I tell people I am diabetic and cannot have any carbs. Most people accept it.
I have one funny anecdote We went to Outback and I told the waitress to take the bread but leave the butter, and ask for extra butter for my broccoli. Later I went to the restroom and I overheard her telling that is no wonder I was so fat, I was eating more butter then broccoli. I wonder if she had figured out why her tip was so stingy.