Myra: Many people are quick to jump on the food-allergy bandwagon and discount environmental allergies, but my understanding is that it's very unlikely for a dog to have food allergies and not also have other types of allergies. In my allergy dog, although he's allergic to seven foods, I think they actually play a small roll compared to the 31 inhalant allergies he has. I've never tried it, but I think as long as his inhalant allergies are kept under control (we do desensitization shots), he could probably eat his offending foods w/o reacting much, if at all. Because keeping the inhalants under control keeps him under his threshold. OTOH, taking away his food allergens (via an elimination diet that we did prior to testing) while his inhalant allergies were flaring did little to help.
I agree that when a dog has allergy symptoms, environmental allergies should be considered first. However, my veterinary dermatologist believes that food allergies are more common than previously thought. On our first visit, she had me put Jessie on an elimination diet. I followed her instructions but doubted if anything would come of it. Well, I was very wrong. As most of you know, it turned out that Jessie has many food allergies, so I would definitely recommend that anyone with an allergy dog try an elimination diet.
As for how much food allergies affect a dog compared to environmental allergies, I think it's on an individual basis. Offending foods definitely affect Jessie more than environmental allergies, and she's more sensitive to some foods than others. Her worst food allergy is salmon and while she's allergic to chicken she can tolerate it longer before she shows a reaction.