A crate of eggs with caution tape on it.
Photo: Trunk Archive

Y ou’ve always enjoyed shrimp, but then one evening after savoring some scampi, you break out in itchy hives. Could it have been the shellfish?

It’s possible.

“Most allergic reactions happen quickly, right after you’ve eaten a food,” says Princess Ogbogu, M.D., associate professor of medicine and director of allergy and immu­nol­ogy at The Ohio State University Wexner Medical Center.

Early allergies to ingredients such as eggs, milk, and soy often resolve during childhood, but they can resurface later. And at any point in your life, you can ­develop new food allergies (which can be life-threatening) or intolerances (which generally cause gastrointestinal distress).

More on Food Allergies

In fact, a study from the Annals of ­Allergy, Asthma & Immunology found that 51 percent of people with allergies developed at least one of them as an adult.

The good news: Food allergies affect less than 4 percent of adults. “The number of people who have a food intolerance is probably higher,” Ogbogu says.

Although the terms “allergy” and “intol­er­ance” (also called “sensitivity”) are frequently used interchangeably, they’re significantly different—and getting the right diagnosis will ensure that you find and follow the correct treatment plan.

Here’s what you need to know to determine why a food is not agreeing with you and how to protect yourself from dangerous reactions or frequent indigestion.

Food Allergy or Intolerance?

A food allergy is an abnormal and overactive response from your immune system to something that it shouldn’t react to.

When you eat—or sometimes even come into contact with—the offending ingredient, your immune system revs up production of antibodies: proteins that fight foreign substances. These indirectly trigger the production of histamines, which start what we recognize as an ­allergic reaction. This whole cascade happens quickly, usually within minutes.

Allergy symptoms can involve your skin, respiratory system, and gastrointestinal system. They may be mildly uncomfortable or downright dangerous, and can vary in severity and type each time you come into contact with the food.

Swelling, itching, hives, shortness of breath, difficulty swallowing, lightheadedness, and/or vomiting within a few minutes to a couple of hours of eating a food may signal an allergy. “If you have such symptoms after a meal, see your physician or an allergist,” Ogbogu says—after seeking emergency care, if needed.

This is especially crucial if you have a reaction after eating shellfish. “It’s rare to develop an anaphylactic ­allergy to most foods as an adult, but shellfish is the exception,” says ­Manish ­Ramesh, M.D., Ph.D., director of the Food Allergy Center at Montefiore Medical Center in the Bronx and Scarsdale, N.Y. (Anaphylaxis is a life-threatening reaction that causes difficulty breathing and loss of consciousness.)

Food allergies can develop in anyone, but they’re more likely to occur in those who have other allergic conditions. “People who have environmental or seasonal allergies, eczema, or asthma are at an increased risk,” ­Ramesh says.

Among adults, shellfish is the most prevalent food allergy, but milk, eggs, soy, peanuts, and tree nuts are also common offenders.

Food intolerances or sensitivities usually don’t involve the immune system. “A food intolerance occurs in the digestive system because it’s the inability of your body to break down certain substances,” ­Ramesh says. Experts don’t understand all the mechanisms involved, but they have decoded the most prevalent one: dairy.

“When you’re lactose-­intolerant, your body isn’t producing enough of the enzyme that breaks down the sugar in milk,” Ogbogu says. The condition becomes more common as you age—an esti­mated 117 million U.S. adults and children 10 and up are lactose-intolerant.

Unlike allergy symptoms, the signs of intolerance are most often digestive, such as gas, bloating, diarrhea, and abdominal cramping.

“The symptoms can vary in intensity depending on how sensitive you are to the food,” Ramesh says. When the only symptom is digestive—such as vomiting—food poisoning could also be the culprit.

Diagnosis and Treatment

Because a serious food allergy has the potential to land you in the hospital, it’s important to get an accurate diagnosis.

Doctors use a skin or blood test (or both) to confirm an allergy. “The two are often interchangeable, and it depends on the food and who’s doing the testing,” Ramesh says.

