LIPID CONTENT AND COMPOSITION OF ANIMAL 559 



% fat (swine) = 100 



(M°i _ 4.914) 

 \sp. gr. / 



Messinger and Steele 193 likewise showed that body fat and body water 

 can be calculated from specific gravity with considerable accuracy. 



(d) The Use of Body Water for the Determination of Body Fat. Since an in- 

 verse relationship is known to obtain between the proportion of body water 

 and that of body fat, 185193194 the determination of the former has been em- 

 ployed in the estimation of the total content of the latter. The proportion 

 of body water is highly variable unless calculated on a fat-free basis. 193 

 According to Pace and Rathbun, 185 water constitutes 72.42 ± 2.11% of the 

 fat-free mass of the guinea pig. Pace and Rathbun 185 summarized figures 

 from the literature for the water content of fat-free tissues of various species, 

 as follows: rat, 72.7%; guinea pig, 73.3%; rabbit, 74.9%; cat, 72.4%; 

 dog, 72.2%; and monkey, 73.3%. The grand average for all species was 

 73.2%. In the case of man, McCance and Widdowson 195 concluded that 

 the fat-free tissues contain 71% of water. Osserman and co-workers, 196 

 making use of the antipyrine method, estimated this figure to be 71.8 ± 

 2.99 for healthy men eighteen to forty-six years of age. Keys and Brozek 

 are of the opinion that the water content of fat-free human tissues should 

 be 72 ± 3 ; moreover, it is suggested that this value is not independent of 

 the fatness of the individual, and that greater deviations are to be expected 

 in emaciated and in obese subjects. 



a'. Test Solutes for the Determination of Water : In order to estimate the 

 proportion of water in the tissues of an individual, the usual procedure is to 

 determine the concentration of a test solute in the blood some time after its 

 administration, and to calculate total water on the dilution principle. In 

 order that the solute may prove satisfactory as a test substance, it must 

 (a) rapidly penetrate and dissolve in all the water of the body, (b) not be 

 adsorbed on, be combined with, or be destroyed by other constituents of 

 the body (or if such is the case, the extent of destruction must be predict- 

 able), (c) be eliminated from the organism at a measurable rate, (d) be non- 

 toxic when given in the required dosage, and (e) be readily and accurately 

 measurable in the blood serum. 166 Several such substances have been used 

 successfully. 



(a') Urea. — It is generally believed that urea is distributed in the body 



»» W. J. Messinger and J. M. Steele, Proc. Soc. Exptl. Biol. Med., 70, 316-318 (1949). 



194 E. Da Costa and R. Clayton, /. Nutrition, 41, 597-606 (1950). 



196 R. A. McCance and E. M. Widdowson, Proc. Roy. Soc. (London), 138 B, 115-130 

 (1951). 



196 E. F. Osserman, G. C. Pitts, W. C. Welham, and A. R. Behnke, J. Applied Physiol, 

 2, 635-639 (1950). 



