Determination of Folate Bioavailability From Brewer's Yeast in a Dual Isotope Randomized Human Intervention Study

An adequate intake of folates is primarily dependent on the amount ingested, but also on the bioavailability of these water-soluble vitamins belonging to the vitamin B9 group. Therefore, we here present a human pilot study with a novel dual isotope design to assess the bioavailability of brewer's yeast folate as a model food. The particular advantage of the study design is the use of yeast intrinsically labeled with N-15, and a new reference folate specifically labeled with C-13. In this trial, 6 volunteers consumed (1) the intrinsically ¹⁵N-labeled yeast, (2) a ¹³C₆-labeled (6S)-5-methyltetrahydrofolate as the reference dose and (3) a folate-free food on one of the three study days. On each study day, 21 plasma samples were taken and analyzed for three different isotopologues of (6S)-5-methyltetrahydrofolate using a ¹³C₅-labeled (6S)-5-methyltetrahydrofolate as quantitation standard in stable isotope dilution assays. The main outcome of the study was that the yeast folates revealed for the five assessed volunteers a mean bioavailability of 74 % relative to 5-methyltetrahydrofolate as the reference dose. The study allows to conclude that intrinsically labeled foods are best suited to accurately measure the bioavailabilities of natural food folates embedded in their natural food matrix.