ITAMIN 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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