The vitamins are, in short, a disorderly bunch. It is almost impossible to define them in a way that comfortably embraces them all. A standard textbook definition is that a vitamin is “an organic molecule not made in the human body which is required in small amounts to sustain normal metabolism,” but in fact vitamin K is made in the body, by bacteria in the gut. Vitamin D, one of the most vital substances of all, is actually a hormone, and most of it comes to us not through diet but through the magical action of sunlight on skin.
