A friend of mine has had great success with using the phrase "Are you trying to tell me I'm fat?" and while they sputter she continues "Because, I already know that - but thanks for the heads up!"
Because, really that's what the OP's co workers are trying to do. They want to fat shame her, but not be obvious. If you call them out on it they might back off.
So much truth, so little time.
These coworkers aren't concerned about your health, they're just getting away with making derogatory statements and assumptions. It's so shocking to people that it's completely possible and *GASP* even probable that a fat individual has the same food or fitness habits as a slender person, but this isn't a discussion on your health because that isn't the core of what your coworkers statements are coming from.
Perhaps in the future you could play the naive card;
"Wow OP I can't believe you eat so healthy!""I don't get it.."
"Well it's just I never would have thought you ate such healthy food..""I don't get what you mean"
By that point the person should be far too engrossed in realizing their blunder to even think of continuing.
You don't need to validate your body to them, it isn't even what they're looking for. They're just looking to be bigoted in a socially acceptable way. Don't ply into their nonsense, don't grace them with an answer, they don't need to or deserve to know about your body.