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.


METHODS
Twenty subjects aged 20-50 y ingested 2 capsules daily for 28 d; each capsule contained approximately 50 nmol [(13)C(6)]hexaglutamyl and approximately 50 nmol [(13)C(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 (13)C(6) and (13)C(11) were measured by using liquid chromatography tandem mass spectrometry, and the ratio of (13)C(6) to (13)C(11) ((13)C(6):(13)C(11)) in plasma on day 28 was used as a measure of their relative bioavailability.


RESULTS
The (13)C(11):(13)C(6) in plasma 5-methyltetrahydrofolate reached equilibrium on day 4 and was 0.66 (95% CI: 0.58, 0.74) on day 28. The (13)C(11):(13)C(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%.


CONCLUSIONS
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.
Melse-Boonstra
Alida
A
Wageningen Centre for Food Sciences, Wageningen, The Netherlands.
Verhoef
Petra
P
West
Clive E
CE
van Rhijn
Johannes A
JA
van Breemen
Richard B
RB
Lasaroms
Johan J P
JJ
Garbis
Spiros D
SD
Katan
Martijn B
MB
Kok
Frans J
FJ
D000284:Administration, Oral; D000328:Adult; D019540:Area Under Curve; D001682:Biological Availability; D002247:Carbon Isotopes; D002853:Chromatography, Liquid / Q000379:methods; D019587:Dietary Supplements; D004305:Dose-Response Relationship, Drug; D005260:Female; D005492:Folic Acid / Q000008:administration & dosage / Q000737:chemistry / Q000493:pharmacokinetics*; D006801:Humans; D007408:Intestinal Absorption / Q000187:drug effects*; D008297:Male; D008875:Middle Aged; D011624:Pteroylpolyglutamic Acids / Q000008:administration & dosage / Q000493:pharmacokinetics; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D053719:Tandem Mass Spectrometry / Q000379:methods; D013763:Tetrahydrofolates / Q000097:blood*; D014803:Vitamin B Complex / Q000032:analysis / Q000737:chemistry / Q000493:pharmacokinetics*
D002247:Carbon Isotopes; D011624:Pteroylpolyglutamic Acids; D013763:Tetrahydrofolates; D014803:Vitamin B Complex; C042605:hexaglutamate folate; D005492:Folic Acid; C005984:5-methyltetrahydrofolate