How Does Running a Marathon Impact Your Body?

On April 21, as the sun stretches across the skyline in Boston, an estimated 30,000 runners will be stretching their limbs in preparation for a 26.2-mile run across several cities and towns. The Boston Marathon, like any endurance event, will test runners’ minds and bodies — but how exactly does running for hours at a time affect specific parts of the body, like a runner’s hormones, heart, and lungs? 

Three University of Colorado Department of Medicine doctors — who have each finished at least one marathon — explain how running long distances can affect a person’s endocrine, cardiovascular, and respiratory systems — and tips on how runners can best prepare for these races. 

A runner’s hormones 

As soon as a person starts running, the endocrine system kicks into gear, helping the body maintain a physiological equilibrium. Seth Creasy, PhD, an associate professor in the Division of Endocrinology, Metabolism, and Diabetes and director of research integration for the CU Anschutz Health and Wellness Center, says it “acts as an internal messaging system” in which hormones are the messengers that help regulate bodily functions like metabolism, stress, and hydration. 

“The endocrine system keeps everything in balance and ensures the body has energy to keep running,” Creasy says. “Hormones help increase breathing, heart rate, and blood flow to ensure enough oxygen is being delivered to the working muscles. In addition, other hormones will be altered to promote energy availability.” 

For example, insulin from the pancreas will decrease to prevent glucose from being stored, he explains. Glucagon, a hormone formed in the pancreas that helps regulate blood sugar levels, will increase to release stored glucose from the liver. Other important hormones, called the antidiuretic hormone (ADH) and aldosterone, will help with hydration levels and blood pressure maintenance. 

Seth Creasy, PhD, ran a trail marathon in Buena Vista, where he learned the importance of not running too aggressively at the beginning of the race and having proper nutrition. Image courtesy of Creasy.

Metabolism impacts and avoiding “the wall” 

A runner’s metabolism also changes throughout the 26.2-mile run.   

“Anaerobic metabolism is a way of providing energy without needing oxygen and is relied on early during the exercise,” Creasy says. “As the exercise continues, the body will lean more heavily on aerobic metabolism. Aerobic metabolism requires oxygen. Early on in exercise, your body will rely more heavily on carbohydrates as a fuel source, but as the exercise continues, this will shift to fats.” 

It’s important that marathon runners remember to eat or drink something. If a runner does not consume calories regularly during the race, then the body will completely deplete its liver and muscle glycogen (a form of stored energy). If this happens, it leads to runners “bonking” or “hitting the wall,” Creasy explains. 

“This is characterized as a period when your body is unable to provide energy fast enough to continue exercise and is associated with feeling dizzy, shaky, weak, and confused,” he says. 

After running a marathon, Creasy estimates it will take one to two weeks for the endocrine system and metabolism to fully recover, as there are “a series of recovery mechanisms post marathon.” 

“The more immediate recovery response would focus on letting the foot off the gas for some hormones that were helping support the continued exercise,” he says. “Over the next few days, some hormones like cortisol may remain elevated as you deal with inflammation and increased muscle breakdown. Things that will help with recovery time include your training level, nutrition, sleep, hydration, and post-race activity levels.” 

An adaptable heart  

When a runner sets out for a long distance, it’s important to be running at a pace that is sustainable and well below their maximum level of effort, explains William Cornwell, MD, an associate professor of cardiology, the sports cardiology director at the CU Anschutz Medical Campus, and director of the cardiac rehabilitation center and cardiac stress lab. Completion of an endurance event, like a marathon, requires a person to maintain a submaximal pace for a long period of time. This means that an individual’s heart rate will be sustained at a moderately increased level compared to the resting value, but still well below the maximum heart rate.  

“The human body is a machine, and the heart is the pump,” Cornwell says. “There should be a commensurate increase in heart rate and blood pressure to deliver the nutrients to the exercising muscles so they can generate energy to move the body.” 

People running a marathon should focus on sustaining a pace that will likely have a cardiac output — or how many liters of blood a heart pumps per minute — of roughly 14 to 16 liters. “Think of a two-liter bottle of Coke — the heart will pump through about seven of those per minute in this scenario,” he says. 

For comparison, a body at rest pumps about six to eight liters and at maximum exercise may pump three to five times that amount. While measuring cardiac output requires technology that the average person doesn’t have access to, runners can track their fitness level through their VO2 max, which is available as a measurement on several smartwatches. 

“The higher your VO2 max, the fitter you are and vice versa,” Cornwell says. “Your cardiac output will increase in proportion to VO2 max.” 

