Publication 502, Medical and Dental Expenses:
https://www.irs.gov/pub/irs-pdf/p502.pdf
The closest mention is for weight-loss programs on page 15 and 17.
You can include the cost of special food in medical expenses only if:
1. The food doesn't satisfy normal nutritional needs,
2. The food alleviates or treats an illness, and
3. The need for the food is substantiated by a physician.The amount you can include in medical expenses is limited to the amount by which the cost of the special food exceeds the cost of a normal diet.
I've been told the same rules applies to diabetics and their dietary needs, so presumably the same rules carry over for other dietary issues like Celiac. Usually the cautionary advisement I hear from tax professionals is to make sure you have all your receipts in case you're audited since most people don't have the tenacity to keep up with those kinds of expenses and it makes the IRS raise an eyebrow when they find somebody who does.