Poor bioavailability of vitamin D₂ from ultraviolet-irradiated D₂-rich yeast in rats

Ultraviolet-irradiated yeast (Saccharomyces cerevisiae) can be used to biofortify bakery products with vitamin D, but in bread, it was not effective in increasing serum 25-hydroxyvitamin D [25(OH)D] in humans, possibly because of the low digestibility of the yeast matrix. We investigated the effects of vitamin D₂-rich intact yeast cells and their separated fraction, yeast cell walls, which we hypothesized to provide vitamin D₂ in a more bioavailable form, on serum 25(OH)D and its metabolites in growing female Sprague-Dawley rats (n = 54) compared to vitamin D₂ and D₃ supplements (8 treatment groups: 300 or 600 IU vitamin D/d, and a control group, 8-week intervention). The D₃ supplement groups had the highest 25(OH)D concentrations, and the vitamin D₂ supplement at the 600-IU dose increased 25(OH)D better than any yeast form (P < .001 for all, analysis of covariance, adjusted for body weight). There were no significant differences between the yeast forms at the same dose (P > .05). Serum 24,25-dihydroxyvitamin D (a vitamin D catabolite) concentrations and the trend in the differences between the groups were in line with 25(OH)D (P < .001 for all). The 24,25-dihydroxyvitamin D to 25(OH)D ratio between the D₂ supplement and the yeast groups did not differ (P > .05). These findings do not support the hypothesis: the ability of the different ultraviolet-treated vitamin D₂-containing yeast forms to increase 25(OH)D did not differ, and the poor bioavailability of vitamin D₂ in the yeasts compared D₃ or D₂ supplements could not be explained by the increased vitamin D catabolism in the yeast-treated groups.