The Difference Between Allergies and Food Intolerance
An allergic reaction happens when the body's immune system mistakes a part of a food for something harmful. Foods that cause the most problems for adults are peanuts, tree nuts (like walnuts or almonds), fish, and shellfish. Any food can cause an allergy, but some other "highly" allergenic foods are: milk, eggs, soy products, and wheat. Allergic reactions usually happen fast—minutes to hours after eating the allergen.
Food intolerance is the digestive system's response to the part of a food or additive that it can't process or that causes irritation. Sugar intolerance, for example, could be from a biological cause in which the body lacks the enzymes needed to process lactose or fructose. It could also be from an additive, such as a sugar substitute in diet foods, that is poorly absorbed by the gut.
The body's response to food intolerance is usually based on portion: A small amount provokes a small reaction; a large portion provokes a larger reaction. The time frame for problems is longer too. The body's reaction to food intolerance can take place immediately or over a few days.