Living in California there is every kind of food and every type of dietary choices imaginable. For anyone to comment on your choice of foods is extremely rude.

If someone made a negative comment about my food choices I would handle it very simply: Look the person straight on the eyes and calmly say, "Oh. How rude." That's it. Not another word about the food. If the other person tries to continue his/her thoughtless behavior, just raise your palm and say, "I don't care to discuss my choices any further." Then CHANGE THE SUBJECT. End of story.