Oral tolerance to food is learned by infants as solid foods are introduced. In the best case, tolerance to regularly eaten food endures throughout the life of the individual. Infants do best on breast milk for the first six months and develop tolerance to solid foods if the are introduced slowly over the next six months. These infants must be fed very carefully and their breast-feeding mothers often have to follow a careful diet to avoid food antigens in the breast milk. More permissive tolerance may not show up until the second year and may be limited for the entire life of that individual.
Many events alter or reduce food tolerance.
An infectious gastroenteritis, for example, may leave the gastrointestinal tract in a hypersensitivity state and the patient reports decreased tolerance to many foods. Those syndromes which manifest immediately after food ingestion are easy to diagnose and specific IgE tests and prick tests are frequently positive. Those with a delayed onset of up to several days are difficult to diagnose and currently available investigations may be unsatisfactory for routine use. Saavedra-Delagado and Metcalfe reviewed mechanisms of food antigens causing gastrointestinal disease. They detailed the pattern of cow's milk induced gastroenteropath y in children, another prototype of food allergic disease, which manifests as chronic diarrhea and is not associated with positive skin tests to cow's milk proteins.