224 III. DIGESTIBILITY OP FATS 



hard and soft fats than did guinea pigs. In fact, Cook 71 states that, al- 

 though mammals in general have high digestibilities (90-98%), low co- 

 efficients of digestibility obtain in guinea pigs, particularly when high 

 levels of fat are given. 



Table 13 records the results of digestibility tests on several species. 



Although the coefficients of digestibility in man and in the lower ani- 

 mals generally show agreement, several differences are immediately ap- 

 parent. One of the most striking variations is in the behavior of castor 

 oil. Thus, castor oil is readily digested in rabbits, rats, sheep, and guinea 

 pigs, and exerts no cathartic effect; this is in striking contrast to its ac- 

 tion in man, where its utilization may be slight, even when a sufficient 

 amount is taken to produce catharsis. It is not known to what extent 

 castor oil is utilized in man when taken in amounts too low to produce 

 diarrhea; one might expect that utilization takes place to some extent 

 under such conditions. Stewart and Sinclair, 74 who reported that castor 

 oil was digested to the extent of 98% in rats, were unable to find any trace 

 of its principal fatty acid, ricinoleic acid, in the phospholipids of the small 

 intestine, liver, or muscle, or in the triglycerides of the liver. These 

 results were interpreted as indicative of the rapid metabolism of this 

 hydroxy acid. 



A second difference between man and the rat as regards the digesti- 

 bility of fat has been noted in the case of rapeseed oil. Deuel, Cheng, 

 and Morehouse 75 found that crude rapeseed oil was digested to the extent 

 of only 77% in the rat, while the digestibility of the refined oil was found 

 to be 82%. This is the lowest digestibility which has been reported for 

 a fat, liquid at ordinary temperatures, other than waxes or fats which con- 

 tain intestinal irritants. This finding is in line with the earlier observa- 

 tion 76 that the rate of absorption of rapeseed oil by the rat is the lowest 

 of any for the common liquid fats. The low digestibility would likewise 

 explain the poor showing of rapeseed oil in the growth tests on rats re- 

 ported by Boer et al. 77 



The poor utilization of rapeseed oil by the rat may well be related to its 

 composition. This fat consists of 40 to 50% of trierucin and not more 

 than 5% of glycerides of saturated acids ; the balance is made up of triolein 

 and trilinolein, as well as of mixed triglycerides. 78 Since no failure of 



74 W. C. Stewart and R. G. Sinclair, Arch. Biochem., 8, 7-11 (194*5). 



75 H. J. Deuel, Jr., A. L. S. Cheng, and M. G. Morehouse, J. Nutrition, 35, 295-300 

 (1948). 



76 H. J. Deuel, Jr., L. Hallman, and A. Leonard, ,/. Nutrition, 20, 215-226 (1940). 



77 J. Boer, B. C. P. Jansen, and A. Kentie, J. Nutrition, 33, 339-358 (1947). 



78 T. P. Hilditch, The Chemical Constitution of Natural Fats, 2nd ed., Wiley, New 

 York, 1947. 



