Fitness fascists are ALWAYS yuppies. Therefore, they are snots who masquerade their elitism and vanity as "concern", especially when it comes to scrutinizing anyone with a BMI over 23 because they feel a smug, deep seated hatred for fat people because it's not glamorous or socially acceptable to be fat.
On the other hand, when an overweight person begins to lose weight, the fitness fascists get their tightie whities in a bunch because the dieter is losing weight quicker than them. It angers them to realize that their idealized and impractical methods of weight loss are faulty at best when actually applied to real life, so they chalk that person's success to "starvation" and resort to fear mongering instead.
They are constantly on a crusade to banish certain foods and are extremely proud of not drinking pop. They're always patting themselves on the back for choking down a carrot and how healthy they are, physically, but in actuality they are some of the most mentally unhealthy tools on the face of the planet, not only chronically acting as though they're on the rag or manifesting in severe symptoms of one personality disorder or the other, but also because they go in fucking shock over swallowing a french fry.
Antonym: A real joy to be around.
I'd rather count calories than eat a bunch of shit I hate and lose weight faster than to listen to one of those fitness fascists and eat food without flavor so that I can lose a pound a month...