Are vitamins D2 and D3 equally effective?
No. Vitamin D3 is substantially more potent than vitamin D2. In the early days of vitamin D research, the only way to evaluate these compounds was to examine their ability to prevent rickets in experimental animals. That was a pretty crude assay, and one couldnt tell the difference between the two forms of the vitamin. So they were assigned the same number of units (IUs) per milligram. But now that we have the serum 25(OH)D to measure, we understand that vitamin D3 may be anywhere from four to 10 times more potent than vitamin D2. Furthermore, vitamin D3 is the natural form, typically made in our bodies if we give them the chance to do so, and hence taking vitamin D3 by mouth is more natural than taking vitamin D2 (which is a purely synthetic product except, as noted above, from what we might get from sun dried mushrooms).
