A dual-label stable-isotopic protocol is suitable for determination of folate bioavailability in humans:: Evaluation of urinary excretion and plasma folate kinetics of intravenous and oral doses of [13C5] and [2H2]folic acid

Stable isotopic protocols for the study of folate absorption were conducted to determine the following: (1) the equivalence of the [C-13(5)] and [H-2(2)] forms of folic acid, and (2) the merits of short-term plasma kinetics from injected and oral doses vs. urinary excretion of [C-13(5)] and [H-2(2)]folates. Another objective was to evaluate the merits of protocols not involving "saturation" of subjects with nonlabeled folate. Oral administration of [C-13(5)] and [H-2(2)]folic acid (similar to 500 nmol each) to adult subjects (n = 4) yielded an equivalent 24-h urinary excretion of similar to 2% of each dose (molar ratio of urinary [C-13(5)]/[H-2(2)]folates = 0.96 +/- 0.055; mean +/- SEM). Expression of urinary excretion as a ratio of [C-13(5)]/[H-2(2)]folates yielded less within-group variability than seen for absolute excretion of each form of labeled folate. In the second study, subjects received 226 nmol of [H-2(2)]folic acid intravenously and 1010 nmol of [C-13(5)]folic acid orally. Isotopic enrichment of plasma [H-2(2)]folates rose rapidly and returned to near basal values by similar to 2 h postdose. In contrast, enrichment of plasma [C-13(5)]folates was detected until 4 h after dose, whereas enrichment values were far lower than seen with [H-2(2)]folate. Adjusting for the difference in dose, the molar response of plasma area under the curve for isotopic enrichment was 15- to 20-fold greater for injected folates. In view of this very limited short-term plasma response even with a relatively large oral dose, presumably due to hepatic first-pass uptake, these findings suggest that plasma kinetics would be of limited usefulness in assessing the relative bioavailability of nutritionally relevant oral doses of labeled folate.