7 weird (but totally normal) side effects of running a marathon

Running 26.2 miles doesn't always make for the prettiest photo opportunity.

Runners run through discarded water cups along Commonwealth Avenue near "Heartbreak Hill" during the running of the 120th Boston Marathon. –Keith Bedford/The Boston Globe

Running a marathon is many things: It’s difficult, inspirational, rewarding, and exhausting. But sometimes, let’s face it, running a marathon can also be downright disgusting. Any runner knows all too well that a 26.2-mile trek can take a toll on your body. From the urge to find a bathroom along the course to those bloody nipples, here are seven common (but not-so-pleasant) potential side effects of going the distance.

Runner’s diarrhea

Ever notice those portable toilets in Hopkinton are always packed? It’s pretty common to have to go No. 2 at the starting line, says Aaron Baggish, M.D., associate director of the cardiovascular performance program at Mass General’s Heart Center and co-medical director for the Boston Marathon. Blame a mix of nerves, excitement, and whatever is in your stomach. Plus, running jostles around everything in your stomach and draws blood away from your gut to your legs, making an accident more likely.

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For the most part, a little GI distress is normal. But pre-race nutrition is important, says Terrence Mahon, a Boston Athletic Association high performance coach. “The night before the race should not be about experimenting with foods that are too spicy or too different than your normal routine,” he says. “This is the one time when simple and boring are good things.”

And consider packing some TP. “You just never know when that irritable stomach becomes a monster and you end up on a mad pursuit for the nearest toilet,” says Mahon. “The last thing you want to find out at that time is that it is out of paper.”

Bloody nipples

“Every year, you see guys coming down Boylston Street with red streaks coming from their chest area,” says Dr. Baggish. Bloody body parts are almost always caused by repetitive abrasions. Prevention happens through trial and error, but for men, adhesive bandages or duct tape over the nipples work well, says Dr. Baggish.

Chafing

Chafing is a runner’s worst enemy, but it’s common on the inside of the legs where your shorts rub together, says Dr. Baggish. Side step the issue by finding shorts that work for you. “The rougher the material the more it chafes the skin and the greater the chance of bleeding,” says Mahon.

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Vaseline and other anti-chafing rubs will be your best friends. Apply them to your inner thighs, nipples, and armpits. And remember, these products work best as preventative measures before any bleeding starts, says Dr. Baggish.

Black toenails

These are also known as the universal sign that you’re a runner, according to Mahon. The culprit(s): shoes that don’t fit, running downhill, or a soggy sneaker that leaves your foot moving around. “Get shoes that fit your biggest toe—remembering that this may not always be your first toe,” says Mahon. You want enough room up front so that you’re able to wiggle your toes around, but not so much that you can slide around, he notes. And repeat after us: Never, ever wear shoes you bought at the Expo the day before.

Blisters

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The recipe for blisters: A hilly course, a hot day, and socks that bunch up. Your foot expands about one-fourth of a size when you run, says Mahon. So you really need that wiggle room. And decent socks matter. “Moisture-wicking is a must; and go with a synthetic blend that hugs your foot well,” he suggests. Simply prone to the painful suckers? Rubbing Vaseline on your feet before lacing up can cut the friction build up, he says.

Tons of sweat

Dr. Baggish says that he fields this question a lot: Can I sweat too much? The way he sees it: “As long as you’re continuing to make sweat, you’re in a good situation.” Docs worry, instead, when people stop sweating, when they are hot but dry. This can be an ominous sign of heat exhaustion, when your core temp is rising to unsafe levels, he notes. Stay hydrated and fueled to fend off heat-related illnesses. Aim to drink four to six ounces every 20 to 25 minutes of running, suggests Mahon.

The bonk

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Any big runner has heard of bonking—or hitting a wall—mid-run. What causes it? “Glycogen (AKA carbs) is a runner’s gasoline,” says Mahon. The problem? “We can only store enough gas in our tanks to last for about 90 minutes to 2 hours of higher intensity running, but the marathon takes longer than that to complete for even the greatest of runners.”

Fortunately, modern sports nutrition has come a long way: carbohydrate drinks, gels, and gummies are common in distance races. “The nutritional support that we get from these options all help to keep our fuel tanks topped off,” Mahon says. Your training runs help you identify which carbs work best for you, he says. So stick to your pace and your plan. A gel or sports drink every 40 minutes of running usually does the trick, too, he notes.

Photos: Scenes from the 2016 Boston Marathon

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