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Vitamins; Overview
Vitamins; Overview RR Trifiletti, Private Practice, Ramsey, NJ, USA r 2014 Elsevier Inc. All rights reserved.
Vitamins are relatively low-molecular-weight organic compounds required by mammals in small amounts in order to sustain normal metabolism. Historically, several vitamins were discovered as ‘curative factors’ for certain deficiency syndromes, such as vitamin C for scurvy. Plants have the necessary metabolic machinery to synthesize many vitamins, whereas this capacity may be limited or nonexistent in mammals. Thus, humans have a dietary requirement for many vitamins and will develop biochemical and often clinical sequelae if a specific vitamin or vitamins are deleted from the diet for an extended period of time. As biochemistry advanced in the first half of the twentieth century, it became clear that some vitamins (the B vitamins, folate, and biotin) were coenzymes or cofactors for a defined type of biochemical reaction, for example, thiamine (vitamin B1) proved to be involved in a variety of oxidative decarboxylation reactions. Other vitamins, such as vitamins C and E, are involved in intracellular redox reactions and have significant antioxidant roles. Vitamin A has a critical role in the biochemistry of vision and has trophic properties in epithelial cells. Vitamin K is involved in the posttranslational carboxylation of many proteins, especially several of the clotting factors. Finally, vitamin D has a critical role in calcium and phosphorus homeostasis. Taken together, the vitamins have critical but varied metabolic roles; their common feature is that they are required in catalytic amounts. This feature separates them from other essential nutritional factors, such as essential amino and fatty acids. Several clinical scenarios may result in vitamin deficiency of sufficient magnitude to produce neurological sequelae. Vitamin deficiency can result from decreased ingestion (or hyperalimentation or infusion) of vitamin, decreased absorption of vitamin, or increased removal of vitamin from the body. Endemic vitamin deficiency is still seen in areas of the world where protein-calorie malnutrition is common. Vitamin deficiency due to decreased ingestion may also occur from failure to appropriately supplement an unusual diet; the ketogenic diet, used to treat various forms of epilepsy, is an example of such a diet requiring vitamin supplementation. Chronic fat malabsorption states can result in significant malabsorption of the fat-soluble vitamins (A, D, E, and K). Ileal resection, which may be present in an infant with necrotizing enterocolitis, can result in malabsorption of a number of vitamins, especially vitamin B12. Chronic hemodialysis is an example of a clinical scenario in which excessive removal of vitamin from the bloodstream could produce untoward clinical effects. Any of the previously mentioned scenarios could lead to deficiency of a number of vitamins, which could make recognition of a specific vitamin deficiency syndrome, such as
Encyclopedia of the Neurological Sciences, Volume 4
pellagra or scurvy, quite difficult. The common practice of prescribing multivitamin preparations for nonspecific indications can further blunt the recognition of vitamin deficiencies by leading to partially treated states. Given this problem, the possibility of vitamin deficiency should be considered in many patients, especially in those in whom one of the clinical scenarios discussed previously is operative. Vitamin ‘deficiency’ is a relative term. Some patients appear to have increased needs for vitamins. Periods of rapid growth may be accompanied by an increased need for vitamins in normal patients. For example, pregnant women require a modest increase in the daily requirement for vitamins relative to the prepartum state. In contrast to these transient periods of increased vitamin needs in normal patients, certain patients have chronically increased vitamin requirements. A number of primary inborn errors of metabolism are the result of the production of mutant enzymes with lower affinity for vitamin-derived cofactors. Consequently, higher vitamin levels may be required to produce cofactor levels adequate for the production of functional enzymes. The net result is a chronic requirement for a much higher amount of a specific vitamin than the normal patient requires. Often, this requirement is more than can be provided by a normal diet and results in a vitamin dependency state. Recognition that vitamin dependency can compound a given primary metabolic disorder is important because the clinical condition may be improved, sometimes dramatically so, by vitamin supplementation at appropriate doses. The key principle is that primary vitamin deficiency can produce secondary metabolic disease and, conversely, primary metabolic disease can result in secondary vitamin dependency. Several hypervitaminoses or vitamin intoxication states have also been well described. These usually arise from accidental ingestions, unusual diets, or deliberately excessive vitamin supplementation. For most vitamins, ingestion of 10–100 times the recommended daily allowance is required before symptoms become apparent.
