A person who contantly provides their opinions on dieting, exercise, and weight loss in an arrogant and pushy manner. They constantly criticize and suppress' others opinions, no matter how factual they may be.
"That stupid Health Nazi lectured me for an hour on why the Atkins diet doesn't work after I told her I've been on it"
A person who forces his or her healthy lifestyle on others through supporting legal bans of anything unhealthy. As well as being an all around A-hole to anyone who isn't like them. Such people generally have a "holier-than-thou" attitude all around, are only looking for yet another reason to look down on people who don't live up to their perceived view of the world.