Is it possible to develop an allergy to all foods?

An allergy is an immune response to components of proteins, so to the extent that a food contains no protein, you can't be allergic to it. There are a number of food ingredients that people often claim make them allergic, such as caffeine, but caffeine is an alkaloid--you can react to it but the reaction is not an allergic response. The best treatment is to avoid caffeine. You can be intolerant of a food without being allergic to it, as many people are with lactose. Some people are told to avoid a broad spectrum of foods based on allergy tests that show a reaction, but the reaction is often not an allergy. Avoiding these foods on the basis of a false positive can make it hard to achieve a balanced diet. This is particularly awful when a growing child stops growing, as often happens. Go to a good allergist if you suspect a food allergy. And if you think you are allergic to ALL foods, you might consult a psychotherapist as well.