Vitamin B6

Vitamin B6 is found in the human body in six different forms: pyridoxamine (amine form), pyridoxal (aldehyde form), and pyridoxine or pyridoxal (alcohol form).¹ Pyridoxine is found in plant foods, while all other forms are mostly in meat, dairy, and dairy products.¹ Vitamin B6 is relatively heat and light-stable, although some losses may occur during cooking.¹

Absorption/Circulation/Excretion: Any vitamin B6 paired with phosphate is freed before absorption with intestinal phosphatase enzymes.¹ Freed vitamin B6 is absorbed in the small intestine via diffusion or a carrier-mediated mechanism.¹ The absorption rate for vitamin B6 ranges between 70-85%; higher absorption rates are associated with lower dietary intake.¹ 

Within intestinal cells, some vitamin B6 can be paired with phosphate (phosphorylation) to keep vitamin B6 from leaving the intestinal cell.¹ For example, pyridoxine and pyridoxal are converted into pyridoxal phosphate via enzymes.¹

Within circulation, pyridoxal phosphate is the primary form found (60%) bound to protein or in red blood cells.¹ Pyridoxamine and pyridoxal are also found in the blood. Vitamin B6 is metabolized in the liver into pyridoxal phosphate from pyridoxal, pyridoxamine, and pyridoxine.¹

Pyridoxal is also catabolized into 4-pyridoxic acid and pyridoxal lactone within the liver and other tissues and excreted in urine.¹

Functions: 

Vitamin B6 in the form of pyridoxal phosphate is used in a variety of metabolic reactions and the synthesis of new amino acids and neurotransmitters. Pyridoxal phosphate is used in the following reactions: 

Transamination - Transforming one amino acid into another by moving an amino group to a carbon skeleton in a Schiff base (a compound with carbon and nitrogen bonds).¹ For example, a donor amino acid donates the amino group to an alpha ketoacid, forming substances such as glutamic acid, oxaloacetate, alanine, pyruvate, etc.¹ The reactions are reversible and amino acids can be passed to other substances to form new molecules.¹

Deamination - Certain amino acids are deaminated (removal of the amino group) using pyridoxal phosphate within deaminase enzymes. 

Decarboxylation - Removal of a carboxyl group from molecules. 

Amino-levulinic acid (ALA)  synthesis - ALA synthetase uses pyridoxal phosphate to create ALA which is further converted into protoporphyrin and heme. Pyridoxal phosphate plays a role in incorporating iron into heme.¹ Deficiency in vitamin B6 can cause hypochromic microcytic anemia to occur.¹

Other reactions that use pyridoxal phosphate include transulfhydration and desulfhydration.¹

Pyridoxal phosphate is also responsible for the synthesis of neurotransmitters (such as dopamine, histamine, serotonin, etc) and the breakdown of glycogen.¹ It is involved in conversions such as methionine to cysteine and tryptophan to niacin.¹ Pyridoxal phosphate is involved in the synthesis of sphingolipids and phospholipids; it is also responsible for linoleic acid conversion into arachidonic acid.¹

Dietary Reference Intake (DRI) - The recommended dietary allowance (RDA) of vitamin B6 for adult men and women over 19 years old is 1.3 milligrams/day.² 

For those over 51 years old, an increase in vitamin B6 is observed at 1.7 mg/day and 1.5 mg/day for men and women.² 

Deficiency/Toxicity - Vitamin B6 deficiency is uncommon but can occur for at-risk individuals, such as those taking medications for tuberculosis or infants fed formula in which vitamin B6 was destroyed.¹ Those who are on corticosteroids, anticonvulsants, or drink alcohol may be at risk for deficiency.¹ Symptoms include CNS abnormalities, poor growth, weakness, dermatitis with lesions around the mouth, etc.¹

Toxicity symptoms (through supplementation) can cause nerve damage and numbness.¹

Dietary Sources of Vitamin B6  

Vitamin B6 is found in both plant and animal products. Pyridoxal and pyridoxamine are found almost exclusively in animal foods, and pyridoxine is found in plant foods. Milling of grains removes some vitamin B6 when the germ and bran are removed, and cooking processes (exposure to heat and alkaline solutions) can decrease vitamin B6 in foods. Examples of food with vitamin B6 are: 

Meat/animal products -  tuna, liver, salmon, chicken, beef, egg 

Grains/legumes/nuts - cereals, kidney beans, walnuts, lentils 

Vegetables/Fruits - bananas, Brussels sprouts, sweet potato, cantaloupes

Source(s):

1. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019).

2. https://ods.od.nih.gov/factsheets/VitaminB6-HealthProfessional/