Because vitamin B6 is present in most foods, dietary deficiency is rare. Secondary deficiency may result from various conditions. Symptoms can include peripheral neuropathy, a pellagra-like syndrome, anemia, and seizures, which, particularly in infants, may not resolve when treated with antiseizure medications. Impaired metabolism (dependency) is rare; it causes various symptoms, including seizures, intellectual disability, and anemia. Diagnosis is usually clinical; no laboratory test readily assesses vitamin B6 status. Treatment consists of giving oral vitamin B6 and, when possible, treating the cause.
Vitamin B6 includes a group of closely related compounds: pyridoxine, pyridoxal, and pyridoxamine. They are metabolized in the body to pyridoxal phosphate, which acts as a coenzyme in many important reactions in blood, central nervous system, and skin metabolism. Vitamin B6 is important in heme and nucleic acid biosynthesis and in lipid, carbohydrate, and amino acid metabolism (see table Sources, Functions, and Effects of Vitamins).
Dietary sources of vitamin B6 include organ meats (eg, liver), whole-grain cereals, fish, and legumes. (See also Overview of Vitamins.)
Etiology of Vitamin B6 Deficiency and Dependency
Dietary vitamin B6 deficiency, though rare, can develop because extensive processing can deplete foods of vitamin B6.
Secondary vitamin B6 deficiency most often results from
isoniazid, cycloserine, hydralazine, corticosteroids, penicillamine)
Excessive loss during hemodialysis
Rarely, secondary deficiency results from increased metabolic demand (eg, in hyperthyroidism).
Rare inborn errors of metabolism can affect pyridoxine metabolism.
Symptoms and Signs of Vitamin B6 Deficiency and Dependency
Vitamin B6 deficiency causes peripheral neuropathy and a pellagra-like syndrome, with seborrheic dermatitis, glossitis, and cheilosis, and, in adults, can cause confusion, electroencephalogram abnormalities, and seizures. Any relationship between pyridoxine and anxiety or depression is uncertain.
Rarely, deficiency or dependency causes seizures in infants. Seizures, particularly in infants, may be refractory to treatment with antiseizure medications.
Normocytic, microcytic, or sideroblastic anemia can also develop.
Diagnosis of Vitamin B6 Deficiency and Dependency
Clinical evaluation
Vitamin B6 deficiency should be considered in
Any infant who has seizures
Any patient who has seizures refractory to treatment with antiseizure medications
Any patient with deficiencies of other B vitamins, particularly in patients with alcohol use disorder or protein-energy undernutrition
Diagnosis of vitamin B6 deficiency is usually clinical. There is no single accepted laboratory test of vitamin B6 status; measurement of serum pyridoxal phosphate is most common.
Pearls & Pitfalls
Treatment of Vitamin B6 Deficiency and Dependency
Pyridoxine
Elimination of risk factors when possible
For secondary vitamin B6 deficiency, causes (eg, use of pyridoxine-inactivating medications, malabsorption) should be corrected if possible.
isoniazid
For deficiency due to increased metabolic demand, amounts larger than the daily recommended intake may be required. For most cases of inborn errors of metabolism, high doses of pyridoxine may be effective.
Key Points
Vitamin B6 deficiency is usually caused by pyridoxine-inactivating medications (eg, isoniazid), protein-energy undernutrition, malabsorption, alcohol use disorder, or excessive loss.
Deficiency can cause peripheral neuropathy, seborrheic dermatitis, glossitis, and cheilosis, and, in adults, confusion and seizures.
Suspect and diagnose based on clinical findings.
