BIOAVAILABILITY FOR HUMANS OF DEUTERIUM-LABELED MONOGLUTAMYL AND POLYGLUTAMYL FOLATES IS AFFECTED BY SELECTED FOODS

Dietary folate exists mainly as polyglutamyl forms that require deconj ugation by Zn-dependent pteroylpolyglutamate hydrolase prior to intest inal absorption. Because deconjugation by pteroylpolyglutamate hydrola se is an essential step in the absorption of dietary polyglutamyl fola tes, factors influencing the deconjugation process may affect folate b ioavailability. This study was conducted to evaluate in vivo the bioav ailability of [H-2(4)]folic acid (d(4)-PteGlu(1)) and [H-2(2)]-pteroyl hexaglutamate (d(2)-PteGlu(6)) administered in solution in water or ci trate buffer or added to selected foods using a single-dose, dual-labe l protocol. In each of six trials, healthy men (n = 7) were given a si ngle oral dose of d(2)-PteGlu(6) and d(4)-PteGlu(1) (677 nmol of each form) blended into orange juice, tomatoes, lime beans, 52 mmol/L citra te (pH 4.1), or water as the control. Urine was collected for 48 h and the isotopic labeling of urinary folates used as criteria of the rela tive bioavailability of administered PteGlu(1) and PteGlu(6). Urinary excretion of d(4)-folates and d(2)-folates derived from the respective oral doses did not differ from the control in any treatment within th e statistical power of this protocol. High relative bioavailability of the polyglutamyl folate was reflected by ratios of urinary d(2)/d(4) folates of similar to 1.0 for control, tomato, lima bean and citrate b uffer trials, whereas the ratio of urinary d(2)/d(4) folates when subj ects consumed orange juice was similar to 33% less than the control ra tio (P < 0.05). These findings suggest that the bioavailability of pol yglutamyl folates in orange Juice would be partially incomplete. Howev er, this would be compensated by the high total folate concentration o f orange juice. The relation of these findings to endogenous dietary f olates requires further investigation.