Food intolerance is a negative reaction to a food that is not related to the immune system or to food poisoning. It is caused by the absence of specific chemicals or enzymes needed to digest a food substance.

Symptoms of food intolerance vary greatly, and can be mistaken for the symptoms of an allergy. It is also sometimes difficult to determine the offending food because the immune system does not react with an intolerance. A deficiency in digestive enzymes can cause some types of food intolerances. Lactose intolerance is a result of the body not producing enough lactase used to break down the lactose in milk. Gluten intolerance results in damage to villi in the small intestine, which makes it difficult for the body to absorb water and nutrients from foods.