Pyridoxine, or vitamin B6, is a coenzyme in reactions of amino acid, carbohydrate, and fat metabolism. Vitamin B6 is actually a term used for a group of vitamins with similar functions: pyridoxine, pyridoxal, and pyridoxamine. All are precursors to pyridoxine coenzyme pyridoxalphosphate (PLP). This coenzyme is involved in reactions involving many systems within the body. PLP has a role in gluconeogenesis through transaminase reactions. The conversion of tryptophan to niacin utilizes an enzyme that requires PLP. In red blood cells PLP is a coenzyme for transaminases. PLP is also involved in the synthesis of several neurotransmitters, such as serotonin, taurine, dopamine, gamma-aminobutyric, and norepinephrine. Intake of vitamin B6 has been associated with immune function. Low intake of B6 results in a decrease of interleukin-2 and lymphocyte proliferation. The effect of B6 on lipid metabolism is still under debate but it may be involved in the conversion of linoleic acid to arachidonic acid.
Pyridoxine is absorbed passively into the jejunum. The bioavailability of vitamin B6 is quite good, more than 75% from foods. Fiber may limit availability. Pyridoxine is transported in the blood bound to albumin and hemoglobin. The majority of B6 is stored in the muscle.
Vitamin B6 metabolism is affected by an alteration in tryptophan metabolism or plasma PLP concentration. Many conditions result in altered tryptophan metabolism. These include asthma, breast cancer, diabetes, and rheumatoid arthritis. PLP plasma concentration is decreased in asthma, alcoholism, breast cancer, diabetes, renal disease, sickle-cell anemia, and smoking.
Drugs that have been shown to effect B6 metabolism include isoniazid, ethinylestradiol, penicillamine, theophylline, and caffeine. Megadoses of vitamin B6, one gram or more per day, have been reported to cause nerve damage.
The DRI for vitamin B6 is: 0.1-0.3 mg for infants, 0.5-0.6 mg for children, 1.0-1.3 mg for adolescents, 1.3-1.7 mg for men, 1.3-1.5 mg for women, 1.9 mg for pregnant women, and 2.0 mg for lactating women. Requirements of vitamin B6 are increased by high protein intake and possibly with exercise.
Good sources of vitamin B6 are meats, grains, and nuts.
Vitamin B6 has been used for Down's syndrome, autism, gestational diabetes, diabetic neuropathy, and depression. Some beneficial effects have been reported in the literature. Therapeutic dosages range from 50 to 100 mg. Doses over 2000 mg have been associated with nerve toxicity exhibited by tingling feet, loss of muscle coordination, and degeneration of nerve tissue.