The food itself can cause problems with skin irritations due to allergies. Changing a dog's food too often will cause indigestion problems. Typically, you should change a dog's food a little a time by mixing the food they already are used to with a little bit of the new food..decreasing the old food, and increasing the new food a little at a time.
If you want to diagnose food allergies .. stick to a food that is for sensitive stomaches and that has the least amount of ingredients .. then slowly experiment with other foods by adding a little at a time .. It will takes a while for your dog to show a reaction to the food .. try a new entry for a week at a time. That way you can pin point what ingredient is causing the reaction.
Changing the dogs' food too many times can cause skin problems. This is because of the indigestion problems experienced as the dog tries to adapt to the certain type of food given. It is important to consult a vet before trying the different foods so that he can advice you accordingly. Read more at http://dogs.about.com/od/dogandpuppyhealth/a/healthydog.htm.