Which vitamin does the body make from sunlight?

Vitamin D is the odd one out among vitamins, because the body can manufacture it rather than having to eat it. Ultraviolet B radiation striking the skin converts a cholesterol derivative in the epidermis into a precursor, which the liver and then the kidneys chemically finish into the active hormone. Its main job is regulating calcium: without it the gut absorbs calcium poorly, and bones fail to mineralise properly, which is what causes rickets in children and softening of bone in adults. The dependence on sunlight explains a lot of geography. At high latitudes the winter sun sits too low for enough UVB to reach the ground at all, so production stops for months regardless of how much time is spent outdoors, and darker skin, which filters more UV, produces less for a given exposure. That is why oily fish, egg yolk and fortified foods matter, and why deficiency is common in northern winters.

Window glass blocks most UVB, so sitting in a sunny room does very little for it.