William Cornwell, MD, has run several marathons and Spartan Races (a blend of endurance running with tackling obstacle courses). The image above is from when he ran the Utah Spartan 50K in 2022. Image courtesy of Cornwell. 

Yet, no matter a person’s fitness level, all the extra work the heart does to move an increased volume of blood through the body has a compound effect over time. The average marathon runner will complete the race around the four-hour mark. 

Cornwell says this is significant because researchers have found that several hours of exercise has a more detrimental impact on the right side of your heart than it does the left side of your heart. This is because the muscle on the right side of the heart is much thinner than the left.

“The left side of your heart has to pump blood throughout your entire body, while the right side of the heart only has to pump blood into the lungs,” he says. “When you exercise for several hours, the pressure in the lungs increases, and over time, the right side of the heart may begin to fatigue. The longer you exercise, the more stress there is on the right side of the heart, and it may be unable to squeeze as strong as before. You don’t really see that on the left side because the left side of the heart is thicker and stronger than the right side.” 

Fortunately, within a week, the heart and cardiovascular system will return to baseline function, meaning both sides of the heart are squeezing strongly again.  

Avoid the side cramps — breathe in, out 

A person’s airways are often the most affected part of the respiratory system when running a marathon, especially if they are running in a dry or polluted environment that can further inflame their airways, explains pulmonologist James Maloney, MD, a professor in the Division of Pulmonary Sciences and Critical Care Medicine. 

“The respiratory muscles, the diaphragm, and the accessory muscles of breathing, particularly the intercostal muscles between the ribs, are also impacted,” he says.  

It’s important to find a pace where runners can have a stable, comfortable breathing pattern, Maloney explains. 

“A normal breathing pattern is when you breathe in and your abdomen goes out a little bit to accommodate your downward moving diaphragms, and then when you breathe out, your abdomen relaxes passively and comes back to a neutral position,” he says. “Some runners may develop paradoxical breathing patterns where their abdomen goes out when they’re breathing out or goes in when they are breathing in — the opposite of what’s normal. This can lead to inefficient ventilation and the dreaded side stitches and side aches.”

Once a runner has the right pace and breathing pattern, Maloney says it’s important to hydrate at fuel stations throughout the race — not just to replenish what’s lost through sweat, but because of what’s lost from the lungs.

“There often is a drying effect on the lungs from all the air moving in and out, so staying hydrated is important to prevent that, particularly in dry Colorado,” he says. “Hydration is also important because there is a fair amount of water being lost from the lung airways just because of the increased ventilation across them. This is less of a problem with marathons in humid climates.” 

After running a marathon, Maloney says there is a 10-15% drop in lung capacity for a few days, meaning a runner’s ability to take a deep breath and blow air out, which is likely because the lungs and breathing muscles are fatigued just like the leg muscles are.  

“It may take a week for those to recover fully,” he says.   

James Maloney, MD, ran his first marathon — the Capital City Marathon in Olympia, Washington — on a sunny, hot day in 1984. Image courtesy of Maloney.

Tips for marathon success  

All three doctors agree that taking the time to properly train for a marathon is key to success, as it will help the runner strengthen their respiratory, cardiovascular, and endocrine systems. 

“The consistent training will prime the body for all the perturbations and challenges posed by prolonged exercise. For example, the body will become better at extracting oxygen, which can then be used to fuel muscles during exercise,” Creasy says. 

Regularly training for a marathon will also help increase liver and muscle glycogen storage capacity, Creasy adds. This will help the body improve fuel utilization, become more efficient at hormone release and hormone responsiveness, and recover faster. 

For people who may have risk factors or health issues — such as asthma, uncontrolled diabetes, cardiovascular disease, hypertension, or chest pain that worsens with exercise — it is important to check in with their primary care provider before running a race to ensure they are fit to run. Maloney recommends anyone over the age of 40 who is not a seasoned runner to do the same.

“Get the input of your primary care physician on how ready and safe you are for training and whether any other screenings or tests might be needed beforehand,” he says. “Also, getting screened for anemia may be valuable, particularly for women who are still menstruating because they are often iron deficient and may be anemic. If you’re marathon training with anemia, then your heart and lungs have to work even harder to provide oxygen to your muscles.”

Running a marathon isn’t for everyone and that’s OK, explains Cornwell, but it’s important for everyone to prioritize regular exercise to stay healthy. 

“Our society is entirely too sedentary. The data would suggest that people, on average, are spending seven to 10 hours a day using mobile devices, their phones, computers, and TVs,” Cornwell says. “You don’t need to incorporate marathons into your lifestyle in order to age gracefully and maintain cardiovascular health, but you do need routine exercise.”