Pyridoxine-5′-β-D-glucoside Exhibits Incomplete Bioavailability as a Source of Vitamin B-6 and Partially Inhibits the Utilization of Co-Ingested Pyridoxine in Humans ,

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Abstract
This research was conducted to investigate 1 ) the bioavailability of pyridoxine-5′-β-D-glucoside (PN-glucoside) relative to that of pyridoxine (PN) in human subjects, and 2 ) the competitive effect of PN-glucoside on the metabolism of co-ingested PN. To evaluate PN-glucoside bioavailability, the subjects were administered a single oral dose of either deuterium-labeled ([2H2 ]) PN (Trial 1) or [2H2 ]PN-glucoside (Trial 2), and the urinary excretion rates of labeled 4-pyridoxic acid (4PA) were measured. The [2H2 ]4PA derived from [2H2 ]PN or [2H2 ]PN-glucoside was excreted mainly in the first 8 h after the dose. Excretion of [2H2 ]4PA during the 48-h postdose period indicated that the bioavailability of PN-glucoside was ∼50% relative to PN, which is consistent with our previous report of 58% bioavailability determined using a different protocol and fewer subjects. To assess the effects of PN-glucoside on PN utilization, the subjects were administered different ratios of nonlabeled PN-glucoside with [2H2 ]PN in Trials 3 and 4. Comparing Trial 1 with Trials 3 and 4, the quantity of nonlabeled PN-glucoside, as a fraction of total vitamin B-6 administered, ranged from 0 to 40% (on the basis of pyridoxine equivalents), with a constant dose of [2H2 ]PN in each. In these trials, the rate but not the total extent of the excretion of [2H2 ]4PA derived from [2H2 ]PN was inversely related to the proportion of co-ingested nonlabeled PN-glucoside. Thus, antagonistic effects of PN-glucoside on PN metabolism do occur in humans, although the effect is less pronounced than that seen previously in rats. Such interactive effects must be considered in evaluating the net bioavailability of dietary forms of vitamin B-6.