796 VII. LIPID DISTRIBUTION IN SPECIFIC TISSUES 



acid, 0.4, 0.6; hexadecenoic acid, 5.8, 4.4; oleic acid, 34.1, 21.6; linoleic 

 acid, 0.7, 0.2; C20-C22 unsaturated acids, 1.2, 1.3; and total unsaturated 

 acids, 42.4, 28.3. 



In the case of cow's milk, Baldwin and Longenecker 497 found that the 

 fatty acid composition of colostrum and that of mature milk were practi- 

 cally identical. On the other hand, human colostrum was found to have a 

 higher phospholipid content and a larger proportion of high molecular 

 weight acids than mature milk, while the opposite condition obtained in 

 the case of the low molecular weight fatty acids. 492497 Phospholipids de- 

 creased from 6.1% of total lipids on the second day to 0.5% of the total in 

 human milk fat obtained between the 22nd and 43rd days. 492 



Bartley et al. m reported that the greatest unsaturation of cow-milk fat 

 occurs at the peak of production, irrespective of the season of the year. 

 There is a decrease in unsaturation up to the fourth or fifth month, fol- 

 lowed by a slight increase in unsaturation toward the end of lactation. 

 Variations in the oleic acid content of the milk were shown to account for 

 the changes in saturation. 



c. Seasonal Changes in the Composition of Milk Fat. Cyclic changes 

 in the fatty acid composition of milk fats occur independently of the varia- 

 tions due to the stage of lactation. For example, Hansen and Shorland 499 

 reported that, beginning in July, there was a progressive increase in the 

 content of C 6 -Ci 4 saturated acids up to November, followed by a slow de- 

 cline to the end of the season. The proportion of Cis-unsaturated acids and 

 of butyric acid varied in the reverse direction, while palmitic acid increased 

 along with the shorter-chain fatty acids. The similarity between the con- 

 stants of butterfats sampled during the same months of different years gave 

 support to the hypothesis that the changes were seasonal. Since the 

 work of Hansen and Shorland was carried on in New Zealand, it would be 

 of interest to determine whether or not the variations were reversed in the 

 northern hemisphere. 



Summer butters have been reported to have nutritive properties not 

 shared by winter butters. Boer and associates 500 ' 601 reported that butterfat 

 is superior to vegetable oils in promoting growth; this they ascribed to the 

 presence of vaccenic acid (A 1112 -octadecenoic acid). However, the results 



447 A. R. Baldwin and H. E. Longenecker, /. Biol. Chem., 155, 407-412 (1944). 



498 E. E. Bartley, J. H. Zaletel, E. W. Bird, C. Y. Cannon, G. H. Wise, and O. Kemp- 

 thorne, J. Dainj Sci., 34, 536-547 (1951). 



499 R. P. Hansen and F. B. Shorland, Biochem. J., 52, 207-216 (1952). 



600 J. Boer, B. C. P. Jansen, and A. Kentie, J. Nutrition, S3, 339-358 (1947). 

 501 J. Boer, B. C. P. Jansen, A. Kentie, and H. W. Knol, /. Nutrition, 33, 359-360 

 (1947). 



