a low level as the dry period ap- 

 proaches. The effect of lactation on the level of blood lipids was found to 

 be independent of the effect of dietary fat. In another study, Maynard 538 

 confirmed these findings by demonstrating that blood lipids are invariably 

 higher in lactating than in non-lactating cows. 



In the early studies of Meigs, Blatherwick, and Cary, 539 it was suggested 

 that the phospholipids in the blood were the source of the milk fat. This 

 hypothesis was based upon the observation that the blood leaving the mam- 

 mary gland contained less phospholipids and more inorganic P than were 

 present in the general circulation. In some cases, Meigs et al. b39 were able 

 to account for the total milk fat secreted by the decrease in phospholipid in 

 the blood as it passed through the mammary glands. 



However, later workers have been unable to obtain decisive evidence 

 that blood phospholipids are the mother substances of milk fats. Although 

 Doulkin and Helman 540 did find a positive correlation between blood lecithin 

 and milk fat, neither Lintzel, 541 working with six goats as well as with a cow, 

 nor McCay and Maynard 542 found any significant difference in phospholipid 

 nor in inorganic P between the jugular venous and arterial blood and that 

 of the mammary gland. Similar results have also been recorded by Gra- 

 ham and co-workers, 543 who found little difference between these levels in 



536 A. Leroy, R. Lecoq, M. Veline, Mme. Valissant, and G. Barjot, Lait, 11, 12-20, 

 144-155, 234-244, 359-368 (1931); Chem. Abst., 25, 5457 (1931). 



537 L. A. Maynard, E. S. Harrison, and C. M. McCay, /. Biol. Chem., 92, 263-272 

 (1931). 



538 L. A. Maynard, Lait, 12, 761-775 (1932). 



639 E. B. Meigs, N. R. Blatherwick, and C. A. Cary, J. Biol. Chem., 87, 1-75 (1919). 



640 A. Doulkin and S. Helman, Lait, 14, 797-808 (1934); Chem. Abst., 29. 835 (1935). 



641 W. Lintzel, Z. Ziicht., Reihe B, Z. Tierzucht. Zuchtungsbiol, 29, 219-242 (1934). 



642 C. M. McCay and L. A. Maynard, /. Biol. Chem., 109, 29-37 (1935). 



643 W. R. Graham, Jr., T. S. G. Jones, and H. D. Kay, Proc. Roy. Soc, B120, 330-346 

 (1936). 



