Vitamin D is both a vitamin and a hormone.
There are two major forms of vitamin D, and both have the word calciferol in their names. Reasonably good evidence tells us that the combination of vitamin D and calcium supplements can be quite helpful for preventing and treating osteoporosis. One researcher has given a strong argument that these recommendations are too low and should be increased considerably, perhaps by as much as a factor of 10. However, this idea has not been universally accepted, and some authorities feel that vitamin D toxicity is a real risk. In many countries, vitamin D is added to milk and other foods like breakfast cereals and margarine, contributing to our daily intake.