William J. Klish, M.D., is Professor of Pediatrics, Head of Pediatric Gastroenterology at Baylor College of Medicine and Chief of the Nutrition and Gastroenterology Department at Texas Children's Hospital in Houston. Dr. Klish has served as chair of the Committee on Nutrition of the American Academy of Pediatrics.

Q: What's the difference between a food allergy and a food intolerance?

A: A food allergy is triggered by certain proteins in food that cause an allergic reaction in an individual when eaten. Symptoms may include hives, wheezing or diarrhea caused by the proteins in milk, peanuts, or shellfish, for instance.

The definition of food intolerance is much broader and includes problems such as lactose intolerance, a common problem defined by abdominal cramping and/or diarrhea due to the inability to properly digest milk sugar, or lactose. Other food intolerances may be as simple as the gassiness that follows the ingestion of beans, cabbage or onions, or the inability to eat spicy food.

Food allergies are by far the most serious form of food intolerance since in rare cases they can be very severe and even fatal. The most common form of allergy in infants is allergy to milk protein. Most infants with this allergy develop diarrhea which occasionally contains blood. Rarely, infants can have a more serious response known as anaphylaxis. This form of allergy causes wheezing, hives and swelling of the face and throat which can interfere with the baby's ability to breathe. This, of course, is a medical emergency.

Milk allergy, particularly the form that causes diarrhea, tends to go away with age. By the end of the first year of life most infants again begin to tolerate milk protein. The anaphylactic form of allergy goes away much more slowly, if at all. Parents should be very cautious about challenging these infants with food products made with milk. Other allergies such as peanut, shellfish, strawberry, etc., are lifelong.

Lactose intolerance, on the other hand, usually does not appear until preschool or early school age. This form of food intolerance is permanent. Most of the adult population of the world is lactose intolerant but doesn't realize it. They only know that they do not like milk. Since lactose intolerance is not an allergy, most people have a threshold below which they can tolerate milk and milk products. For this reason most of us can enjoy a bit of ice cream or eat yogurt.

Some food intolerances, such as the gassiness caused by green apples, depend on not only how much is eaten but what is eaten along with the food. If gas-forming foods are released from the stomach slowly, they may be more completely absorbed, thereby having less effect. Eating the offending food in smaller amounts, more slowly, or with other foods can help a child to tolerate it better.