Here are a few studies but we have more in our Mushroom Vitamin D article.
Keegan, R. J. H., Lu, Z., Bogusz, J. M., Williams, J. E., & Holick, M. F. (2013). Photobiology of vitamin D in mushrooms and its bioavailability in humans. Dermato-Endocrinology, 5(1), 165–176.
Phillips, K. M., Horst, R. L., Koszewski, N. J., & Simon, R. R. (2012). Vitamin D4 in Mushrooms. PLoS ONE, 7(8), e40702.
Holick, M. F., Biancuzzo, R. M., Chen, T. C., Klein, E. K., Young, A., Bibuld, D., … Tannenbaum, A. D. (2008). Vitamin D2 Is as Effective as Vitamin D3 in Maintaining Circulating Concentrations of 25-Hydroxyvitamin D. The Journal of Clinical Endocrinology & Metabolism, 93(3), 677–681.
