What Happens to Your Body When You Run a Marathon

Participating in a marathon is a worthy goal that should be approached mindfully and with a firm plan of attack. Running such long distances, even with adequate preparation, precipitates several physiologic changes and challenges to the body.

Marathon running is a great way to build your immune system, burn fat and improve mental outlook. There is also potential stress on the body that comes with running 26.2 miles at once. Some of those stress factors can include increased body temperature, stress on the kidneys, lower-body soreness and trouble sleeping after the race.

If you are thinking of running a marathon, here are seven things to consider:

With a marathon, your body temperature rises throughout the duration of the run such that by the end of the race the body’s core temperature will have risen from the normal level of about 98.6ºF to temperatures as high as 102–103ºF (similar to the temperature you might have with the flu or other illness). Adequate hydration during and after your run, plus careful attention to the dangers of hyper- and hypothermia, ensures a safe finish.

Toward the end of a marathon is when you may start to feel the effects of tissue damage on your muscles and joints, especially going downhill. Your foot is cushioning each step and with each stride there may be small injuries to the feet. As many marathon runners know, it is not uncommon to lose a few toenails due to the micro-trauma of each stride on the foot. Properly fitting shoes should minimize this, but may not completely solve the issue for every runner.

The kidneys may suffer as a result of running a marathon. Yale researchers have shown that immediately after the run, upwards of 80% of marathon participants showed some form of mild-to-moderate insult to kidney function (also known as Stage 1 Acute Kidney Disease). Gradual exercise generally does not cause acute renal failure. Only strenuous exercise, coupled with not paying attention to water supplementation during exercise, can easily lead to kidney injury; even acute renal failure. Adequate hydration and paying attention to one’s body are the best preventative approaches.

This may be obvious, but it’s still worth mentioning: As you start your marathon, calorie demands may approach 600–800 calories an hour very early in the event. The average body stores 500 grams of glycogen, which equates to approximately 2,000 calories of glucose available for immediate mobilization. On average, every mile run burns 100–125 calories of glucose which means 20 miles in the entire glycogen/glucose supply has been exhausted. This is when most runners hit the infamous “wall,” also known as the point where they feel like they can’t go any further. Fueling properly with sport gels and electrolyte drinks throughout the run helps, but on rare occasions, hypoglycemia can lead to a runner passing out.

Many people have trouble falling asleep after running a marathon, in spite of feeling very tired. One reason for this is elevated levels of the stress hormone cortisol as a result of putting the body through the challenges of the distance.

Your legs may begin tightening and cramping during or immediately after the race, but delayed onset muscle soreness, DOMS, typically kicks in the day after the race and is worst over the next 2–3 days. There isn’t much you can do to prevent or deal with DOMS, though some people take anti-inflammatories to lessen the pain.

There are many benefits to running. “Running marathons improves your immune system so your body functions are more effective and efficient at fighting off germs,” says Dr. Michael J. Barber, board-certified internist, cardiologist and electrophysiologist. “You can also lose weight: Running long distances is a way to burn lots of calories and, if a nutrition program is incorporated into training, can eventually result in decreased body fat.” Running marathons also provides tremendous mental benefits including increased confidence, improved cognition and concentration, decreased stress and improved general overall outlook.

About the Author

Adrienne Jordan
Adrienne Jordan

Adrienne has been a lifestyle writer for publications like National Geographic Traveler USA Today, BBC Travel, Travel & Leisure, American Way and Men’s Journal. Some of the most standout experiences in her travels include gorilla trekking in Uganda, cruising the fjords of Norway, swimming in the devil’s pool in Zambia and road tripping through South Africa. When she is not traveling, she enjoys painting, photography and attending fashion and theatre events in her home base of New York City.