CHAPTER 111 

 THE DIGESTIBILITY OF FATS 



1. Introduction 



To many people, the terms "absorption" and "digestibility" are synon- 

 omous, and connote the same physiological phenomenon. However, in 

 the eyes of the biochemist and physiologist, absorption and digestibility 

 constitute two distinctly different properties of foodstuffs, although they 

 are often related to each other. 



By "absorption" one refers to the process by which foodstuffs are re- 

 moved from the small intestine. It is usually considered to be a function 

 of time. On the other hand, the term, "digestibility," signifies the over- 

 all utilization of a foodstuff in the gastrointestinal tract. It cannot be 

 expressed on a rate basis, but it is usually considered to be an index of the 

 completeness with which the material is removed from the intestine during 

 its passage through the body. 



Substances which are slowly absorbed frequently have a low digesti- 

 bility. However, the latter condition may not be attained unless the 

 slowly absorbed material is ingested in large amounts. Fructose and cot- 

 tonseed oil are two substances which are somewhat slowly removed from the 

 intestine. However, both of these foodstuffs are practically completely 

 absorbed under normal conditions. In the comparison of the rates of 

 absorption recorded in Chapter II, rather wide variations between the 

 absorption rates of a number of fats have been noted. However, it will be 

 found that in most instances the fats have a similar practically complete 

 digestibility, which bears no relationship to the rate of absorption. 



The coefficient of digestibility is the term used to indicate the extent of 

 digestibility. It represents the percentage of the ingested foodstuff which 

 gains entrance into the body and hence is not lost in the feces. In the 

 case of fats, the coefficient of digestibility is expressed by the following 

 ratio: 



[Fat ingested] — [Fat excreted (corrected for metabolic fat)] 1fn 

 Fat ingested 

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