Jeff Tweedy, leader of the bands Wilco and Uncle Tupelo, struggled with an addiction to prescription painkillers and cigarettes. After successful rehab and several years of clean living he had this to say about his life:
“I've never felt better. I've never been healthier. … I run four or five miles, four or five times a week, but I broke both my legs running too much last summer. I had stress fractures in both my tibias from running too much. You know, once you're an addict, you're always an addict, so just because I found something good to do doesn't mean I'm not going to hurt myself doing it.”
[from an interview on pitchforkmedia.com, 2007]
Yes, exercise can activate the pleasure circuit. And so, it can become a substrate for addiction as well. A portion of this addiction may come from the production of endorphins, the brains own morphine-like molecules that can activate dopamine release in the pleasure circuit though an indirect action. Does that make exercise a virtue or a vice (or a little of both)?