METHODS FOR STUDY OF DIGESTIBILITY OF LIPIDS 203 



(5) The Use of Inert Chemical Substances as Indices of Digestibility 



In addition to the several methods listed above for the determination 

 of digestibility, which involve a timed collection of feces, it is likewise 

 possible to obtain essentially the same information from random samples 

 of feces when the substance which is being tested for digestibility is fed 

 concomitantly with a completely indigestible substance. One needs only 

 to know the proportion of the test substance and of the indigestible ma- 

 terial in the food and in a sample of feces to calculate the coefficient of 

 digestibility of the test material. 



There are several prerequisites if a substance is to be used as the in- 

 digestible component of the diet. In the first place, the material must be 

 completely indigestible under all dietary conditions. Furthermore, it 

 must not be toxic, and it is essential that it be completely inert insofar 

 as the test substance is concerned. An indigestible substance which causes 

 diarrhea, or which produces an increased peristalsis, would obviously affect 

 the extent of digestibility of substances concomitantly present in the in- 

 testine. Moreover, a substance such as mineral oil would be unsatis- 

 factory for fats, since it would dissolve most fatty material and so alter 

 the extent of digestibility. This behavior on the part of mineral oil has 

 been repeatedly demonstrated with ^-carotene; this carotenoid becomes 

 almost entirely ineffective as a provitamin A when fed concomitantly with 

 mineral oil. 



a. Ferric Oxide as an Inert Material in the Determination of Digesti- 

 bility. The use of iron oxide (Fe 2 3 ) as an inert substance in the deter- 

 mination of digestibility was first proposed by Bergeim. 8 Heller and 

 associates 9 were able to demonstrate that, when rats were used as the 

 experimental animals, the iron oxide method gave fairly accurate results. 

 However, Moore and Winter, 10 as well as Knott and co-workers, 11 re- 

 ported that the iron oxide procedure yielded unsatisfactory results with 

 cattle. This finding was confirmed by Hale, Duncan, and Huffman, 12 

 who ascribed the irregular results to variations in the amount of iron oxide 

 present in various parts of the gastrointestinal tract. The results of 

 Bell and Crampton 13 with swine were satisfactory, on the whole, except 



8 0. Bergeim, J. Biol. Chem., 70, 29-33 (1926). 



9 V. G. Heller, C. H. Breedlove, and W. Likely, /. Biol. Chem., 79, 275-282 (1928). 



10 L. A. Moore and O. B. Winter, J. Dairy Sci., 17, 297-305 (1934). 



11 J. C. Knott, H. K. Murer, and R. E. Hodgson, /. Agr. Research, 58, 553-556 (1936). 



12 E. B. Hale, C. W. Duncan, and C. F. Huffman, Proc. Amer. Soc. Animal Production, 

 32, 389-393 (1939). 



13 J. M. Bell and E. W. Crampton, Sci. Agr., 27, No. 1, 42-49 (1947). 



