576 VI. OCCURRENCE OF LIPIDS IN THE ANIMAL 



rat at birth, and at fifteen, forty-five, and seventy days of age. Their data 

 are summarized in Table 12 (page 574). 



The most striking change in the composition of rats during the first 

 seventy days of life is the tremendous increase in the fat content. Thus, 

 the percentage of total lipids practically doubles over this interval, in- 

 creasing from 21.3% at birth to 40.7% at seventy days of age. When one 

 takes into consideration the great increase in body weight which occurs 

 during this period, one finds that the actual amount of lipid material ac- 

 cumulated is much greater than is indicated by the increased percentage. 



The neutral fat continues to be the principal lipid constituent. In this 

 category, there is more than a three-fold increase, namely from 11.7% at 

 birth to 35.5% at seventy days. The neutral fat fraction is deposited in 

 adipose tissues, in contradistinction to the essential lipids, which are con- 

 cerned with cell structure. In this latter group of lipids, the cholesterol 

 esters are highest at birth, and lowest at fifteen days, after which they re- 

 turn to their initial level at seventy days. In the case of phospholipids and 

 free cholesterol, a synthesis must occur, since the total content of the body 

 is greatly increased after seventy days. However, since the rate of in- 

 crease is somewhat less than for protein and other structural elements, the 

 percentage of the above lipids is somewhat reduced as the animal becomes 

 older. 



On the other hand, cerebrosides continue to increase in amount during the 

 entire period of the test, while the proportion of cephalin rises up to the 

 forty-fifth day. The increased concentration of both of these components 

 can be explained largely by the fact that the muscle tissue, which contains a 

 relatively high amount, represents a higher percentage of body weight in the 

 seventy-day-old rat than in the newborn animal. Thus, according to 

 Donaldson, 259 the musculature at birth accounts for 24% of the body weight 

 while, at seventy-eight days of age, it comprises 41% of the total body sub- 

 stance. 



Although considerable variations obtained in the amount of fat laid 

 down by rats from the twenty-second to the eighty-eighth day of life when 

 they were fed a high-fat vs. a high-carbohydrate diet, the distribution of 

 lipids was quite similar. Whereas the total lipids amounted to 16.58% at 

 twenty-two days, the figures were 44.80% for those on an adequate diet, 

 49.10% for those on the high-fat diet, and 36.01% for those on the high- 

 carbohydrate diet, after each had been fed 3000 Calories of their respective 

 diets. On the other hand, the proportion of essential lipid components on 



259 H. H. Donaldson, The Rat, Memoirs Wistar Inst. Anat. Biol., 2nd ed., Philadelphia, 

 1924. 



