What Running a Marathon Does to Your Body

The elite men's runners race near the start of the 118th Boston Marathon Monday, April 21, 2014 in Hopkinton, Mass. (AP Photo/Michael Dwyer)
The elite men's runners race near the start of the 118th Boston Marathon in 2014.

A marathoner crossing the finish line rarely looks picture perfect: There are at least 26.2 kinds of stains a runner can accrue over the course of the race. But there’s even more happening below the surface, and slow muscles are just the beginning. Here’s what a marathon does to different parts of your body:

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More blood and oxygen flow to your noggin, which strengthens neuron connections and keeps you sharp, says Jen Schwartz, M.D., a runner and sports medicine doctor at Beth Israel Deaconess. A flux of endorphins and “feel good” hormones can lead to “runner’s high,” and Harvard research has found that you’ll release BDNF, a brain-health protein. Long term, runners enjoy improved concentration and memory, too. In fact, research has linked brain size to how long and hard an animal can run. A PloS One study even suggests the large size of the human brain is linked to endurance exercise.


“When you’re sitting at rest, your heart pumps three to five liters per minute,” says marathoner Aaron L. Baggish, M.D., associate director of Mass General’s Cardiovascular Performance Program. “During a marathon, it has to maintain a cardiac output about three to five times higher than that.” There are two ways to increase blood flow from the heart, he says: Squeeze out more blood each time your heart contracts, or speed up how many times your ticker beats per minute. You do both over 26.2 miles, and while this is a stress on the heart, “for a prepared athlete, it’s a stress that can be well tolerated,” Baggish says.


Not only does your breathing rate go up, but your lungs receive more blood, helping to increase your “tidal volume”—the volume of air moving in and out of your lungs, says Schwartz. Over time, runners become more efficient not only at exchanging oxygen and CO2, but also at taking in more air in one breath, she says. Breathing according to your stride (three steps per inhale, three per exhale) can reduce how hard your muscles have to work during a run and improve endurance performance, according to a different PLoS One study.


Your stomach takes a double whammy: Intestines jostle around and your body sends blood away from your gut to places like your legs, explains Krista Austin, Ph.D., who worked with 2014 Boston Marathon winner Meb Keflezighi. This can cause inflammation, hormone changes, and even the leaking of toxins—all of which can lead to nausea, says Schwartz.

“You also begin to dehydrate—and in order for your gut to function right, you need to be hydrated,” Austin says. Load up on high glycemic carbs, though, and you could cramp up or find yourself in need of a bathroom—stat. A steady stream of low-glycemic carbs and fluids can help you perform without pain, says Austin.


Your muscle fibers grow as you train. You’re left with a surge of slow-twitch fibers (great for lower intensity, higher endurance exercise), more blood vessels, and more mitochondria (cells where energy is produced). During the race, glycogen—the fuel you’ll use—is stored in your muscles more effectively than it is in those of non-endurance runners. You’ll be well-fueled by increased blood flow to the muscles and more heart contractions. But this uptick in activity can also lead to the build-up of waste products like lactic acid, says Schwartz. Hence some soreness the day after the race.


Blisters, chafing, calluses and lost toenails are familiar problems for marathoners. Also: sweaty feet. Pounding the pavement can also cause decreased blood counts in the feet and even the trauma or breakdown of red blood cells, something called “foot strike hemolysis,” says Schwartz. Musculoskeletal problems and tendon issues can also result from long runs. What can you do about it? Rest up. After the race, of course.

Read more coverage of the 2015 Boston Marathon .