THIS! I also do not start or engage in any conversation about food, weight, dieting, or body image. If someone brings it up, I change the subject. If you engage in diet/food/body talk you have to realize that you are participating in the evaluation of someone's body or food choices and inviting comments about yourself as well.
If they are friends I say "Never ask a fat man about his diet,you will soon be bored beyond belief"... I f they are just acquaintances, I change the subject
It's my business
If they really insist I go into a technical discussion on the relative merits of the body mass index and the fat mass index... all leading up to a joke about why the Navy prefers the Fat mass index (they don't want any pencil neck geeks in their ranks)
My biggest problem is that my very sweet beloved wife talks about everything thing under the sun. Before the weight loss I've had complete strangers come up to me me and ask " How's that left testicle doing... descended all the way yet". Ya know, she a nurse. It's one thing when that question comes from a smelly gym buddy in a locker room, for a guy, it's quite another when it comes from the mouth of a 20 something female nurse trainee. Gotta be the residual sexism I've been struggling with