During a skin test, drops of allergen extracts are applied to your skin, which is then pricked. If you’re allergic to the food, a raised bump will usually appear within 15 minutes. A blood test measures levels of an antibody called IgE that is produced in response to allergens.

If the tests are inconclusive, a doctor may have you try an oral food challenge, where you eat a small quantity of a potential allergen under medical supervision.

The treatment plan for a food allergy is simple yet life-changing: Avoid any contact with the food that’s causing problems. Be extra cautious when eating out (alert your waiter), and carefully read labels when purchasing packaged foods.

“One allergic reaction doesn’t predict the next,” Ogbogu says. “Even if you’ve had a fairly mild reaction in the past, a future reaction could be more severe.”

That’s why many doctors will prescribe self-injectable epinephrine (Adrenaclick, EpiPen, or generic) when you test positive for a food allergy. “If your physician recommends an EpiPen, it’s impor­tant to carry it with you at all times,” Ogbogu says. “It could save your life in the event of an accidental exposure that causes anaphylaxis.”

Pinpointing food intolerances is more challenging than diagnosing allergies. Aside from lactose and fructose intolerance (which can be confirmed via a breath test), most food sensitivities can’t be accurately detected through lab work. But if you have gastrointestinal symptoms after eating, you should still see your primary care doctor to rule out other causes, such as inflammatory bowel disease.

Skip home tests, which are frequently inaccurate and may cause you to eliminate a food from your diet unnecessarily. Many, Ramesh says, measure an antibody that has little to do with food allergies or sensitivities.

Your doctor may ask you to try an elimination diet—where you stop eating many types of potentially problematic foods, then reintroduce them one at a time—to zero in on the food that might be causing your digestive problems.

He or she might also suggest an easier plan that combines journaling with eliminating just one food at a time. “Keep a diary of everything you eat and how you feel afterward,” Ogbogu says. “If you have symptoms every time you eat a certain food, try eliminating it from your diet to see if you feel better.”

When you’ve found an offender, you may want to try introducing it back into your diet in small amounts—some people with food intolerances can handle limited quantities of the trigger item with no symptoms.

Different forms of the offending food may also be tolerable. For example, if you’re lactose-intolerant, you might be able to eat certain cheeses or yogurts without feeling sick. And if you’re sensitive to fructose, you may find that you can handle certain fruits but not high-fructose corn syrup.

Smart Swaps for Problem Foods

Dairy, nuts, shellfish, and wheat are the most common foods that cause adverse reactions. But cutting them from your diet may rob your body of calcium, protein, fiber, and omega-3 fatty acids. Talk to your doctor if you’re not sure you’re getting enough of these nutrients.

Here, Jen Bruning, M.S., R.D.N., L.D.N., a spokesperson for the Academy of Nutrition and Dietetics, explains how to replace them.

Calcium. This nutrient is abundant in dairy, and it’s crucial for maintaining your teeth and bones. Milk-free calcium-rich sources include dark, leafy vegetables, such as broccoli rabe and collard greens; fish with soft bones, such as sardines and canned salmon; and fortified ­orange juice and almond milk.

Protein. This macronutrient helps your body make bones, muscles, cartilage, skin, and blood. It’s also important to keep hormones and enzymes functioning properly. If you’re limiting dairy or cutting out seafood, eat lean meats, beans, peas, nuts, seeds, and whole grains to ensure that you’re getting enough protein.

Fiber. Going gluten-free? “Choose grains like oats, quinoa, and sorghum to replace some of the fiber in your diet,” Bruning says. Legumes, vegetables, and fruits are also rich in the nutrient. Fiber boosts bowel health and regularity, and may help lower cholesterol and control blood sugar.

Omega-3 fatty acids. These may help protect against heart disease. If you can’t eat omega-3-rich fish and shellfish, load up on walnuts, and chia and flax seeds. 

Editor’s Note: This article also appeared in the October 2018 issue of Consumer Reports On Health.