If I were with some of my true friends they wouldn't make fun of me like that. If I'm with a bunch of people that I don't like anyway I don't say anything at all I just use my middle finger until they stop taunting me. Then I tell them about some big words that relate to my diet and tell them everything I know in a manner that makes them feel really stupid. Then they start to ignore me and I am happy again. I can't believe that the waiter complained about your diet. I would of liked to piss him off and ask for the manager because he was being an asshole. If I'm with family I just ask politely like you did and then if there is nothing in your case I sit there and stare at my plate and I don't take any shit from them. I either stare at my empty plate or explain to them for the millionth time how I eat. Also I could act like I do have food and just keep talking and laughing and having a good time just ignoring people when they say something stupid, ignorant or negative. There are many ways to go about it, some piss people off a little more than others but you got to get back at them some of time and have a little fun doing it.
