I would confuse them...the next time they comment, "I can't believe your eating a salad.." eagerly say oh yea, wow, that's what my doctors been after me about. He wants me to lay off the healthy low fat stuff.
The coworker will say "why?"
Then you reply with " because my doctor is concerned I am far too thin, dangerously underweight almost. He has been lecturing me about I need to put in at least ten pounds.....gosh, maybe you all are right....do you have any potato chips or anything like that?"
Coworker will scratch head and probably say "ohhh....." And walk off in a confused daze.
Every time they say only a salad, or I can't believe how healthy or low fat you eat, confuse the heck out of them...then thank them