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Q: I react badly to milk products and thought I had a milk allergy. But my doctor said it was a type of food intolerance, not a true food allergy. What’s the difference?

A: When you’re intolerant to a particular food, it’s often because your body lacks an enzyme needed to break down a component in that food. Or the food intolerance may just be sensitivity to an additive or gluten. But food intolerances do not involve the immune system.

You likely have lactose intolerance, the most common food intolerance. Your intestines produce little or no enzyme called lactase. Without enough lactase, you can’t break down lactose, the main sugar in milk. This leads to bloating, gas and loose stools.

Gluten sensitivity is another common food intolerance. It’s very different from celiac disease. While some of the symptoms of abdominal discomfort and fatigue are similar, gluten sensitivity does not involve the immune system.

Some people are sensitive to food additives. For example, eating foods with sulfites (found in wine, dried fruits and canned goods) can lead to flushing and wheezing. Or a person with sensitivity to monosodium glutamate (MSG) may experience headaches, palpitations, or numbness after eating foods flavored with it.

A true food allergy involves your immune system. Your body recognizes a normally innocuous food, such as peanuts, as a potentially harmful foreign invader. It goes into defensive mode, producing high levels of an antibody called immunoglobulin E (IgE). Food allergies usually start early in life. But it’s not impossible for them to appear for the first time later on.

People with true food allergies need to avoid the food completely. Even very small amounts can lead to severe and even life threatening reactions.

While food intolerances can be quite uncomfortable, they are rarely serious. Usually you don’t need to completely eliminate the food from your diet. Symptoms can be avoided by limiting the amount of food to which you are sensitive. For example, you may find that a small portion of ice cream or pizza doesn’t bother you, while a glass of milk would wreck your day.

(Howard LeWine, M.D., is an internist at Brigham and Women’s Hospital in Boston and assistant professor at Harvard Medical School. For additional consumer health information, please visit www.health.harvard.edu.)

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