If only humans could make their own vitamin D. Oh, wait
The major natural source of vitamin D is synthesis of cholecalciferol in the lower layers of the epidermis of the skin, through a photochemical reaction of UVB light, from the sun exposure (specifically UVB radiation) or UVB lamps.
In the winter, only 10 percent of the body is exposed, and nearly 2 hours of sun exposure at noon is needed to produce a sufficient amount of vitamin D.
If only humans could make their own vitamin D. Oh, wait
Sure you can, but there’s also winter months where you can’t really go outside.
Thanks for pointing that out.
Source
I thought less time was required.