FACTORS INFLUENCING THE UTILIZATION OF VITAMINS

1. Availabilty: Not all of a vitamin may be in absorbable form. For instance some of the nicotinic acid in maize is bound in such a way that it is not absorbed from gut. Fat-soluble vitamins may fail to be absorbed if the digestion or absorption of fat is impaired.

2.      Antivitamins: These are known to be present in some natural foods. Several synthetic analogues of vitamins have proved to be highly poisonous (e.g. aminopterin, desoxypyridoxine), presumably because they block the enzymes with which the true vitamins are concerned.

3.      Provitamins: Substances occur in foods which are not themselves vitamins but are capable of conversion into vitamins in the course of digestion. Thus the carotenes are the provitamin of vitamin A and, to some extent at least, the amino acid tryptophan can be converted to nicotinic acid.

4.      Biosynthesis in the Gut: The normal bacterial flora of the gut is capable of synthesizing significant amounts of certain vitamins (e.g. vitamin K, nicotinic acid, riboflavin, vitamin B_{12 ,} folic acid). Bacteria are also capable of extracting vitamins from the ingested food and retaining them until excreted in the faeces. It is unlikely that bacterial activity significantly affects the amounts of vitamins available to a healthy human body with the exception of vitamin K. Bacteria are more likely to reduce than to increase the amounts of vitamins available for absorption, as is clearly demonstrated in the blind loop syndrome.

These considerations indicate how difficult it may be in practice to define the nutritive value of a diet in respect to a given vitamin by simple reference to a chemical analysis of its vitamin content, and that the true requirements for a given vitamin are likely to vary very much frome one individual to another.

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