Due to health issues (ulcerative colitis) I have to avoid all grains, lactose, sugar and starchy vegetables - this concerned me but I walked the Frances in 2013 over 6 weeks and found plenty of excellent food that I could eat along the way - From Pamplona onwards always had hard cheese, tinned fish, pate, fresh fruit, dried fruit, nuts and pork scratchings (aka pork rinds, chicharrones in Spanish and an excellent energy food!) in my pack to snack on, and the salads in Spain are fabulous - I just left the inevitable bread and used my (very bad) Spanish to explain "soy alergica de harina, leche y azucar"
Even eating out it was very easy to stick to plain grilled meat or fish with salad and vegetables. A lot of the menu del dia had a main course of grilled fish or meat with vegetables or salad.
In fact I went home feeling healthier than I had in the previous 12 months!