Vitamin B6

Scientific studies on the relationship between vitamin B6 and disease prevention:
A review article (a collection of scientific studies on a specific topic) of randomized, placebo-controlled double blind clinical trials (RCTs) will answer the following question:
"Do taking dietary supplements make sense?" Yes for a positive conclusion and no for a negative conclusion.

A review article (a collection of scientific studies on a specific topic) of cohort studies or case-control studies will answer the following question:
"Should I change my diet?".

1. Vitamin B6 supplements reduce nausea and vomiting during pregnancy
2. Dietary intake of vitamin B6 and folate reduces stroke
3. Oral vitamin B supplementation does not prevent cognitive decline in cognitively unimpaired individuals
4. Higher vitamin B2 and B6 dietary intake decreases ER-/PR- breast cancer
5. 1 μg/day dietary B12 intake increases esophageal cancer
6. High blood vitamin B6 levels reduce colorectal cancer
7. High dietary vitamin B9, D, B6 and B2 intake reduces risk of colorectal cancer

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• Vitamin B6 is a water-soluble vitamin that is present in the body as six vitamers: the alcohol pyridoxine (pyridoxol), aldehyde pyridoxal, amine pyridoxamine and their respective 5’-phosphorylated esters.
• Animal-derived vitamin B6 consists mostly of phosphorylated pyridoxal and pyridoxamine while that from plants consists largely of free and bound pyridoxine.
• Phosphorylated B6 vitamers are converted to their free bases by intestinal alkaline phosphatases and these are then absorbed from the upper small intestine by a carrier-mediated system.
• In the liver pyridoxine, pyridoxamine and pyridoxal are phosphorylated by pyridoxal kinase to their 5’-phosphate esters and pyridoxine phosphate and pyridoxamine phosphate oxidized by pyridox(am)ine phosphate oxidase (PNPO) to form pyridoxal phosphate (PLP).
• The metabolically active coenzyme form of vitamin B6 is pyridoxal 5’-phosphate (PLP).
  PLP is the major form that is used by pyridoxine-dependent enzymes. These enzymes catalyse more than 100 essential biochemical reactions in human metabolism.
• Vitamin B6 is a cofactor in the conversion of tryptophan to 5-hydroxytryptamine and of methionine to cysteine.
• Vitamin B6 acts as a cofactor for 2 enzymes, cystathionine B-synthase and cystathionase, which convert homocysteine to cysteine.
• Vitamin B6 is involved in cellular replication, antibody production and the function of the nervous system.
  Vitamin B6 is also involved in the biosynthesis of several neurotransmitters, including serotonin, gamma amino-butyric acid (GABA), dopamine and noradrenaline and so has a role the regulation of mental processes and mood.
  Furthermore, vitamin B6 is also involved in sodium-potassium balance, histamine metabolism, the conversion of tryptophan to niacin, the absorption of vitamin B12 and the production of hydrochloric acid in the gastrointestinal tract.
• Vitamin B6 deficiency is unusual in humans.
  Vitamin B6 deficiency may be a factor in hyperhomocysteinaemia (>12 μmol/L), which is associated with an increased risk of cardiovascular disease.
  Vitamin B6 deficiency has also been reported to be associated with immune dysfunction, kidney stones, cancer and carpal tunnel syndrome.
• High intakes of vitamin B6 from food sources have not been reported to cause adverse effects.
• The bioavailability of vitamin B6 from animal products is quite high, reaching 100% for many foods. In general, the bioavailability from plant foods is lower.
  The bioavailability is generally in the region of 75% in a mixed diet.
• Recommended intakes of vitamin B6 are based on protein intake. The recommended intakes have been set at 15 mcg/g protein for adults. This is equivalent approximately 1.2-1.8 mg/day.
  Pregnant and lactating women and older people, who have low vitamin B6 levels, can usually increase their intake through a high-protein diet.
• The half-life of pyridoxine is 15-20 days.
• Vitamin B6 is stored in the liver, with about 50% also being present in muscle, bound to glycogen phosphorylase. Pyridoxine is also stored in the brain.
• Vitamin B6 in plasma is mainly PLP (60%), PN (15%) and PL (14%). These vitamers circulate bound to albumin.
• The PLP concentration is a direct indicator for the activity of vitamin B6 in the organism.
• The main form of vitamin B6 used in supplements is pyridoxine.
• The total body storage for adults is 167 mg or sufficient for 2-6 weeks.
  Muscle glycogen phosphorylase is a major repository of vitamin B6 in the body, accounting for at least 80% of the total body pool.
• The tolerable upper level (UL) is 25 mg vitamin B6 per day.