Urinary excretion of [2H4]folate by nonpregnant women following a single oral dose of [2H4]folic acid is a functional index of folate nutritional status

Abstract

In a 10-wk study with nonpregnant women (21-27 y, n = 5-6 per group), subjects were fed a diet containing ~68 nmol/d (30 μg/d) folate from food that was supplemented with folic acid in apple juice to yield a constant intake of 454, 680 or 907 nmol/d (200, 300 or 400 μg/d) to evaluate folate status and long-term in vivo kinetics. Reported here is an additional phase of this protocol conducted to determine the relationship between short-term urinary excretion after a single isotopically labeled dose and various measures of folate nutritional status. It was hypothesized that urinary excretion from a single [glutamate-2H4]folic acid ([2H4]folic acid) dose would increase in proportion to folate nutritional status due to saturable cellular uptake and retention processes along with saturation of renal reabsorption. Each subject was given 1.13 μmol (500 μg) of [2H4]folic acid orally on the morning of d 70 of the study, followed by a complete 24-h urine collection. Urine was analyzed to determine the isotopic enrichment of urinary folate by gas chromatography-mass spectrometry and the concentration of urinary folate by HPLC. Urinary excretion of [2H4]folate was greatest at the 907 nmol/d intake and was positively correlated with serum folate concentration but was not correlated with erythrocyte folate. Excretion of [2H4]folate tended to be greatest when plasma homocysteine concentrations were low (<8 μmol/L), although this relation was not significant. These results suggest that 24-h urinary excretion after a single oral dose of isotopically labeled folate is a functional indicator of folate nutritional status that complements other measures of folate nutriture.