Lactase is an enzyme that digests lactose, a sugar naturally found in milk products. The inability to digest lactose affects about 75 percent of the world’s population and is highly prevalent in African Americans, Asian Americans, Caucasian Americans of Mediterranean and Jewish descent.

Hispanics, and Native Americans: 25 percent of Americans are lactose intolerant, including 15 percent Caucasians, 70 percent of African Americans, 74 percent of Native Americans, 53 percent of Mexican Americans, and 90 percent of Asian Americans.

Lactose intolerance is caused by an enzymatic deficiency and is not a milk allergy, which is the inability to digest milk proteins. Lactose intolerance causes a wide variety of symptoms including abdominal cramping, acne, bloating, diarrhea, gas, headaches, and nausea.

Most people with lactose intolerance fail to recognize that the food they eat has any relationship to how they feel. There are two ways to test for lactose intolerance—a self-test and a laboratory test. To self-test, you must restrict intake of all dairy products for at least ten days.

Obvious sources include milk, yogurt, cheese, ice cream, creamed soups, frozen yogurt, powdered milk, and whipped cream. Less obvious sources are bakery items, cookies, hot dogs, lunchmeats, milk chocolate, most nondairy creamers, pancakes, protein powder drinks, ranch dressings, and anything that contains casein, caseinate, lactose, sodium caseinate, and whey.

If you’re not sure what’s in a food, avoid it during the testing period. It’s probably best to eat all your meals at home or prepare all food yourself. If lactose intolerance is causing your problems, you will probably notice your symptoms have disappeared significantly.

Reintroduction of dairy products will trigger a return of symptoms. However, the results may be inconclusive because you may be sensitive to other foods in addition to dairy—in which case your symptoms might not change much. A laboratory test would then be indicated.

Your doctor can order a simple, noninvasive hydrogen breath test to pinpoint if lactose intolerance is causing your problems. This challenge test is ideal for people who find it difficult to complete a self-test or are confused about their findings.

After you breathe into a bag to collect a baseline sample, you drink a small amount of a lactose solution and breathe into a different bag. Lab technicians measure the levels of hydrogen and methane gas you exhaled in both samples. Normal methane levels are 0 to 7 parts per million (ppm).

If levels increase at least 12 ppm between the two samples, it indicates lactose intolerance, even if your hydrogen production is normal. Normal hydrogen levels are 10 ppm. Levels of 20 ppm or more are commonly found in people with lactose intolerance. When both methane and hydrogen are measured, false results are narrowed considerably.

Use of antibiotics, enemas, and laxatives are common reasons for false negative results, which occur 5 percent of the time. This test has a few false positive results that are generally caused by eating high-fiber foods before the test, exposure to cigarette smoke, or sleeping during testing.