It makes a bit more sense when you look at the rest of their behavior. People who take drugs (especially psychedelics) tend to look down on people who don't as being prudes, unenlightened, or just ignorant. When you look at it that way, drug-users and health nuts have something in common; they like to use their way of life to feel superior to other people.

(oh, and make sure to only give 5-htp after the high wears off. Otherwise, you increase the risk of serotonin syndrome.)