A vitamin is a chemical substance found mostly in foods. It is necessary for the proper working and growth of the body. Vitamins take part in chemical reactions that help bring food energy to the body’s cells.
In these reactions, vitamins serve as catalysts, substances which speed up or aid reactions, but which are not changed themselves by the reactions. You might think of light as a catalyst in picture-taking, you can’t take a photograph without light, but the light itself isn’t changed by the process of picture-taking.
Since vitamins aren’t changed by the chemical reactions in which they take part, they aren’t “used up” by the body, and therefore can be used over and over again. But there are some vitamins which the body doesn’t store, and these must be taken fresh every day.
The body can store extra vitamins A, D, E, and K, but all other vitamins are flushed out of the body when they’re not quickly used.