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The Scary Side Effect of Running In the Heat

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Before you run on a sunny summer day, you do your prep work—you slather on sunscreen (right?) and fill your post-workout water bottle. There might be more to consider, though: New research published in the Journal of Physiology suggests that a rise in body temperature could increase the risk of dangerous heart problems in some people.

Researchers looked at proteins called sodium channels, which are made by various genes and play a key role in the heart's electrical signaling. After exposing the sodium channels to rising temperatures, theose made by a mutated gene, which could be present in people who have a family history of or predisposition to heart problems, became dysfunctional. That's a problem because disruptions in your heartbeat—known as cardiac arrhythmias—can kill you in an instant. (See What Running In the Heat Does to Your Body.)

"We don't exactly know why this particular form of the sodium channel is more temperature sensitive than others, but we suspect that it has to do with the way the protein interacts with calcium," says study author Peter Ruben, Ph.D., a professor of department of biomedical physiology and kinesiology at Simon Fraser University. Your body uses sodium, potassium, and calcium in a delicate balance to send signals between cells.

The mutation in this gene is just one form of long QT syndrome, a condition that throws your heartbeats off track. Long QT syndrome affects about one in 2,000 people, according to Ruben. "There may be other forms of the disease that are also temperature sensitive," he says. "We don't know because not all of the forms of the disease have been tested the way we did."

Even if your heart is healthy, it's smart to stock up on water when you run outside. "The usual rules of staying hydrated—like keeping your electrolytes balanced—are very important for everyone, but particularly for anybody who has any family history of this kind of disease, because they're just a little bit more susceptible to extremes in conditions," says Ruben. (Also, be aware of these 11 Heat-Related Workout Inflictions.)

Still, don't panic: Since these conditions are rare, most people who exercise outdoors won't need further evaluation, says Sumeet Chugh, M.D., medical director of the Heart Rhythm Center at Cedars-Sinai Medical Center. "In some situations, these run in the family and others have significant symptoms, which should prompt health evaluation," he says. In addition to fainting, symptoms could include dizziness, palpitations, or shortness of breath disproportionate to your workout.

If a cardiologist has ever diagnosed you with long QT syndrome, ask your doctor whether it's safe to work out in hot weather. But you should do the same if you have a family history of sudden cardiac death. One more thing: If you tend to faint often, tell your doctor. While many things—such as dehydration or nervous system problems—can cause fainting, heart trouble can also be a culprit. Arrhythmias can cause fainting by limiting blood flow to your brain, Ruben explains.

If you don't have a family or personal history of heart problems, you still need to be careful running in hot temperatures. Drink plenty of water before, during, and after your run, and don't forget the sunscreen! After all, the only thing worse than a scorching sunburn is one that leaves a racer-back tan line.

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