Vitamin B₆ Definition

Vitamin B₆ is a water-soluble B complex vitamin that is required by the body to remain healthy. Humans cannot make vitamin B₆, so they must get it from foods in their diet. Vitamin B₆ is also called pyridoxine.

Vitamin B₆ is a coenzyme or cofactor, meaning that it functions in conjunction with many different enzymes and is necessary for more than 100 different enzyme reactions that are involved in human metabolism. Thus, vitamin B₆ has a broad range functions that affect many different bodily systems. It is involved in the metabolism of neurotransmitters that carry the impulses between nerves, and thus is essential for nervous system function. It is involved in the formation of new skin and other cells. It is involved in the conversion of stored carbohydrates into energy and helps keep blood sugar (glucose) within a normal range. Vitamin B₆ is required for producing hemoglobin that carries oxygen in red blood cells. It is important for breaking down proteins into amino acids, so that the more protein that is consumed, the higher the vitamin B₆ requirement. Vitamin B₆ is important for the functioning of the immune system, because it is involved in the production of antibodies for fighting disease. Vitamin B₆ is involved in brain development in the fetus and infant. It is important for the production of DNA, the genetic material of all cells, and, along with vitamin B₁₂ and folic acid (vitamin B₉), it helps regulate the levels of an amino acid in the blood called homocysteine that may be linked to heart disease.

Vitamin B₆

SOURCE: Office of Dietary Supplements. National Institutes of Health. “Vitamin B₆: Fact Sheet for Health Professionals.” https://ods.od.nih.gov/factsheets/VitaminB6-HealthProfessional (accessed April 19, 2018).

B₆ is unique among vitamins in that either a deficiency or an excess can cause peripheral neuropathy, a condition of nerves outside of the central nervous system that is characterized by muscle weakness, numbness, and pain. Vitamin B₆ deficiency can also cause problems with the skin and the blood, including a type of anemia in which there are too few functioning red blood cells.

Vitamin B₆ was discovered in the 1930s and is one of the best-studied of the vitamins. Few vitamins and minerals have such a broad and diverse range of activities in the body as vitamin B₆.

There are three different forms of vitamin B₆—pyridoxine, pyridoxal, pyridoxamine—and three derivatives of each of these. All forms of vitamin B₆ are converted in the body into the same active molecule, pyridoxal 5’-phosphate. Vitamin B is a water-soluble vitamin. Unlike the fat-soluble vitamins A, D, E, and K, it is not stored in the body but rather is excreted in urine.

The Institute of Medicine (IOM) of the U.S. National Academy of Sciences has developed dietary reference intake (DRI) values for vitamins and minerals. DRIs consist of three sets of numbers. The recommended dietary allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97%–98% of the population. The adequate intake (AI) is an estimate of the daily requirement when there is not enough information to determine an RDA. The tolerable upper intake level (UL) is the average maximum amount that can be taken daily without risking negative side effects. These amounts are in milligrams (mg) for vitamin B₆:

Vitamin B₆ is found in many foods including chicken, pork, liver, fish, nuts, milk, potatoes, dark green vegetables, dried beans, avocados, bananas, wheat germ, and whole grains. It may be added to fortified breakfast cereals, breads, flour, and other grains. However, heating and freezing foods reduces their vitamin B₆ content. As much as 60%–80% of the vitamin B₆ in vegetables is lost during canning, and about 40% of B₆ in fruit is lost by canning. Processing of meat and grains also reduces their vitamin B₆ content.

Approximate vitamin B₆ contents of some common foods include:

Enzymes are proteins that promote (catalyze) chemical reactions within the body. Coenzymes or cofactors are molecules that are required to join with enzymes for their activity. Some of the more than 100 enzyme reactions that require vitamin B₆ include:

Vitamin B₆ deficiency is very rare in the United States, although it is associated with a few rare genetic disorders. Alcoholics are at higher risk for developing vitamin B₆ deficiency, as are the elderly and those who take certain medications or who have certain inflammatory disorders. People with kidney disorders or kidney transplants or with autoimmune disorders such as rheumatoid arthritis, celiac disease, Crohn's disease, ulcerative colitis, or inflammatory bowel disease sometimes develop vitamin B₆ deficiency. Internationally, malnutrition, lack of a varied diet, or a diet limited to non-enriched grains increases the risk of vitamin B deficiency.

Symptoms of vitamin B₆ deficiency develop slowing. They include skin and tongue sores and inflammation, scaly lips, or cracks at the corners of the mouth. Other symptoms include itchy rashes and anemia. Very low levels of vitamin B₆ can cause irritability, depression, confusion, and a weakened immune system. Infants with vitamin B₆ deficiency may become irritable or develop very sensitive hearing or seizures.

The tolerable upper intake levels for vitamin B₆ are quite high—approximately 50 times the RDA, although there are no confirmed health benefits from large daily vitamin B₆ supplements. Even at high doses of vitamin B₆ supplements, few side effects have been reported, although these can include nausea, vomiting, and breast soreness. Very high doses (above 200 mg/day) over a long period can result in numbness in the arms and legs; burning, tingling, or pin-pricking of the hands and feet; problems with balance and coordination; and other sensory changes. These symptoms usually disappear within several months of stopping vitamin B₆ supplementation.

Vitamin B₆ reduces the effectiveness of tetracycline antibiotics, of the seizure drug phenytoin, and of levodopa used to treat Parkinson's disease. The tuberculosis drugs cycloserine (Seromycin) and isoniazid and penicillamine used to treat rheumatoid arthritis reduce the level of vitamin B₆ in the blood and may require B₆ supplementation on the advice of a physician. However, vitamin B₆ supplements may interact with cycloserine and worsen any seizures or nerve cell damage caused by the drug. Certain epilepsy drugs may both decrease vitamin B₆ levels, and vitamin B₆ may reduce the effectiveness of the drugs for controlling seizures. Theophylline, used to treat asthma and other lung diseases, can reduce vitamin B₆ levels and cause seizures.

Parents should be aware that the RDA and UL for vitamins and minerals, including vitamin B₆, are much lower for children than for adults. Accidental overdose can occur if children take adult vitamins or dietary supplements.

See also Arthritis diet ; Fertility ; Fertility diet ; Folate ; Vitamins .