We often use the words allergy and intolerance interchangeably to describe a negative reaction to eating a certain food. “A food allergy involves the immune system. Food intolerance does not,” says dual board-certified allergist and gastroenterologist John Leung, MD, AGAF, FACAAI, adjunct associate professor at the Friedman School and founder of Boston Specialists. “This is an important distinction, because immune-mediated reactions have the potential to be life-threatening.”
Food Allergy. Approximately eight percent of children and 11 percent of adults in the U.S. have a known food allergy.
One of the ways your complex immune system fights intruders is by creating antibodies—proteins in the blood that recognize specific unwanted germs or substances. One type of antibody is immunoglobulin E (IgE). “The majority of food allergies are IgE-mediated,” says Leung. For an unknown reason, your body thinks a food or food component is dangerous and creates IgE antibodies. The next time you eat that food, the IgE “army” is called up, which signals cells to release chemicals to fight the “invader.” Unfortunately, these chemicals cause symptoms like eczema, hives, runny nose, trouble breathing, swelling, and even anaphylaxis—a severe, potentially life-threatening allergic reaction.
There are other immune-mediated allergic responses that do not involve IgE antibodies. One well-known example is celiac disease. “In celiac disease, immune cells called T-cells react to the protein gluten, which is found in wheat, rye, and barely,” says Leung. “This reaction damages the lining of the small intestine.” Other less well-known non-IgE mediated allergies include eosinophilic esophagitis (EoE), which causes narrowing of the esophagus, difficulty swallowing, and possible choking, and FPIES (Food Protein Induced Enterocolitis Syndrome), a life threatening allergic reaction that causes vomiting, diarrhea, dehydration, and, potentially, hypovolemic shock in infants.
IgE-mediated reactions typically occur immediately. Non-IgE mediated immune reactions can take up to 48 hours to develop.
Food Intolerance. About 25 percent of people in the U.S. have some kind of food intolerance. These reactions do not involve the immune system. They are not life threatening, but they can cause life-disrupting discomfort.
The most well-known example is lactose intolerance. Lactose is a milk sugar. The body makes an enzyme called lactase that breaks down this sugar during digestion so it can be absorbed. If the body does make enough of this enzyme, the lactose cannot be fully digested and is instead fermented by gut bacteria, causing gas, bloating, pain, and diarrhea.
Dietary fructose intolerance (also called fructose malabsorption), in which the intestines cannot absorb the sugar fructose, is often a trigger for symptoms of irritable bowel syndrome (IBS).
“Intolerances are not driven by immune reactions, so they are not allergies,” says Leung. “Even experts often use these terms incorrectly. Celiac disease, for example, is often called gluten intolerance, even though it is immune-mediated.” Food sensitivity is another term often loosely applied to intolerances.
Diagnosis. IgE-mediated reactions can be diagnosed with a blood test. Skin prick tests can also be used: A health-care provider puts a tiny bit of the allergen on the skin and then scratches or pricks the skin so the allergen can get into the body. If you are allergic, you will develop a small red bump at the site within about 15 to 20 minutes.
Non-IgE mediated immune reactions are harder to diagnose. “We do not currently have research-supported tests that can detect non-igE mediated immune responses,” says Leung. Several companies advertise tests for immunoglobulin-G (IgG) responses, but these have not been scientifically proven to do what the companies claim they do. Due to the lack of evidence to support it, many organizations, including the American Academy of Allergy, Asthma & Immunology, have recommended against the use of IgG testing.
Lactose and fructose intolerance can be diagnosed using breath tests that measure gases created when gut bacteria ferment undigested or unabsorbed sugars in the intestines.
Diagnosis of celiac disease is done by looking at the small intestines (endoscopy) and running a special blood test.
If diagnosable diseases and allergic conditions have been ruled out, an elimination diet is another way to determine which foods, if any, are causing your symptoms. If you suspect a particular food or ingredient is causing your discomfort, you can try removing it from your dietary intake and see if your symptoms resolve.
A true elimination diet, which involves removing almost all foods from the diet and adding them back in a prescribed way over time, should only be performed under the guidance of a health-care professional to avoid unnecessary restrictions and possible malnutrition.
Treatment. With IgE-mediated food allergies, avoiding even trace amounts of the allergen is essential. That said, some people allergic to milk or eggs can tolerate small amounts if they are fully cooked. (This is not true for all allergens.) Non-IgE mediated allergies may require complete avoidance as well. Immunotherapy—where the immune system is exposed to small amounts of an allergen in a medical setting—is being tested as a treatment for food allergies, but it is not yet approved by the U.S. Food and Drug Administration.
With food intolerances, avoidance will prevent discomfort, but is not a matter of life and death. Importantly, people with an intolerance can often consume some amount of the trigger food without symptoms. People with lactose intolerance have the option of taking lactase enzymes before eating dairy or choosing lactose-free dairy products.
What to Do. While IgE-mediated allergies are severe and readily apparent, non-IgE mediated reactions have symptoms that might not be obviously related to eating.
If you often feel unwell after eating, or if you have unexplained symptoms like fatigue, “brain fog,” and headaches that cannot be attributed to other conditions, see a health-care professional. Your primary care provider is a good place to start. You may then be referred to an allergist, gastroenterologist, and/or registered dietitian for testing, diagnosis, and treatment.