A dual-isotope-labeling method of studying the bioavailability of hexaglutamyl folic acid relative to that of monoglutamyl folic acid in humans by using multiple orally administered low doses

Background: The bioavailability of dietary folate may be hampered by the need of the glutamate moieties to be deconjugated before absorption. Previous studies comparing the bioavailabilities of polyglutamyl and monoglutamyl folic acid had inconsistent results. Objective: The objective was to estimate the bioavailability of polyglutamyl relative to that of monoglutamyl folic acid by using a sensitive stable-isotope approach that allowed for the administration of multiple low doses in humans. Design: Twenty subjects aged 20-50 y ingested 2 capsules daily for 28 d; each capsule contained approximate to 50 nmol [C-13(6)]hexaglutamyl and approximate to 50 nmol [C-13(11)]monoglutamyl folic acid. Amounts of the isotopically labeled compounds in the capsules were verified by various methods. The degrees of isotopic enrichment of plasma 5-methyltetrahydrofolate with C-13(6) and C-13(11) were measured by using liquid chromatography tandem mass spectrometry, and the ratio of C-13(6) to C-13(11) (C-13(6): C-13(11)) in plasma on day 28 was used as a measure of their relative bioavailability. Results: The C-13(11): C-13(6) in plasma 5-methyltetrahydrofolate reached equilibrium on day 4 and was 0.66 (95% CI: 0.58, 0.74) on day 28. The C-13(11): C-13(6) content in the capsules varied between 1.18 and 1.96. After correction for this ratio, the estimated bioavailability of hexaglutamyl relative to that of monoglutamyl folic acid was >= 78%. Conclusion: Multiple dosing of low amounts of labeled folic acid is a sensitive, accurate, and efficient method of measuring the relative bioavailability of folic acid compounds, provided that the administered doses can be reliably assessed.