I was diagnosed with Celiac Sprue in 2016 a few months prior to leaving the military. Nothing like being told in your late twenties "oh, by the way, you can no longer have bread, beer, or general happiness."
After four years of dealing with this illness, I have learned to cope but I still miss donuts...
Now, preparing for my through-hike of the Appalachian Trail, I did my research and listened to everyone I could about what to take on the trail, as far as food. The biggest issue is that there is not a ton of cost effective ways to get the calories you need on trail while not being able to eat wheat or soy (yes, soy too).
So here is what I did eat:
Certain Clif Bars
Oatmeal (not quakers since they are not GF for some reason)
*GF Tortillas (Corn or Spinach) with Peanut Butter and Honey
Cap'n Crunch or Lucky Charms (because I'm an adult)
*GF Tortillas with Pepperoni, Cheese Sticks, Spinach, Special Sauce (or Mustard) and Fritos (yes Fritos)
Cheese (usually Gouda, White Cheddar, or Horseradish)
Trail Mix (be careful with these though, they are not all GF and any that have M&Ms have soy)
and Anything that I would eat for dinner could also be part of lunch (If you are hungry, you eat, and your body tells you what to eat)
Dinner (aka Feast Nights)
Mashed Potatoes usually Instant
Olive Oil (sometimes)
Fruit Leather, Gummies, Roll-ups, Gushers, or just plain ol' Fruit
Tuna (or Salmon)
Hot Sauce or Jalapeno Relish
Salt and Vinegar Chips
Cashews or Almonds
*GF Tortillas are not always available along the trail, I was lucky to find them at Walmart if there was one.
Now this is what I ate, I would supplement with other things as I found them, Angry Orchard Ciders in tall cans or the occasional Bratwurst or Hot Dogs... This is not an all inclusive list, either. There is also the SuperFoods list that you generally wouldn't eat altogether, but on trail they make sense. (If you disagree or have any to add, please comment below)
SuperFoods for Through-hiking:
Chocolate Milk (Protein, Sugar, fats, plus it tastes soooooo gooood) Find the Soy Free ones and you're golden
Fritos Assorted Flavors (Carbohydrates) Crush these up and add them to your potatoes for some crunch, add them to wraps at lunch to make up for the energy you are burning all day, if you get sick of the flavor just switch to another one....
My friend Crash swears by Pork Rinds, I would rather starve but it makes sense (Protein, Fats)
Gushers (Sugar, Calories) For a quick boost I would grab a King Size pack of these. I also used them in super dry areas (like Pennsylvania) to trick my body into thinking it was hydrated....
Ice Cream (Protein, Sugars, Fats, and basically all the same benefits of chocolate milk, just frozen) Again, read your labels some are made with soy and others have wheat in them.
When reading labels remember that the "servings" make a difference. If it says 9g of protein per serving on the label but there are 10 servings in the box and you eat all of the things PLUS the box you actually had 90g of protein and some cardboard.
Also, check the ingredients. There is a section below the ingredients usually in BOLD that should tell you what allergens are in the products BUT read all of the ingredients to be sure. I have bought food, taken it on trail, then had an allergic reaction because I didn't see the "contains wheat" as part of an ingredient. It happens, don't sweat it.
You aren't worried about limiting calories anymore. If you are on trail, and you are counting calories to "slim down" or "lose weight" you are going to die (or burn out). If you are looking at calories at all, it should be to make sure you are getting enough.
When I started out I was eating between 2000-3000 calories a day. Around the midway point (Harpers Ferry-ish) I was ingesting between 4000-5000 a day. And by the time I got to Maine I would eat constantly and was never full.
This was about 6000-7000+ calories a day.
Oh, and I lost 52 lbs doing this.
Don't forget to eat all the things while hiking, know what your body can handle, and Taco Bell will destroy your soul in the worst way on a Zero.
This is just a brief overview of the food I ate on trail, is this comprehensive? No, is this what you should eat on your five day round trip? Also No. BUT, if you are planning to do the Appalachian Trail and are looking for ideas, I hope this is a good start for you.