LIPID CONTENT AND COMPOSITION OF ANIMAL 573 



(5) Factors Altering the Distribution, Content, and Composition of Tissue 



Lipids 



a. The Effect of Age. (a) Distribution of Tissue Lipids as Affected by 

 Age. In young animals of both sexes, the subcutaneous fat comprises 85 

 to 90% of the total storage fat. The quantities of intramuscular, genital, 

 and perirenal fat are especially low in young rats when compared with the 

 values in the adult animals. The proportion of subcutaneous fat was re- 

 duced to 65% in all tests when the animals were at the 150 g. level; this 

 decrease was accompanied by an increase in the proportion of storage fat in 

 the other fat depots. A still further readjustment occurs in the 250 g. rat. 

 The same relationship exists, irrespective of whether or not the rats have 

 been receiving a low fat diet (corn) or a high-fat diet (Crisco). These data 

 are summarized in Table 11. 



(b) Content of Tissue Lipids as Affected by Age. No reliable data are 

 available on the relative fat content of the tissues of children during infancy 

 or childhood. Reynolds 253 noted that the thickness of the subcutaneous 

 fat, as determined by x-ray pictures of the calf of the leg, is increased during 

 infancy; a decrease occurs during childhood, and this is followed by another 

 rise during adolescence, and a postadolescent leveling off. 



However, considerable data demonstrate in an unequivocal manner that 

 the fat content of the adult increases progressively with advancing years. 

 Making use of tritium as the test solute, for the indirect determination of the 

 fat content, Prentice et al. 2i0 found that the water content of four young men 

 averaging 22.2 years of age was 60.4%, as contrasted with a value of 52.1% 

 for fifteen men who had a mean age of 44.5 years. Similar variations were 

 obtained by Edelman et al. 2bi over a wider age span when deuterium oxide 

 was employed as the test solute. Thus, the following values were obtained 

 for the water content of human tissues: 25.0 yr. (34 men), 61.1%; 40.9 yr. 

 (10 men), 55.4%; and 66.0 yr. (6 men), 54.3%. In the case of women, the 

 same trend was observed but there was a consistently lower water content 

 (and presumably a higher fat content) throughout, viz., 28.8 yr. (18 wo- 

 men), 51.2% ; 47.3 yr. (6 women), 48.2%; and 73.0 yr. (5 women), 46.2%. 



The results obtained by the use of specific gravity measurements, by 

 Brozek, 255 for five age groups have given results in conformity with those 

 listed above; the fat was computed by the formula of Rathbun and Pace. 186 

 These data are as follows: 20.3 yr., 9.9% fat; 25.2 yr., 14.4% fat; 46.0 



253 E. L. Reynolds, Anat. Record, 100, 621-630 (1948). 



264 I. S. Edelman, H. B. Haley, P. R. Schloerb, D. B. Sheldon, B. J. Friis-Hansen, 

 G. Stoll, and F. D. Moore, Surg. Gynecol. ObsleL, 95, 1-12 (1952). 

 266 J. Brozek, Federation Proc, 11, 784-793 (1952). 



