Food intolerance is a condition in which people react to foods in a way that does not affect the immune system. It is different to a food allergy. Two common intolerances are to lactose and food additives. Symptoms of food intolerance include nausea, stomach pain, gas, cramps, bloating, vomiting, heartburn, diarrhea, headaches, and irritability. To identify the problem foods a person may keep a food diary or go on an elimination diet. Once the trigger has been identified, they must then reduce or avoid the problem food.