Can people be allergic to exercise?

Imagine a woman has a few drinks one night, pops some ibuprofen in the morning for her hangover, and goes for a run. She's never had an issue with alcohol, ibuprofen or exercise before. But this time, some combination of the three triggers a life-threatening allergic reaction known as anaphylaxis.

This is a real case that happened to a patient of Dr. Andrew Murphy, an allergist at Suburban Allergy Consultants in Pennsylvania. So, is it possible to be allergic to exercise?

Not technically, Murphy said. When a person is exposed to something they're allergic to, a protein from the allergen, for example peanuts, grass or cat dander, interacts with antibodies on immune cells in their body. The immune cells fire off chemicals like histamine that cause sneezing, hives, wheezing and other symptoms. There's no protein that enters the body when a person works out, so it's not possible to be allergic to running. 

But, exercise can trigger the same array of symptoms in people with a rare condition called exercise-induced anaphylaxis. "It's not necessarily antibody-mediated, but exercise is a trigger and the allergy [immune] cells are still activated," Murphy told Live Science.

Related: Why do I get sore days after I’ve worked out?

Exercise alone is enough to trigger an allergic reaction in some people, but the way it does so is unknown. One theory proposes that exercise causes a release of endorphins, which triggers certain immune cells to release chemicals such as histamine, according to a 2010 review in the journal Current Allergy and Asthma Reports. 

Other people, like Murphy's patient, have a condition known as food-dependent exercise-induced anaphylaxis, or FDEIA. For them, eating a certain food before working out can cause symptoms such as wheezing or hives — even if they have no reaction to that food when lounging. In the case of Murphy's patient, the "food" was either alcohol, ibuprofen, or a combination of the two. There are several theories why certain foods in combination with exercise can cause an allergic reaction, including that working out makes the gastrointestinal tract more permeable, allowing allergens to come into better contact with the immune system. Ibuprofen can further increase permeability and the body's uptake of foreign allergens, and it may directly interact with immune cells, according to the review.

Exercise-induced anaphylaxis is rare. About 2% of people in the Western world experience anaphylaxis, and 5% to 15% of cases are exercise-induced, according to the review. The food-dependent version of the condition is less common and represents between a third and a half of all cases. Not many children come into the office with either condition, Murphy said, and it generally appears in teens and adults. 

Treatment for food-dependent exercise-induced anaphylaxis is simple: Don't eat for four hours before and after working out, according to a review in the journal Expert Review of Clinical Immunology. Another option is to avoid the triggering foods entirely, if those foods are known. 

When exercise is the sole trigger, managing the condition can be harder than managing FDEIA, but doctors can help patients develop specialized exercise regimens, Murphy said. The amount of exertion someone with the condition can do safely varies. A casual stroll can trigger a reaction in some people, but others can ride a bike with no issues. Each patient must work with a doctor to figure out their own limits. Murphy always prescribes an EpiPen in case of emergency, and antihistamines can help prevent episodes, according to a 2001 review in the journal American Family Physician.

Serious reactions in response to exercise are rare. In 25 years of practice, Murphy has only seen one person pass out from it. "I usually see more people with cutaneous [skin-related] symptoms like hives, or they have some lip swelling or get wheezing with it," he said.

Originally published on Live Science.