By Melissa Dahl

Does all that exertion on the elliptical machine make you nauseous? Have you ever been convinced that if you spend even one more minute on the treadmill, you will actually die? Maybe it’s not all in your head.

A few people are actually allergic to exercise, and in very rare cases, a sweaty workout could be enough to kill them.

Exercise-induced anaphylaxis is a fairly rare condition which can cause hives, fainting, vomiting and difficulty breathing during a workout, and the symptoms can last up to four hours after it. In some cases, it can be triggered by certain foods eaten before exercise, like peanuts, shellfish, eggs or even, in two reported cases, celery. But this isn't just your average food allergy, an expert explains.

"These are people who will not have this reaction unless they exercise right after eating this food," says Dr. Jacqueline Eghrari-Sabet, an allergist in private practice in Montgomery Village, Md., and a spokeswoman for the American Academy of Allergy, Asthma and Immunology. "Eating shellfish and sitting there? Nothing. But eating shellfish and exercising? For these people, it's bad news."

As you exercise and your heart rate speeds up, your blood starts whizzing through organs much faster, and therefore more frequently, than it normally does. With every trip your blood takes to your stomach, it's picking up more, say, celery bits. For those with exercise-induced anaphylaxis, the normal amount of celery antigens picked up by the blood isn't enough to bother them. But while exercising, the extra celery bits their blood is picking up causes an allergic reaction.

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Running and jogging are the most likely to trigger an attack, but other strenuous activities like dancing, volleyball, skiing and even yard work can also cause a reaction.

Since the 1970s, only 1,000 cases of exercise-induced anaphylaxis have been documented – and among those cases, one death. Experts believe that’s because many people with this condition are able to recognize the symptoms quickly and keep it under control by waiting a couple hours after they eat to work out and bookending their workouts with a slow warm-up and cool-down.

"It'll usually happen when you're really exercising," says Eghrari-Sabet. "I don't think you're going to get it when you're bowling. But if you're doing cardio or a hip hop class, then, yes."

Others suffer from the less serious exercise allergy cholinergic urticaria, a common type of heat rash, which differs from anaphylaxis in that it starts and ends with the skin reaction – no nausea or difficulty breathing for these folks. Ten to 20 percent of the population will experience some form of it during their lives. Besides exercise, sun exposure, spicy foods or even getting too emotionally worked up can cause an itch attack.

The condition can strike spontaneously, so even if you’ve been exercising all your life with nary a rash, you can unexpectedly break out in hives. Even some marathon runners have suddenly come down with a bad case of the itches after jogging, explains Eghrari-Sabet. Women are most susceptible to the condition, and the average age for its first appearance is 16. (A handy way to get out of gym class?)

Unfortunately for people seeking an excuse not to break a sweat, most dermatologists and allergists send their patients with exercise-related allergies right back to the locker room.

“If they come to me, I’m not going to tell them not to exercise,” says Dr. Bruce Robinson, a Manhattan dermatologist. Instead he advises patients to pick a less strenuous regimen or a cooler place to work out.

Because the itchy sensation happens when body temperature suddenly rises, it can be eased by warming up and cooling down slowly, before and after every workout. Or try swimming for your normal cardio routine, which will keep the body temperature cool. If a food allergy is the culprit, don’t eat for a couple of hours before your workout.

Although serious side effects are rare, some experts believe, that exercise-induced anaphylaxis often goes undiagnosed. So if you start to feel itchy while working out, watch out. It’s probably best to avoid death by treadmill at all costs.

For more on exercise allergies and other workout quandries, read our Smart Fitness column.