LIPID CONTENT AND COMPOSITION OF ANIMAL 575 



yr., 22.2% fat; 50.0 yr., 24.0% fat; and 54.6 yr., 25.2% fat. In a later 

 study made by Brozek and associates 256 on sixty-two women, who were in 

 age groups 24.2, 39.1, and 56.0 years, the specific gravities were 1.0459, 

 1.0336, and 1.0218, which correspond to values of 26.1, 32.4, and 38.8% of 

 the body weight accounted for by fat, as calculated by the Rathbun-Pace 

 formula. 186 When a linear prediction equation was employed, the mean 

 values for fat in the case of women twenty-five, thirty-five, forty-five, and 

 fifty-five years of age were reported to be 26.5, 30.5, 34.5, and 38.5%, re- 

 spectively. 



The average skinfold measurements made at ten sites showed mean 

 values of 19.5, 23.3 and 24.9 mm., respectively, with increasing age. 166 



(c) The Composition of Tissue Lipids as Affected by Age. Marked al- 

 terations also occur in the chemical composition, as well as in the total 

 amount of the body lipid during growth and aging. Such changes as those 

 which occur during the early days of fife are, without question, of a physio- 

 logic nature. On the other hand, it is considerably more difficult to eval- 

 uate the alterations in tissue composition in the aged, inasmuch as many of 

 the changes are the result of disease, and hence are unphysiologic and unpre- 

 dictable. Moulton, 257 who has carried out extensive studies in the field, 

 concluded that mammals "show a rapid decrease in relative water content, 

 and an increase in protein (nitrogen) and ash content from earliest life until 

 the time of chemical maturity is reached." However, the interpretations 

 of Moulton may be open to some question, since they are all based upon 

 data calculated on the fat-free basis. This same investigator likewise notes 

 that the most striking change in the chemical composition of the body of 

 mammals associated with growth and development is an increase in the fat 

 content. Although it is probably justifiable to exclude adipose tissue from 

 the discussion, it is questionable whether one should exclude the so-called 

 essential lipids which represent the element constant (see page 597). Wil- 

 liams and associates 148 showed that, although the proportion of total lipids 

 and likewise of essential lipids does decrease with age, cerebrosides and 

 cholesterol esters actually increase coincident with a decrease in phospholip- 

 ids and free cholesterol. McCay 258 has written an excellent review on the 

 biological and medical aspects of the subject of aging. 



Williams et al. 148 determined the content of various lipids in the whole 



2S6 J. Brozek, K. P. Chen, W. Carlson, and F. Bronczyk, Federation Proc., 12, 21-22 

 (1953). 



267 C. R. Moulton, J. Biol. Chem., 57, 79-97 (1923). 



268 C. M. McCay, in E. V. Cowdry, Problems of Aging, Biological and Medical Aspects, 

 2nd ed., Williams & Wilkins, Baltimore, 1942. 



