VITAMIN B-6

The chemical name of vitamin B₆ is pyridoxine hydrochloride; 2-methyl-3-hydroxy-4,5-bis (hydroxy-methyl) pyridine. Other forms of vitamin B₆ include pyridoxal, and pyridoxamine. In the body all three of these compounds can be phosphorylated. About 70-80% of the vitamin B₆ in the body is located in muscle bound to glycogen phosphorylase, an enzyme involved in releasing glucose from glycogen. About 10% is located in the liver; the remainder is distributed among the other tissues.

B₆ is one of the most versatile enzyme cofactors. It is involved in breaking more types of chemical bonds than most cofactors. It is listed in Enzyme Nomenclature as a component of approximately 120 enzymes including at least one entry in 5 of the 6 major enzyme classes. Pyridoxal phosphate is a cofactor in the metabolism of amino acids and neurotransmitters and in the breakdown of glycogen. Pyridoxal phosphate can bind to steroid hormone receptors and may have a role in regulating steroid hormone action. Pyridoxal phosphate can be converted to pyridoxamine phosphate which can also serve as an enzyme cofactor. 4-pyridoxic acid is the major excretory product.

Deficiencies: Alterations in the function of the nervous system evidenced by electroencephalography are among the earliest symptoms of vitamin B-6 deficiency. Severe deficiency may produce seizures, dermatitis, glossitis, cheilosis, angular stomatitis and anemia. Frank deficiencies are rare, but subclinical deficiencies may exist, especially in women and the elderly.

Diet recommendations: Adequate Intakes (AI) have been set at 0.1 mg/d for infants up to 6 mo. and 0.3 mg/d for 6 mo. to 1 yr. Recommended Dietary Allowances (RDAs) (mg/d) for children have been set at 0.5 (1-3 yr.), 0.6 (4-8 yr.), and 1.0 (9-13 yr.). For men the RDAs (mg/d) are 1.3 (14-50 yr.) and 1.7 (51+ yr.). For women the RDAs (mg/d) are 1.2 (14 - 18 yrs.), 1.3 (19-50 yr.), 1.5 (51+ yr.), 1.9 during pregnancy, and 2.0 during lactation.

Food sources: White meats (poultry, fish, pork), bananas and whole grains are good sources of vitamin B6.

Clinical uses: Pyridoxine-dependent seizures and some types of sideroblastic anemias respond to vitamin B6 supplementation. Vitamin B6 in conjunction with folate and vitamin B12 helps to lower plasma homocysteine, a risk factor for heart disease. Vitamin B6 supplements may be required in conjunction with a number of drugs which have the side-effect of altering vitamin B6 metabolism. Increased concentrations of pyridoxal phosphate in plasma are used as one of the criteria for diagnosing hypophosphatasia. Because vitamin B6 metabolism is altered in a variety of disease states, there have been suggestions that vitamin B6 supplements may be beneficial in many other conditions. However, convincing scientific support is not currently available.

Recent research: Current studies involve the bioavailability of pyridoxine glycosides, which can account for a significant fraction of the vitamin B6 in some plant products; improved methods of assessing vitamin B6 status and requirements; and alterations in vitamin B6 metabolism in various pathological conditions, particularly heart disease and homocysteine.

For further information:

Leklem, J. E. (1990) Vitamin B6. In: Handbook of Vitamins (Machlin, L. J., ed.), 2nd ed., pp. 341 - 392, Marcel Dekker, New York, NY

Committee on the Scientific Evaluation of Dietary Reference Intakes, Institute of Medicine (1998) Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic acid, Biotin, and Choline. National Academy Press, Washington, DC.

Prepared By:

Stephen P. Coburn, Ph.D.
Department of Biochemistry
Fort Wayne State Devlp Center
Fort Wayne, IN 46835
Phone: 219-485-7554 mx2636
FAX: 219-485-2863
Email: coburn@ipfw.edu

James E. Leklem, Ph.D.
Professor, Dept. of Nutrition and
Food Management
Oregon State University
Milam 108
Corvallis, OR 97331-5103
Phone: 541-737-0969
FAX: 541-737-6914
Email: leklemj@ccmail.orst.edu

Robert D. Reynolds, Ph.D.
Associate Prof. Dept of Human
Nutrition and Dietetics
University of Illinois at Chicago
1919 W. Taylor Street, M/C517
Chicago, IL 60612
Phone: 312-996-1207
FAX: 312-413-0319
Email: reynolds@uic.edu