An upset stomach that stays upset. Headaches that come back again and again. Persistent rashes. Aching joints. With the increasing number of reports on food intolerance, it's enough to make you wonder: "Could that be what's wrong with me?"

Food intolerance, like food allergies, causes an adverse reaction to something you ate. If it's a true allergy, however, the signs are pretty hard to miss. The body treats the substance as an invader and floods the system with antibodies; symptoms range from hives and runny nose to vomiting, difficulty breathing, and even death.

An intolerance is subtler, and can develop at any age. People with gluten intolerance, otherwise known as celiac disease (CD), for example, suffer slowly as the lining of their small intestines becomes less able to absorb nutrition from food. "It takes an average of 11 years to diagnose celiac disease," says Peter H.R. Green, MD, director of the Celiac Disease Center and professor of clinical medicine at Columbia University Medical Center in New York. "Some people are completely asymptomatic."

Better diagnostic tools and more awareness in the medical community are two reasons food allergies and intolerance are in the news. If you've been curious about whether you might have a food intolerance, read on to learn about finding out for sure.

Wheat/gluten
According to the National Institutes of Health (NIH), some 2 million Americans have CD, which is caused by an immune reaction to gluten, the protein found in many grains—wheat (including spelt and kamut), barley, triticale, and rye. Oats contain no gluten but are sometimes contaminated by residues from other grains.

Jenny Lass, coauthor of Grain-Free Gourmet (Whitecap, 2006), dropped to 105 pounds before she was diagnosed with CD in 2001. Long-term celiac sufferers often end up malnourished because their damaged intestines don't absorb enough nutrients. But even on a gluten-free diet, Lass had "terrible fatigue, mouth sores, and joint pain." Her former family doctor, unaware that some celiac patients don't respond to a gluten-free regimen, "just shrugged it off," she says. Only when she began following what is known as the specific carbohydrate diet (www.scdiet.org), which restricts all grains, lactose, starches, and refined sugars, did she feel better.

>Symptoms
CD's symptoms vary from constipation to diarrhea, weight loss to weight gain. Low levels of nutrients can give celiac patients anemia or osteoporosis, and sufferers may be prone to other autoimmune disorders, including thyroid disease and rheumatoid arthritis. Once present, it's not something you ever outgrow.

>Tests
Simple blood tests can now identify CD by measuring immunoglobulin A (IgA), anti-tissue transglutaminase (tTGA), and IgA anti-endomysium antibodies (AEA). After positive blood test results, sampling intestinal tissue confirms the CD diagnosis.

CD is hereditary, so diagnosed patients should encourage parents, siblings, and children to be tested. Between 5 percent and 15 percent of family members will also test positive—one of the reasons diagnosis rates are increasing so rapidly, according to the National Digestive Diseases Information Clearinghouse.

>If you've got it
Eliminate gluten from your diet. The initial learning curve can feel a bit steep: Gluten hides in a surprising multitude of places, including vitamins and even stamp adhesive. Check ingredient lists for the following: modified food starch; wheat germ and bran; graham, gluten, or durum flour; oat bran, bulgur, farina, or semolina; or anything made with malt extract or flavorings.

Then explore the bulk section. Kelly Keough, a sugar- and wheat-free chef, cookbook author, and TV host, suggests trying alternative grains, such as millet or quinoa. She uses her food processor to grind buckwheat groats, golden flax, and almonds into a meal for baking. Her favorite gluten-free flour is a mix of brown rice flour, potato starch, and tapioca flour, together with xanthan gum.

>For the rest of us
Mix up the grains you feed your family for variety and better health. For gluten-free products, try Amy's Kitchen rice crust pizza or French Meadow Bakery's gluten-free tortillas with organic flaxseed. Nutballz and Spring Bakehouse cookies are free from most common allergens, including soy, wheat, gluten, dairy, and eggs.

Milk/lactose
Lactose, or milk sugar, is by far the most common cause of food intolerance. As much as 70 percent of adults develop lactose intolerance, according to the NIH. Its origin is an enzyme deficiency that tends to worsen as you age, so if milk troubled you a little as a kid, it's likely to be off limits by middle age.

>Symptoms
If the body lacks sufficient lactase—the enzyme that converts lactose, or milk sugar, into a digestible substance—the undigested lactose ferments in the gut. This lactose intolerance causes diarrhea, gas, and nausea; sufferers report bloating and cramps shortly after eating. By contrast, a full-blown milk allergy (which is triggered by proteins and most often seen in infants) causes itchy rashes, vomiting, and hives.

>Tests
Cutting out dairy often is both diagnostic tool and cure: If a dairy-free diet curbs the symptoms, many doctors dispense with further testing. Because a healthy person converts lactose into glucose, the same glucose-tolerance blood tests used to diagnose diabetes can be helpful. (Patients take a lactose-laden drink, then blood glucose levels are measured over time.) A breath-hydrogen test can confirm the diagnosis by measuring by-products of undigested lactose in the breath.

>If you've got it
As with gluten intolerance, the treatment for lactose intolerance is simple: no lactose. Happily, this needn't mean cutting out dairy, according to Lass and her coauthor, Jodi Bager. Most people can digest some dairy products, especially those low in milk sugar. Yogurt and many aged cheeses are good candidates, because lactose gets consumed by yogurt's active cultures, as well as by cheese's aging process. (Lass' favorite cheese: "pure" Italian Parmesan, which she says is "not polluted with other ingredients.")

Like gluten, lactose hides in packaged foods, including lunchmeats, salad dressings, and candies. Check labels for words such as whey, curds, milk by-products, dry milk solids, and nonfat dry milk powder. Even "nondairy" products such as coffee creamer may include ingredients derived from milk.

Over-the-counter lactase enzymes, sold as tablets and liquids under such brand names as Lactaid, can ease symptoms. Other products, such as DairyCare and Lactagen, claim to condition the gut by gradually exposing it to a combination of probiotics and prebiotics, including lactose itself.

>For the rest of us
Eating more yogurt would benefit nearly everyone: It's full of healthy probiotic bacteria that aid digestion. If you seem to have trouble digesting cow dairy, try yogurts made from "alternative" milks, such as Redwood Hill Farm's goat milk yogurt, Woodstock Water Buffalo's water buffalo milk yogurt, or Willow Hill Farm's sheep milk yogurt. Substitute low-fat or fat-free yogurt for milk, cream, sour cream, or buttermilk in recipes; you'll reap creaminess with fewer calories.