Vitamin D (D3 cholecalciferol) is created under the skin by ultraviolet light (UVB) exposure. In northern climates such as in Canada, in the winter we simply are not exposed to enough sunlight to manufacture the adequate amount of Vit D.
Bare skin needs to be directly exposed to UVB rays between 10am-2pm for at least 15 minutes each day to produce enough vitamin D. Lotions, windows and clothing can all prevent the manufacturing of Vit D. Vit. D also takes a fair amount of time (up to 48 hours) to travel through the skin into the bloodstream, so it is actually is possible to wash away Vitamin D while showering.
Hence, we always suggest supplementing with a high quality oral Vit D, particularly D3.