Food intolerances may sometimes be mistaken for food allergies. An allergy involves the immune system. Intolerance is an adverse reaction that does not involve the immune system. Food intolerance is often related to dose or frequency of exposure to the particular food. A reaction may not always occur with each exposure to the food or food additive, and the build-up effect is highly variable. Reactions may be immediate or delayed by up to 20 hours, making diagnosis difficult.
Symptoms can mirror those of a food allergy (e.g. hives, eczema), but can also be more general (headaches, bloating, wind, behavioural problems), making diagnosis problematic. Common intolerances include foods containing naturally-occurring substances such as lactose (the sugar found in milk), or high in added substances, such as amines, salicylates or glutamate.