American Journal of Clinical Nutrition, Vol 12, 162-169, Copyright © 1963 by The American Society for Clinical Nutrition, Inc.

Some Factors Affecting the Absorption of Vitamins

¹ From the Food and Drug Laboratories, Department of National Health and Welfare, Ottawa, Canada

It has been shown that the dosage-excretion patterns of vitamins differ. Many factors, most of which are not fully understood, are involved in vitamin absorption. These include mode of administration, pharmaceutical form and size of the dose as well as site of absorption of the vitamin. The size of dose significantly influences the percentage of excretion of thiamine and vitamin B₁₂ but has no effect on that of ascorbic acid, niacinamide or riboflavin. Oral doses of thiamine above 2.5 mg. appear to be largely unabsorbed, and there seems to be no justification for their use in vitamin therapy.

Available information indicates that administration of the total dose in several divided portions has no effect on the excretion of riboflavin, ascorbic acid or niacinamide. This would indicate that absorption of these vitamins takes place by passive transport. Divided doses of thiamine and vitamin B₁₂ are absorbed to a significantly greater extent than single doses of these vitamins.

The pharmaceutical form, which significantly influences the rate of disintegration of the product in vivo, can be a limiting factor in vitamin absorption. Contrary to the claims made, none of the so-called "sustained-release" vitamin preparations on the market produced more efficient utilization of thiamine or riboflavin or sustained urinary excretion of these vitamins. The fact that some of these preparations disintegrated in vitro but were only partially available to the body suggests that both thiamine and riboflavin absorption is limited to the upper part of the intestinal tract. Absorption of these vitamins from the rectum is negligible.

It may be concluded that since the absorption mechanisms of vitamins differ markedly and since their absorption is so readily influenced by the pharmaceutical form in which they are compounded, there is need for much more information to ensure that vitamin therapy is placed on a sound physiologic basis. Such information is of particular importance when attempts are made to modify absorption rates.