Urbain P., Singler F., Ihorst G., Biesalski H.K., Bertz H. Bioavailability of vitamin D2 from UV-B-irradiated button mushrooms in healthy adults deficient in serum 25-hydroxyvitamin D: a randomized controlled trial. Eur J Clin Nutr. 2011;65(8):965–971.
Rationale
Mushrooms synthesize D2 from ergosterol under UV light — industry marketed "vitamin D mushrooms" before any direct human bioavailability comparison existed. This study provided it: mushroom D2 in food versus a D2 tablet versus placebo, head to head.
Methods
A randomized placebo-controlled single-blind trial in winter (to minimize sunlight confounding): 26 young adults with baseline 25(OH)D ≤50 nmol/L received weekly portions of soup made with UV-B-irradiated button mushrooms (28,000 IU D2/week), a D2 supplement, or placebo, for 5 weeks.
Results
Serum 25(OH)D rose at essentially identical rates in the mushroom-soup and tablet groups — +3.9 and +4.7 nmol/L per week — and not in placebo. Bioavailability of mushroom D2 was not inferior to a pharmaceutical dose.
Conclusions
Vitamin D2 in UV-irradiated mushrooms is absorbed as well as a supplement — the foundational human evidence for "vitamin D mushrooms."
Worth noting
Small (n=26), short (5 weeks), but the design is exactly right: same molecule, food matrix vs tablet, deficient participants, winter. The later prediabetics study shows where this result stops working — cooking losses and real-world dosing. Together they define both the promise and the limits of the food-based D2 approach.