Maybe your friend is skipping gym meetups for a date with Grey’s Anatomy and a glass of pinot noir — or they might be allergic to exercise like they told you. Yes, it is an actual ailment.

The condition is called exercise-induced anaphylaxis, or EIA, and it’s much worse than waking up with sore muscles. Flares can occur during or after exercise and the reaction causes flushing, wheezing, nausea, swelling of hives, and various other symptoms.

While the symptoms are reason enough to worry, the allergy is rare. It only affects about 2% of the western population. If you’re still concerned about any reactions you experience from exercise, there are a few things to know before tossing out the gym bag.

Exactly why it occurs is unclear
Maria Castells, an allergist at Brigham and Women’s hospital, says researchers and doctors alike are not sure why it occurs. While there are many theories, testing them proves to be difficult due to the inability to recreate the various situations in the lab. “So there’s no mouse model and no human model of the ideas,” Castells says.

Scientists have discovered, however, that the same exercises don’t always produce the same results. The only exercise not yet associated with having a negative reaction is swimming.

There are various triggers
If you’re thinking that going easy in the gym is a simple resolution, unfortunately it’s not so simple. Regular activities like walking or doing yard work can produce reaction symptoms. For about 30 to 50 percent of sufferers, the reaction is food-dependent and exercise-induced. This means the symptoms are a result of a colaboration of certain foods combined with exercise. Some reactions may also arise with the use of pain relief drugs. There have been cases of women who are in menstruation and the high levels of estrogen provokes the reaction.

“There are a variety of things that it might be,” Castells says. “And for a portion it’s nothing, really, just the exercise itself.” When it comes to how much exercise is needed to produce a reaction, that too varies from individual to individual; however, those who are in shape are less vulnerable to a reaction.

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