I just went through the Atlanta Airport in June, going to the GIG conference in Richmond.
I got a note, from my doctor. It said that I have Celiac, that I am on a medically restricted diet and that I need to bring my own food with ice pack.
I put the note in the soft sided lunch pack that had yogurt, cheese, hard boiled eggs and an apple in it. I packed in my purse crackers and a breakfast bar. I bought soda there.
They never even questioned me. If they did, I would have asked to speak to a supervisor.
Now, this trip was before the attacks in England. They will be in high alert now. But I would still do the same thing. Pack something, bring a note and ask to see the supervisor if there are any questions.