Vitamin B-6 requirements of humans

Abstract

Vitamin B-6, also referred to as vitamin B 6, is the generic descriptor for all 3-hydroxy-2-methylpyridine derivatives possessing the biological activity of pyridoxine. Convincing evidence of the essentiality of the vitamin was demonstrated when hyperirritability and epileptiform-type seizures were reported in infants fed autoclaved commercial liquid formula low in vitamin B-6; administration of the vitamin corrected these symptoms. Vitamin B-6 status and requirements have been assessed using several methodologies. Dietary assessment usually consists of 24-h recall and/or 2–7-d intake records with nutrient composition being estimated using values given in the US Department of Agriculture's Handbook 8 series. Frequently used biochemical assessment methodologies include xanthurenic acid excretion after a tryptophan load, urinary 4-pyridoxic acid level, urinary and plasma total vitamin B-6 levels, erythrocyte alanine and aspartate aminotransferase activity coefficients, and plasma pyridoxal-5′-phosphate concentration. The most accepted vitamin B-6 status parameter is plasma pyridoxal-5′-phosphate concentration; however, more than one method should be utilized in evaluating status. The minimum vitamin B-6 requirement of men as evaluated by different researchers using various methodologies seems to be about 0.65 to 1.25 mg/d; similar estimates have been made for women. Estimates of vitamin B-6 requirements of pregnant and lactating women range from about 2.1 to 7.6 mg/d; pregnancy may affect many of the parameters utilized for assessing vitamin B-6 status. Controlled feeding studies utilizing infants, children, and adolescents are mainly lacking. Recommended Dietary Allowances (RDAs) for vitamin B-6 have existed since 1968; these RDAs are based primarily on protein intake. A few other countries also have recommended intake values for the vitamin. Vitamin B-6 metabolism and requirements may be altered in several diseases and pathological conditions. The symptoms for some but not all patients with these diseases/conditions have been reported to improve when therapeutic doses of the vitamin are given. Vitamin B-6 has been shown to be toxic. Dosages above 200 mg pyridoxine hydrochloride/d are not recommended.