The metabolism of small doses of vitamin B-6 in men

The metabolism of small doses of pyridoxine (PN) and of equimolar doses of PN, pyridoxal (PL) and pyridoxamine (PM) was studied in five men. Fasting subjects were given 0, 0.5, 1, 2, 4 and 10 mg pyridoxine HCl and 19.45 mumoles of PN, PM and PL; one dose was administered a week. Plasma total vitamin B-6 (B-6) and pyridoxal phosphate (PLP), and urinary B-6 and 4-pyridoxic acid (4PA) were determined in timed blood and urine samples collected after each dose. The n doses had a significant P less than 0.01) overall effect on all of these measurements; the relationship between PN level and the subjects' responses was linear (P less than 0.01). Plasma B-6 peaked at 0.5 or 1 hour after the PN doses; PLP at 0.5, 1 or 3 hours, depending on size of dose. Plasma B-6 but not PLP approached fasting levels 3 to 5 hours after the 0.5- to 4-mg PN doses; both plasma B-6 and PLP were still elevated 24 hours after 10 mg PN. In general, the rate of urinary B-6 and 4PA excretion was maximal the first 3 hours after the doses. With increasing PN doses, the percent of the dose recovered as urinary B-6 and 4PA decreased from 9 to 7% and 63 to 35%, respectively. Immediately following PL, plasma B-6 and urinary 4PA rose steeply indicating the rapid plasma clearance and oxidation of this B-6 vitamer. Responses to PM were generally slower than for PN or PL, suggesting that PM is absorbed more slowly or metabolized differently, or both, than PL or PN. A dose of at least 1 mg of B-6 is necessary to obtain measurable changes in vitamin B-6 metabolism.