See also: Amino Acid Disorders. Neuropathies, Nutritional. Thiamine (Vitamin B1) and Beri-Beri. Vitamin A. Vitamin D
Further Reading Pfeffer G, Majamaa K, Turnbull DM, Thorburn D, and Chinnery PF (2012) Treatment for mitochondrial disorders. Cochrane Database of Systematic Reviews. Issue 4. Art. No.: CD004426. DOI: 10.1002/14651858.CD004426.pub3 Snodgrass SR (1992) Vitamin neurotoxicity. Molecular Neurobiology 6: 41–73.
doi:10.1016/B978-0-12-385157-4.00115-9
721
Vitamins are relatively low-molecular-weight organic compounds required by mammals in small amounts in order to sustain normal metabolism. Historically, several vitamins were discovered as ‘curative factors’ for certain deficiency syndromes, such as vitamin C for scurvy. Plants have the necessary metabolic machinery to synthesize many vitamins, whereas this capacity may be limited or nonexistent in mammals. Thus, humans have a dietary requirement for many vitamins and will develop biochemical and often clinical sequelae if a specific vitamin or vitamins are deleted from the diet for an extended period of time. As biochemistry advanced in the first half of the twentieth century, it became clear that some vitamins (the B vitamins, folate, and biotin) were coenzymes or cofactors for a defined type of biochemical reaction, for example, thiamine (vitamin B1) proved to be involved in a variety of oxidative decarboxylation reactions. Other vitamins, such as vitamins C and E, are involved in intracellular redox reactions and have significant antioxidant roles. Vitamin A has a critical role in the biochemistry of vision and has trophic properties in epithelial cells. Vitamin K is involved in the posttranslational carboxylation of many proteins, especially several of the clotting factors. Finally, vitamin D has a critical role in calcium and phosphorus homeostasis. Taken together, the vitamins have critical but varied metabolic roles; their common feature is that they are required in catalytic amounts. This feature separates them from other essential nutritional factors, such as essential amino and fatty acids. Several clinical scenarios may result in vitamin deficiency of sufficient magnitude to produce neurological sequelae. Vitamin deficiency can result from decreased ingestion (or hyperalimentation or infusion) of vitamin, decreased absorption of vitamin, or increased removal of vitamin from the body. Endemic vitamin deficiency is still seen in areas of the world where protein-calorie malnutrition is common. Vitamin deficiency due to decreased ingestion may also occur from failure to appropriately supplement an unusual diet; the ketogenic diet, used to treat various forms of epilepsy, is an example of such a diet requiring vitamin supplementation. Chronic fat malabsorption states can result in significant malabsorption of the fat-soluble vitamins (A, D, E, and K). Ileal resection, which may be present in an infant with necrotizing enterocolitis, can result in malabsorption of a number of vitamins, especially vitamin B12. Chronic hemodialysis is an example of a clinical scenario in which excessive removal of vitamin from the bloodstream could produce untoward clinical effects. Any of the previously mentioned scenarios could lead to deficiency of a number of vitamins, which could make recognition of a specific vitamin deficiency syndrome, such as
Encyclopedia of the Neurological Sciences, Volume 4
pellagra or scurvy, quite difficult. The common practice of prescribing multivitamin preparations for nonspecific indications can further blunt the recognition of vitamin deficiencies by leading to partially treated states. Given this problem, the possibility of vitamin deficiency should be considered in many patients, especially in those in whom one of the clinical scenarios discussed previously is operative. Vitamin ‘deficiency’ is a relative term. Some patients appear to have increased needs for vitamins. Periods of rapid growth may be accompanied by an increased need for vitamins in normal patients. For example, pregnant women require a modest increase in the daily requirement for vitamins relative to the prepartum state. In contrast to these transient periods of increased vitamin needs in normal patients, certain patients have chronically increased vitamin requirements. A number of primary inborn errors of metabolism are the result of the production of mutant enzymes with lower affinity for vitamin-derived cofactors. Consequently, higher vitamin levels may be required to produce cofactor levels adequate for the production of functional enzymes. The net result is a chronic requirement for a much higher amount of a specific vitamin than the normal patient requires. Often, this requirement is more than can be provided by a normal diet and results in a vitamin dependency state. Recognition that vitamin dependency can compound a given primary metabolic disorder is important because the clinical condition may be improved, sometimes dramatically so, by vitamin supplementation at appropriate doses. The key principle is that primary vitamin deficiency can produce secondary metabolic disease and, conversely, primary metabolic disease can result in secondary vitamin dependency. Several hypervitaminoses or vitamin intoxication states have also been well described. These usually arise from accidental ingestions, unusual diets, or deliberately excessive vitamin supplementation. For most vitamins, ingestion of 10–100 times the recommended daily allowance is required before symptoms become apparent.
See also: Amino Acid Disorders. Neuropathies, Nutritional. Thiamine (Vitamin B1) and Beri-Beri. Vitamin A. Vitamin D
Further Reading Pfeffer G, Majamaa K, Turnbull DM, Thorburn D, and Chinnery PF (2012) Treatment for mitochondrial disorders. Cochrane Database of Systematic Reviews. Issue 4. Art. No.: CD004426. DOI: 10.1002/14651858.CD004426.pub3 Snodgrass SR (1992) Vitamin neurotoxicity. Molecular Neurobiology 6: 41–73.
doi:10.1016/B978-0-12-385157-4.00115-9
721