Study conclusion: vitamin D2 and vitamin D3 have a different effect on serum concentration of vitamin D metabolites and biochemical indices of calcium metabolism.2
National Osteoporosis Society (NOS) position on vitamin D3 vs. vitamin D2:5
1. Houghton L, Vieth, R. The case against ergocalciferol (vitamin D2) as a vitamin supplement. Am J Clin Nutr October 2006, vol 84, no. 4, 694-697.
2. Tjellesen L, Hummer L, Christiansen C, Rødbro P. Serum concentration of vitamin D metabolites during treatment with vitamin D2 and D3 in normal premenopausal women. Bone Miner. 1986:1:407-413.
3. Trang HM, Cole DE, Rubin LA, Pierratos A, Siu S, Vieth R. Evidence that vitamin D3 increases serum 25-hydroxyvitamin D more efficiently than does vitamin D2. Am J Clin Nutr. 1998; 68: 854-858.
4. Bischoff-Ferrari H et al. Prevention of Nonvertebral Fractures With Oral Vitamin D and Dose Dependency. Arch Intern Med. Vol 169 (No. 6) March 23, 2009.
5. National Osteoporosis Society. Vitamin D and Bone Health: A Practical Clinical Guideline for Patient Management. April 2013. V1.1.