Yet another one of our laundry list of B vitamins, Vitamin B6, also known as pyridoxine, is water-soluble and very important in metabolism. Discovered in 1934 by a Hungarian doctor named Paul Gyorgy its active form is called pyridoxyl phosphate. It is essential for multiple parts of metabolism of amino acids and for gluconeogenesis which is the body’s ability to make glucose for energy using glycogen and fat stores in the body. It is also essential for neurotransmitter synthesis which are the brain’s communication molecules. It also aids in the synthesis of histamine and hemoglobin. Lastly it directly affects gene expression and when present can decrease the expression of glucocorticoids and increase the synthesis of albumin. All making this vitamin very important in many essential functions of the body.
Vitamin B6 is found in a wide variety of food including meat, vegetables (most notably carrots, and spinach), bananas and nuts. In plant sources it is in the form pyridoxine which is quite stable but in meats and animal products it is in the form of pyridoxal or pyridoxamine which are far less stable and prone to break down during cooking and food processing. Yet again, in grains it is mostly in the outer shell and germ so it is lost in highly processed grains (so I will say it again, if you eat grains eat the WHOLE grain).
Deficiency causes mostly skin manifestations including seborrheic dermatitis (dry, itchy, scaly, flaky, red patches of skin on the face and head), tongue inflammation, angular chelitis (inflammation of the corners of the mouth), and intertrigo (inflammation of the skin folds). It can also cause neurologic manifestations such as sleepiness and neuropathy. Deficiency is rare in the healthy population but diseases such as diabetes, HIV, kidney disease, and rheumatoid arthritis seem to put people at risk for deficiency even if they take in enough Vitamin B6. Medications such as steroids and seizure medications can also deplete vitamin B6.
Toxicity has never been seen from food sources but at doses of 1000 mg a day (770 times the recommended daily allowance) in supplement form has been reported to cause a neuropathy that can lead to pain and numbness in the legs. The recommended daily allowance is 1.3 mg. Likely closer to 50 mg per day is conducive to good health and it should be taken in conjunction with the other B vitamins in a B complex if it is going to be taken in supplement form.
Studies of Vitamin B6 to treat diseases are lacking. A 1992 study by Deijen et al. was a well designed placebo controlled trial and showed that vitamin B6 improved memory, most prominently long-term memory. A 2002 study by Bryan et al had the same finding. A 1999 study by Wyatt etl al. reviewed 9 studies with over 900 patients and found that a dose of 100 mg per day of Vitamin B6 may be beneficial for treating the symptoms of PMS. However they do mention that many of the studies were of poor quality so larger studies should be done. Some have thought that Vitamin B6 would be useful in treating depression given that it is needed to help make neurotransmitters including serotonin. A review in 2005 by William et al of the available placebo controlled trials for vitamin B6 in depression found no statistically significant benefit. Two studies have shown that Vitamin B6 may help alleviate morning sickness during pregnancy. The first was a 1991 study by Sahakian et al. and the second was in 1995 by Vutyavanich et al. Both showed statistically significant benefit.
Overall like many of the B vitamins Pyridoxine has gone mostly unstudied in its role to treat disease. It does seem to possibly have some small effect on memory and nausea during pregnancy and is likely safer than pharmaceutical drugs during pregnancy if taken in low doses. PMS also seems to be a promising use. Both of these need larger better studies to establish a clear role. But mostly I feel it is a mistake to take each of the B vitamins in isolation. Their metabolism is intricately woven together and taking too much of one can deplete or compete with the others. I feel it is best to take them as nature usually gives them to us in B complex form. Of course the safest and best way is by eating plenty of vegetables and nuts.
References
Medical Revolt 
I am enjoying your vitamin researches very much! Thanks
B6 before bedtime gives vivid dreams.