Vitamin B6 is a water-soluble vitamin and is part of the vitamin B complex group.‡
FROM WILKEPEDIA: Vitamin B6 is widely distributed in foods in both its free and bound forms. Good sources include meats, whole grain products, vegetables, nuts and bananas. Cooking, storage and processing losses of vitamin B6 vary and in some foods may be more than 50%, depending on the form of vitamin present in the food. Plant foods lose the least during processing as they contain mostly pyridoxine which is far more stable than the pyridoxal or pyridoxamine found in animal foods. For example, milk can lose 30-70% of its vitamin B6 content when dried. Vitamin B6 is found in the germ and aleurone layer of grains and milling results to the reduction of this vitamin in white flour. Freezing and canning are other food processing methods that results in the loss of vitamin B6 in foods.
The classic clinical syndrome for B6 deficiency is a seborrhoeic dermatitis-like eruption, atrophic glossitis with ulceration, angular cheilitis, conjunctivitis, intertrigo, and neurologic symptoms of somnolence, confusion, and neuropathy.
While severe vitamin B6 deficiency results in dermatologic and neurologic changes, less severe cases present with metabolic lesions associated with insufficient activities of the coenzyme pyridoxal phosphate. The most prominent of the lesions is due to impaired tryptophan-niacin conversion. This can be detected based on urinary excretion of xanthurenic acid after an oral tryptophan load. Vitamin B6 deficiency can also result from impaired transsulfuration of methionine to cysteine. The pyridoxal phosphate-dependent transaminases and glycogen phosphorylase provide the vitamin with its role in gluconeogenesis, so deprivation of vitamin B6 results in impaired glucose tolerance.
Adverse effects have only been documented from vitamin B6 supplements and never from food sources. This article only discusses the safety of the common supplemental form of vitamin B6 pyridoxine (for a full discussion please see pyridoxine). Toxicologic animal studies identify specific destruction of the dorsal root ganglia which is documented in human cases of overdosage of pyridoxine. Although vitamin B6 is a water-soluble vitamin and is excreted in the urine, doses of pyridoxine in excess of the RDI over long periods of time thus result in painful and ultimately irreversible neurological problems.
The primary symptoms are pain and numbness of the extremities, and in severe cases difficulty walking. Sensory neuropathy typically develops at doses of pyridoxine in excess of 1,000 mg per day. However, there have been a few case reports of individuals who developed sensory neuropathies at doses of less than 500 mg daily over a period of months. None of the studies, in which an objective neurological examination was performed, found evidence of sensory nerve damage at intakes of pyridoxine below 200 mg/day. This condition is usually reversible when supplementation is stopped.
Vitamin B6 has been used to treat nausea and vomiting in early pregnancy for decades, commonly in conjunction with other medications such as metoclopramide or doxylamine. Alone, it has been found safe and effective, though any woman’s prenatal caregiver must help guide treatment for these symptoms.
At least one preliminary study has found that this vitamin may increase dream vividness or the ability to recall dreams. It is thought that this effect may be due to the role this vitamin plays in the conversion of tryptophan to serotonin.
The intake of vitamin B6, from either diet or supplements, could cut the risk of Parkinson’s disease by half according to a prospective study from the Netherlands. “Stratified analyses showed that this association was restricted to smokers,” wrote the authors.
Pyridoxine has a role in preventing heart disease. Without enough pyridoxine, a compound called homocysteine builds up in the body. Homocysteine damages blood vessel linings, setting the stage for plaque buildup when the body tries to heal the damage. Vitamin B6 prevents this buildup, thereby reducing the risk of heart attack. Pyridoxine lowers blood pressure and blood cholesterol levels and keeps blood platelets from sticking together. All of these properties work to keep heart disease at bay.
Nutritional supplementation with high dose vitamin B6 and magnesium is one of the most popular alternative medicine choices for autism but randomised control trials have had mixed results and small sample sizes mean that no conclusions can be drawn as to the efficacy of this treatment.
Some studies suggest that the vitamin B6-magnesium combination can also help attention deficit disorder, citing improvements in hyperactivity, hyperemotivity/aggressiveness and improved school attention.
A lack of the vitamin may play a role in sensitivity to monosodium glutamate (MSG), a flavor enhancer. This sensitivity can cause headaches, pain and tingling of the upper extremities, nausea, and vomiting. In both of these syndromes, supplementation of pyridoxine alleviates symptoms only when people were deficient in the vitamin to begin with.
If people are marginally deficient in vitamin B6, they may be more susceptible to carpal tunnel syndrome. Carpal tunnel syndrome is characterized by pain and tingling in the wrists after performing repetitive movements or otherwise straining the wrist on a regular basis. Vitamin B6 has been shown in at least two small-scale clinical studies to have a beneficial effect on carpal tunnel syndrome, particularly in cases where no trauma or overuse etiology for the CTS is known.
Vitamin B6 has long been publicized as a cure for premenstrual syndrome (PMS). Study results conflict as to which symptoms are eased, but most of the studies confirm that women who take B6 supplements have reductions in bloating, breast pain, and premenstrual acne flare, a condition in which pimples break out about a week before a woman’s period begins.There is strong evidence that pyridoxine supplementation, starting ten days before the menstrual period, prevents most pimples from forming. This effect is due to the vitamin’s role in hormone and prostaglandin regulation. Skin blemishes are typically caused by a hormone imbalance, which vitamin B6 helps to regulate.
Mental depression is another condition which may result from low vitamin B6 intake. Because of pyridoxine’s role in serotonin and other neurotransmitter production, supplementation often helps depressed people feel better, and their mood improves significantly. It may also help improve memory in older adults. However, the effectiveness as treatment for PMS, PMDD, and clinical depression is debatable.
It is also suggested that ingestion of vitamin B6 can alleviate some of the many symptoms of an alcoholic hangover and morning sickness from pregnancy. This might be due to B6’s mild diuretic effect. Though the mechanism is not known, results show that pyridoxamine has a therapeutic effects in clinical trials for diabetic nephropathy.
Larsson et.al. have shown that vitamin B6 intake and pyridoxal phosphate (PLP) levels are inversely related to the risk of colon cancer. While in their study the correlation with B6 intake was moderate, it was quite dramatic with PLP levels where the risk of colon cancer was nearly decreased in half. 
‡ These statements have not been evaluated by the FDA. These products are not intended to diagnose, treat, cure or prevent any disease.