Vitamin B6 is an essential, water-soluble nutrient the human body requires but cannot synthesize. Since the body does not store large reserves, it must be consistently obtained through diet. Vitamin B6 is a group of six related compounds, collectively called vitamers. The biologically active form is pyridoxal 5′-phosphate (PLP), which functions as a coenzyme to facilitate a vast number of metabolic processes. PLP is involved in over 100 enzyme reactions.
Biological Roles in the Human Body
The most prominent role of Vitamin B6 is its participation in the metabolism of amino acids, the building blocks of proteins. PLP acts as a coenzyme for transamination and decarboxylation reactions, which are fundamental processes for both breaking down and creating amino acids. This function is particularly important for the body to utilize dietary protein and manage nitrogen balance effectively.
PLP-dependent enzymes are also involved in the synthesis of several neurotransmitters that regulate brain and nerve function. For example, it is required for the creation of serotonin, which regulates mood and sleep, and dopamine, involved in motivation and reward. The vitamin also assists in the production of gamma-aminobutyric acid (GABA), a major inhibitory neurotransmitter that helps calm the nervous system.
Beyond amino acid and nervous system function, Vitamin B6 is necessary for the formation of hemoglobin, the protein in red blood cells that transports oxygen. It is a cofactor for the enzyme 5-aminolevulinic acid synthase, which is required for the synthesis of heme. Without sufficient B6, the body can develop microcytic anemia, characterized by small, pale red blood cells.
The vitamin supports immune function by promoting the production of lymphocytes and interleukin-2, which are important components of the body’s defense system. PLP also links protein and carbohydrate metabolism by acting as a coenzyme for glycogen phosphorylase. This enzyme releases glucose from stored glycogen in the muscles and liver.
Dietary Intake and Sources
Because the body cannot produce Vitamin B6, the Recommended Dietary Allowance (RDA) guides daily intake to ensure adequate levels. For adult men up to 50 years old, the RDA is 1.3 milligrams (mg) per day, increasing to 1.7 mg for men over 50. Adult women aged 19 to 50 require 1.3 mg daily, which increases to 1.5 mg after age 50.
The best sources of B6 are widely distributed across different food groups, making deficiency rare in the general population. Meats and fish are rich sources, with high concentrations found in beef liver, tuna, salmon, and poultry. Starchy vegetables such as potatoes and non-citrus fruits like bananas are also excellent contributors to daily intake.
Many foods, including fortified breakfast cereals, also contain added Vitamin B6. Bioavailability, or the proportion of the vitamin that the body can absorb and utilize, is generally high, with approximately 75% of the B6 from a mixed diet being absorbed. However, some plant sources contain a glycosylated form of pyridoxine, which has a reduced bioavailability compared to the forms found in animal products.
Adverse Reactions and Toxicity
Unlike fat-soluble vitamins, it is virtually impossible to consume toxic levels of Vitamin B6 solely through food intake. Toxicity is almost exclusively associated with the long-term, excessive consumption of B6 supplements, often taken at doses far exceeding the RDA. The primary concern with high-dose B6 supplementation is the development of a condition known as peripheral neuropathy.
Peripheral neuropathy is a type of nerve damage that affects the nerves outside the brain and spinal cord. Symptoms typically begin with sensory changes, such as numbness, tingling, or a burning sensation, most often affecting the hands and feet in a “stocking-glove” pattern. In more severe cases, high intake can lead to a loss of coordination and difficulty walking.
To prevent this nerve damage, the established Upper Tolerable Intake Level (UL) for adults is set at 100 mg per day. While some people may tolerate higher doses without immediate effects, chronic intake above this UL increases the risk of adverse neurological reactions. When toxicity is suspected, stopping the supplement is the first step, though the nerve damage may take time to resolve and can sometimes be permanent.
It is important to distinguish toxicity symptoms from the effects of a severe B6 deficiency, which can include dermatitis, confusion, and a form of anemia. While deficiency is generally rare, taking high doses of B6 supplements causes a distinct injury to the sensory nerves, which is a key difference from the symptoms of low intake.