METHODS FOR STUDY OF DIGESTIBILITY OF LIPIDS 197 



color of the second marker appeared, were retained for analyses, and were 

 considered to represent total feces for the experimental period. 



The following procedure was used for the analysis of the feces. The ma- 

 terial was first dried to a constant weight in an oven at 95 °C. The total 

 dried weight of the feces was recorded, and they were ground so that uni- 

 form samples might be prepared for analysis. In the U.S.D.A. studies, 

 fat determinations were made on aliquot samples of the dried feces in the 

 Soxhlet extractor, using diethyl ether as the solvent. 



a. Correction for Metabolic Fat or Metabolic Lipid. In order to make 

 allowance for ether-soluble materials in the feces other than that arising 

 from the fat ingested, a correction must be made for the so-called "meta- 

 bolic fat" or "metabolic lipid." This consists of the fatty materials pres- 

 ent in bacterial residues, digestive juices and secretions, and in the epi- 

 thelial cells of the stomach and intestinal tract, which will be present in the 

 feces along with any undigested food fat. Since it is impossible to sepa- 

 rate the undigested food fat from the metabolic lipid, an indirect, method 

 is necessary to estimate its quantity. 



To determine the factor to be employed to correct for metabolic lipid, 

 Langworthy and Holmes 2 carried out tests with a fat-free basal diet similar 

 to that used in the other studies except that the blanc-mange contained 

 no added fat. Under such conditions, the lipid in the stools, after cor- 

 rection was made for the minimal quantities of undigested fat 3 in the 

 whole-wheat crackers (10%) and milk fat (5%) in the blanc-mange, was 

 considered to be metabolic lipid. The figures for total fecal fat and meta- 

 bolic lipid, respectively, in the four subjects were 11.8 and 9.3 g., 8.3 and 

 6.7 g., 16.8 and 14.9 g., and 11.7 and 9.9 g. Expressed in percentage of the 

 weight of the water-free feces, the values were 6.84, 7.70, 16.02, and 9.00%, 

 respectively; the average correction figure for metabolic lipid was 9.89 g. 

 per 100 g. of dried feces. 



(2) Procedure of Deuel, Johnson et al. 4 



The procedure employed by Deuel, Johnson et al. 4 is quite similar to 

 that used in the earlier U.S.D.A. tests, with several modifications designed 

 to give greater accuracy in the collection of feces and to render the diets 

 more palatable. 



2 C. F. Langworthy and A. D. Holmes, U. S. Dept. Agr., Bull. No. 310, 1-22 (1915). 



3 W. O. Atwater and A. P. Brvant, State of Connecticut, Twelfth Ann. Re-port Storrs 

 Agr. Expt. Sta., Storrs, Conn., 73-110, 111-1*23, 124-141 (1899); also cited by C. F. 

 Langworthy and A. D. Holmes, U. S. Dept. Agr., Bull. No. 310 (1915), p. 19. 



4 H. J. Deuel, Jr., R. M. Johnson, C. E. Calbert, J. Gardner, and B. Thomas, J. 

 Nutrition, 38, 369-380 (1949). 



