Vitamin Nutrition - General Observations
( Originally Published 1956 )
IN MANY ways it is unfortunate that a number of the chemical substances which are dietary essentials have been grouped together in a single category designated vitamins. If the entire group of 15 or more vitamins is considered together, the only characteristic that all members have in common is essentiality in small amounts in the diet due to the body's inability to manufacture them from simpler compounds. Even this statement is not entirely true. Many animals can synthesize ascorbic acid, and all animal species tested, including man, manufacture niacin from the amino acid tryptophan. In some in-stances, synthesis of a vitamin by bacteria in the intestinal tract may furnish a sufficient amount of a particular vitamin, e.g., vitamin K, to largely eliminate the necessity of its inclusion in the diet.
Modern nutritional science has advanced to such an extent that each of the vitamins should be considered separately in terms of its chemical structure, requirement and function in the body. While there are close functional interrelationships between many of the vitamins, others are not so related. In addition, there are as close or closer, interrelationships between vitamins and minerals, and between vitamins and amino acids as there are among the vitamins themselves. Pathologic conditions which influence the requirement or utilization of one vita-min may not affect that of another. There is little scientific justification for the therapeutic use of all of the vitamins in a single prescription except in situations of gross dietary inadequacy or in pathologic states requiring pro-longed parental feeding. On the other hand, it is often essential to furnish, at the same time, each of the vitamins which function in various stages of a given metabolic process. In view of the above, diagnosis of vitamin malnutrition must be considered from the standpoint of individual chemical compounds and not the group as a whole. This is true in spite of the fact that certain of the vitamins, the "B" group in particular, are found closely associated in nature and that multiple deficiency is more often en-countered than deficiency of a single factor. Appreciation of the complexity of deficiency and of the interrelation-ships among nutrients is necessary if correct diagnosis is to be achieved. -
Vitamins, or their derivatives, have been shown to function in metabolism in combination with proteins in the promotion of catalysis of certain body processes. Most commonly, vitamin derivatives are coenzymes while proteins form the corresponding apo-enzymes. It seems logical to assume that altered biochemical reactions pre-cede clinical signs of vitamin deficiency. If this is the case, tests of specific metabolic function should permit diagnosis before overt physical abnormalities make their appearance. As yet, few tests of metabolic function are available. Appropriate experimental materials, such as specimens of tissue, are not easily obtained. In the limited studies which have been conducted, metabolic alterations have been found to be quantitative rather than qualitative and suitable norms and variability among individuals around these norms have not been established. In addition, as Snell has indicated, most of the enzymes examined thus far "have not shown readily measurably changes be-fore other more readily observable deficiency symptoms appear". This last statement may be explained best by presuming that the proper enzymes have not been chosen for study. It would not be anticipated that all enzymes dependent on a given vitamin would decrease in activity at the same rate as deficiency progresses. Enzymes requiring the highest concentration of coenzyme for activation would be most rapidly affected and hypo-function of these enzymes presumably should lead to the earliest clinical signs of deficiency (17). When enzymes unusually sensitive to vitamin deficiency are discovered, it may be possible to devise suitable "load" tests which will cause the enzymes to work at maximum capacity. Resulting metabolites can be measured and findings in experimental and control subjects compared. Such tests would obviate the necessity for analysis of tissue samples. A few procedures of this type have been proposed relative to vitamin BB (p. 119) and thiamine It might be anticipated that no single test of specific metabolic function will suffice in detecting deficiency or adequacy of a given vitamin under all conditions.
Minimal daily requirements of some of the vitamins essential to man have been determined; little is known of optimal needs. Undoubtedly, numerous factors influence requirement. For vitamin-requiring bacteria, the optimal intake of a vitamin cannot be determined apart from the total makeup of the ration. It seems likely that this may apply to vitamin needs of animals and man. Variation in requirement may be expected with changing physiologic conditions and in pathologic states.