Vitamin B-6

Vitamin B-6 is found as 3 forms: pyridoxine hydrochloride [2-methyl-3-hydroxy-4, 5-bis (hydroxy-methyl) pyridine], pyridoxal and pyridoxamine. About 70-80% of the vitamin B-6 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. Vitamin B-6 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 plays a role in the immune system; thus adequate intake is important. 4-pyridoxic acid is the major excretory product.

Deficiencies:
Alterations in the function of the nervous system evidenced by electroencephalography are among the earst 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.

Clinical uses:
Pyridoxine-dependent seizures and some types of sideroblastic anemias respond to vitamin B-6 supplementation. Vitamin B-6 supplements may be required in conjunction with a number of drugs which have the side-effect of altering vitamin B-6 metabolism. Increased concentrations of pyridoxal phosphate in plasma are used as one of the criteria for diagnosing hypophosphatasia. Because vitamin B-6 metabolism is altered in a variety of disease states, there have been occasional suggestions that vitamin B-6 supplements may be beneficial in many conditions. However, at this time there is no consensus that vitamin B-6 is beneficial in conditions other than certain genetic defects.

Diet recommendations:

Age (years) mg/day
Infants	0-0.5	0.3
	0.5-1.0	0.6
Children	1-3	1.0
	4-6	1.1
	7-10	1.4
Females	11-14	1.4
	15+	1.6
Pregnant		2.2
Lactating		2.1
Males	11-14	1.7
	15+	2.0

Food sources:
White meats (poultry, fish, pork), bananas and whole grains are good sources of vitamin B-6. A glucoside form of pyridoxine in certain plant (not animal) products may limit availability of B-6.

Toxicity:
Excessive acute or chronic exposure to vitamin B-6 can be a neurotoxin. It appears that in most individuals oral intakes of less than 500 mg/day can be tolerated. Larger intakes should be avoided. Because individuals may vary in their susceptibility to toxicity, a physician should monitor intakes in excess of the Recommended Daily Allowances listed above.

Recent research:
Current studies involve the bioavailability of pyridoxine glycosides, which can account for a significant fraction of the vitamin B-6 in some plant products; improved methods of assessing vitamin B-6 status and requirements; and alterations in vitamin B-6 metabolism in various pathological conditions.

For further information:

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

Raiten, D.J. ed. (1995) Vitamin B6 Metabolism in Pregnancy, Lactation, and Infancy. CRC Press, Boca Raton, FL.

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