LIPID CONTENT AND COMPOSITION OF ANIMAL 553 



been required over the test period. These data justify the conclusion of 

 the authors that protein can change to carbohydrate. 



4. The Lipid Content and Composition of the Animal as a Whole 



(1) Methods for Determining the Lipid Content of Animals, 

 Including Man 



Two general types of methods are available for the assessment of the fat 

 content of animals, namely by direct analysis of the animal tissues, or by 

 tests in which the fat content may be calculated indirectly. The latter 

 procedure has the advantage that it can be carried out more rapidly, and 

 that it can be employed not only once, but repeatedly with live animals 

 without injury to them. A comprehensive review of the methods which 

 are available for the estimation of body fat in man and in animals was 

 compiled by Keys and Brozek. 166 



a. Direct Analyses. The determination of the lipid content of the body 

 as a whole, or of specific tissues, is a slow and laborious process. Although 

 considerable data have been obtained with laboratory animals by this 

 process, the results available for human subjects are negligible. The pro- 

 cedure involved consists in analyzing the entire animal or an aliquot part 

 of the homogenized tissue for the several components, namely water, pro- 

 tein, minerals, and fat. The partition of water into extracellular and intra- 

 cellular fractions is recommended, and will provide important additional 

 information. A further refinement of the technic involves the separation 

 of the total fat into the structural portion (as in the nervous system) and 

 the depot or storage fat. 



b. Indirect Methods for the Estimation of Body Fat. Although the 

 direct method for estimating the composition of animal tissues is an accu- 

 rate one, it is unsatisfactory for application to living individuals. Several 

 indirect methods are available, however, which include determination of 

 body weight, estimation of the extent of fat deposits in the subcutaneous 

 fat layer, determination of body water following the administration of 

 water-soluble compounds, or the use of fat-soluble dyes. Each of these 

 procedures has its advantages and disadvantages. 



(a) The Use of "Standard" Weight Tables. In addition to the subjective 

 impressionistic appraisal of the relative fatness or leanness of a patient by 

 the clinician, the most widely employed criterion for establishing the nu- 

 tritive state is by comparison of the actual weight with the so-called 



166 A. Keys and J. Brozek, Physiol. Revs., S3, 245-325 (1953). 



