Toxicity. Prolonged exposure to sunlight may cause sunburns but it does not produce an excess of vitamin D, but megadoses of orally administered vitamin can lead to hypervitaminosis. In children, hypervitaminosis D may take the form of metastatic calcifications of soft tissues such as the kidney; in adults, it causes bone pain and hypercalcemia. As a point of some interest, the toxic potential of this vitamin is so great that in sufficiently large doses it is a potent rodenticide!
A deficiency of water-soluble vitamin C leads to the development of scurvy, characterized principally by bone disease in growing children and by hemorrhages and healing defects in both children and adults. Sailors of the British Royal Navy were nicknamed “limeys” because at the end of the 18th century the Navy began to provide lime and lemon juice to them to prevent scurvy during their long sojourns at sea. It was not until 1932 that ascorbic acid was identified and synthesized. Unlike vitamin D, ascorbic acid is not synthesized endogenously in humans, who therefore are entirely dependent on the diet for this nutrient. Vitamin C is present in milk and some animal products (liver, fish) and is abundant in a variety of fruits and vegetables. All but the most restricted diets provide adequate amounts of vitamin C.
Function. Ascorbic acid acts in a variety of biosynthetic pathways by accelerating hydroxylation and amidation reactions. The most clearly established function of vitamin C is the activation of prolyl and lysyl hydroxy-lases from inactive precursors, allowing for hydroxylation of procollagen. Inadequately hydroxylated procollagen cannot acquire a stable helical configuration or be adequately crosslinked, so it is poorly secreted from the fibroblasts. Those molecules that are secreted lack tensile strength, are more soluble, and are more vulnerable to enzymatic degradation. Collagen, which normally has the highest content of hydroxyproline, is most affected, particularly in blood vessels, accounting for the predisposition to hemorrhages in scurvy. In addition, a deficiency of vitamin C suppresses the synthesis of collagen polypeptides, independent of effects on proline hydroxylation. Vitamin C also has anti-oxidant properties. These include an ability to scavenge free radicals directly and the participation in metabolic reactions that regenerate the anti-oxidant form of vitamin E.
Deficiency States. Consequences of vitamin C deficiency are illustrated in
Fig. 8.22 . Fortunately, because of the abundance of ascorbic acid in foods, scurvy has ceased to be a global problem. It is sometimes encountered in affluent populations as a secondary deficiency, particularly among elderly persons, people who live alone, and chronic alcoholics—groups often characterized by erratic and inadequate eating patterns. Occasionally, scurvy appears in patients undergoing peritoneal dialysis and hemodialysis and among food faddists.
Toxicity. The popular notion that megadoses of vitamin C protect against the common cold or at least allay the symptoms has not been borne out by controlled clinical studies. Such slight relief as may be experienced probably is a result of the mild antihistamine action of ascorbic acid. The large excess of vitamin C is promptly excreted in the urine but may cause uricosuria and increased absorption of iron, with the potential for iron overload.
Other vitamins and some essential minerals are listed and briefly described in
Tables 8.9 and 8.10 . Folic acid and vitamin B12 are discussed in Chapter 12.
VitaminKCofactorinhepaticcarboxylationofprocoagulants—factorsIIBleedingdiathesis(prothrombin),VII,IX,andX;andproteinCandproteinS
VitaminB1(thiamine)Aspyrophosphate,iscoenzymeindecarboxylationreactionsDryandwetberiberi,Wernickesyndrome,Korsakoffsyndrome