Ask them to stop eating carbage for a week and watch their reaction:

Oh! I couldn't live without bread/chocolate/whatever! No cereal? What would I eat for breakfast! You don't drink milk? How do you get calcium, etc., etc., blah, blah, blah...

Whaddya mean you can't live without it? It's just food! Does that not sound like an addict talking? Addicts think they can't live without alcohol/crack/heroin/whatever. But a piece of bread? Doesn't something sound terribly wrong about that?

Karen