Dietary vitamin B-6 restriction does not alter rates of homocysteine remethylation or synthesis in healthy young women and men.

BACKGROUND The effects of vitamin B-6 status on steady-state kinetics of homocysteine metabolism in humans are unclear. OBJECTIVE The objective was to determine the effects of dietary vitamin B-6 restriction on the rates of homocysteine remethylation and synthesis in healthy humans. DESIGN Primed, constant infusions of [(13)C(5)]methionine, [3-(13)C]serine, and [(2)H(3)]leucine were conducted in healthy female (n=5) and male (n=4) volunteers (20-30 y) before and after 4 wk of dietary vitamin B-6 restriction (<0.5 mg vitamin B-6/d) to establish whether vitamin B-6 status affects steady-state kinetics of homocysteine metabolism in the absence of concurrent methionine intake. Effects of dietary vitamin B-6 restriction on vitamin B-6 status, plasma amino acid concentrations, and the rates of reactions of homocysteine metabolism were assessed. RESULTS Dietary vitamin B-6 restriction significantly reduced plasma pyridoxal 5-phosphate (PLP) concentrations (55.1 +/- 8.3 compared with 22.6 +/- 1.3 nmol/L; P=0.004), significantly increased plasma glycine concentrations (230 +/- 14 compared with 296 +/- 15; P=0.008), and significantly reduced basal (43%; P < 0.001) and PLP-stimulated (35%; P=0.004) lymphocyte serine hydroxymethyltransferase activities in vitro. However, the in vivo fluxes of leucine, methionine, and serine; the rates of homocysteine synthesis and remethylation (total and vitamin B-6-dependent); and the concentrations of homocysteine, methionine, and serine in plasma were not significantly affected by dietary vitamin B-6 restriction. CONCLUSIONS Moderate vitamin B-6 deficiency does not significantly alter the rates of homocysteine remethylation or synthesis in healthy young adults in the absence of dietary methionine intake.