Vitamin B6

Vitamin B6 refers to a group of chemically similar compounds—pyridoxine, pyridoxal, and pyridoxamine—all converted to the active coenzyme form pyridoxal 5′-phosphate (PLP). Discovered in the 1930s, vitamin B6 plays essential roles in amino acid metabolism, gluconeogenesis, neurotransmitter synthesis (serotonin, dopamine, GABA), hemoglobin formation, and modulation of steroid hormone action. Dietary sources include both animal (poultry, fish, organ meats) and plant (whole grains, legumes, nuts, bananas) foods. In the body, PLP acts as a cofactor for enzymes involved in transamination, deamination, and racemization critical for protein and amino acid metabolism. Vitamin B6 contributes to homocysteine metabolism via cystathionine β-synthase, reducing cardiovascular risk. Inadequate intake or impaired absorption leads to deficiency, manifesting as microcytic anemia, dermatitis, glossitis, depression, irritability, and neuropathy. Populations at higher risk include the elderly, alcoholics, individuals with malabsorptive disorders, and those on medications like isoniazid or oral contraceptives. The Recommended Dietary Allowance for adults ranges from 1.3 to 1.7 mg/day. Therapeutic doses up to 100 mg/day may be used for conditions like carpal tunnel syndrome or PMS, but chronic intake above the Tolerable Upper Intake Level of 100 mg/day can cause sensory neuropathy. PLP is absorbed in the jejunum and binds to albumin for transport; excess is catabolized to 4-pyridoxic acid and excreted in urine, underscoring its water-soluble nature.

Vitamin B6 functions as the coenzyme pyridoxal 5′-phosphate (PLP). PLP-dependent enzymes include aminotransferases responsible for amino acid interconversion and deaminases involved in amino acid catabolism. In neurotransmitter biosynthesis, PLP is a cofactor for aromatic L-amino acid decarboxylase and glutamate decarboxylase, producing serotonin, dopamine, and GABA. For homocysteine remethylation, PLP is essential for cystathionine β-synthase, catalyzing homocysteine conversion to cystathionine, thereby lowering plasma homocysteine. PLP also participates in glycogen phosphorylase activation for gluconeogenesis and glycogenolysis. In heme synthesis, PLP-dependent δ-aminolevulinic acid synthase catalyzes the condensation of glycine and succinyl-CoA, initiating porphyrin formation. Furthermore, vitamin B6 facilitates sphingolipid synthesis and modulates steroid hormone action by influencing gene expression. Absorbed in the jejunum via passive diffusion, vitamin B6 binds to albumin and crosses cell membranes to be phosphorylated by pyridoxal kinase. Excess is catabolized to 4-pyridoxic acid and excreted in urine.

Vitamin B6 is abundant in various foods. Animal-based sources include poultry (chicken, turkey), fish (salmon, tuna), and organ meats (liver). Plant sources encompass whole grains, legumes (chickpeas, lentils), nuts (walnuts, peanuts), and fruits like bananas and avocados. Fortified cereals also provide significant amounts.

Vitamin B6 deficiency can manifest as microcytic anemia due to impaired hemoglobin synthesis and neurologic symptoms including peripheral neuropathy, seizures, irritability, and depression. Dermatologic signs include cheilosis, glossitis, and seborrheic dermatitis. Elevated homocysteine may increase cardiovascular risk.

Recommended Dietary Allowances for vitamin B6 are established by the Institute of Medicine (US) and vary by age, sex, and life stage to prevent deficiency and support metabolic functions.

Vitamin B6 is available as pyridoxine hydrochloride tablets, pyridoxal 5′-phosphate (active form) supplements, and fortified foods. Bioavailability depends on form and gastrointestinal function, with PLP being immediately active.