Some research suggests that vitamin D3 is far more potent than vitamin D2 in humans; we know that there are differences in their potency in other animals. However, the relative potency has not be definitively proven. Vitamin D2 was only added to the human diet in the 1930s, when it was discovered that scientists could easily make this form from plants and fungi. Without enough research on the metabolism of both forms of vitamin D, we believe that the best form of vitamin D is the one our ancestors produced and consumed for millennia.