344 THE BIOCHEMISTRY OF B VITAMINS 



however be prevented by nutritional riboflavin-5'-phosphate or the 

 dinucleotide coenzyme form. 19 



Only scant knowledge exists in other cases. Free B vitamins are 

 generally believed to be readily absorbed in the intestine, but probably 

 not as such. It is not certain whether cozymase and inositol phosphate 

 are absorbed directly or first split to simpler products, but both nicotinic 

 acid and its amide are apparently absorbed. Recent evidence indicates 

 that coenzyme A is probably hydrolyzed prior to pantothenate absorp- 

 tion. 20 By analogy with thiamine and riboflavin, it seems possible that 

 many of these vitamins undergo phosphorylation prior to absorption; 

 this point merits further investigation. Since absorption processes and 

 "thresholds" are apparently major factors in influencing the individual 

 differences in efficiency of B vitamin utilization and requirements, it is 

 indeed surprising that more extended study of this subject has not been 

 undertaken to date. 



It is readily apparent that a large number of factors may influence 

 the ability to absorb the B vitamins. Diets high in fats may mechanically 

 prevent ready access of the vitamin to the absorptive membrane. 

 Pharmaceutical derivatives of riboflavin have been prepared with such 

 low solubility as to be poorly absorbable. In the case of renal resorption 

 of amino acids, competition may exist between amino acids that are 

 resorbed by a similar mechanism for the limited metabolic activity of 

 the membrane, 21 and it seems quite logical that an excess of metabolites 

 that are transported across the intestinal wall by a mechanism (e.g., 

 phosphorylation) similar to that involved with a vitamin may diminish 

 the absorbability of that vitamin. This factor may explain the observa- 

 tion that the presence of food in the gastrointestinal tract diminishes the 

 absorption of calcium pantothenate in dogs. 22 Certainly any factor that 

 influenced the phosphorylation activity of the intestine would have this 

 effect, and this consideration may well be advanced as one other that 

 favors a balanced diet containing a variety of nutrients. At present, how- 

 ever, it is entirely impossible to estimate what portion of vitamin mal- 

 nutrition is due to absorption difficulties. It is known that wide variations 

 do exist in the ability to use dietary components, and further extended 

 studies are necessary to show what effect the state and nature of the 

 nutrition have on the ability of an individual to utilize B vitamins. We 

 have already noted (p. 300) that large amounts of B vitamins occur in 

 the feces, and it is most uncertain what portion of this material is of 

 exogenous origin and whether its presence is due to its inavailability to 

 the animal or to its not being required. Over 60 per cent of the B vitamins 

 of feces are said to be water-soluble, and should be available. 23 Perhaps 

 by a better understanding of the absorptive process, we may some day 



