What happens to our body when we run a marathon?

Running on the streets has become a phenomenon in Brazil: in 2024, there were almost 3,000 official races in the country, an increase of 29% compared to the previous year, according to the Brazilian Association of Street Race Organizers. Millions of Brazilians are challenging themselves to long-distance races, such as the 42.195-kilometer marathon.

The benefits of running include improved cardiovascular fitness, weight control and blood sugar levels. However, along with the gains, concerns arise. The death of a 20-year-old man during the Porto Alegre International Marathon on June 7th has rekindled awareness of the risks associated with extreme effort. After all, completing such a race requires extreme adaptation in the human body, much more than just putting on your sneakers and running to the finish line.

During a marathon, the body needs to adapt quickly to cope with prolonged effort. “The muscles consume a lot of energy and oxygen, the heart beats faster to pump blood, and breathing accelerates to maintain effective gas exchange. Body temperature rises and there is a great loss of water and mineral salts through sweat, in the process of cooling the body,” explains cardiologist Luciana Janot, a reference physician for Rehabilitation and Sports Medicine at Hospital Israelita Albert Einstein.

While the lungs increase their volume and respiratory rate, the heart pumps up to four to six times more blood per minute compared to when it is at rest to provide sufficient oxygen to the muscles. The metabolism acts like a true orchestra, with the use of carbohydrates and fat alternating according to the athlete's energy demand and fitness, to maintain the necessary energy until the end of the race. “Physical exercise is a major physiological stressor, and the marathon represents a more prolonged stress,” summarizes the cardiologist.

The brain also plays a crucial role during running, developing a greater tolerance to discomfort and helping athletes maintain focus and emotional control to deal with progressive fatigue. Research published in 2017 in the International Journal of Exercise Science showed that during long-distance running, levels of cortisol (a hormone that helps the body deal with situations of physical or emotional exertion) and alpha-amylase (an enzyme that is a marker of acute stress) initially increase but can be suppressed over extreme distances—a neuroendocrine response that can help athletes maintain performance under prolonged stress.

Time to rebalance

Right after crossing the finish line, the body enters a critical period of recovery. According to Janot, it is common to feel muscle pain, intense fatigue and even changes in sleep and appetite. “These reactions are normal and are part of the natural repair process,” he explains.

In the first 24 to 48 hours after the race, the body begins to reorganize itself, initiating an inflammatory response to repair muscle micro-injuries, eliminate toxins produced and recover energy reserves. Biological markers, such as creatine kinase (an enzyme that helps produce energy in muscle cells) and troponin (a protein that indicates cardiac stress), tend to be at high levels, indicating significant effort and overload.

The immune system also needs time to rebalance itself. “In the days following a marathon, the body is more susceptible to infections, so it’s important to respect the rest period,” says Janot. This period of vulnerability, known as the “open window,” can last from 24 to 72 hours after the event. A meta-analysis published in 2024 in Exercise Immunology Review concluded that athletes have a high risk of developing respiratory infections during this period due to the temporary suppression of immunity induced by extreme effort.

Complete muscle recovery varies from person to person. “It varies between 10 and 20 days, depending on the athlete’s fitness, the intensity of the effort, their diet and the quality of their sleep,” explains the cardiologist. This time reinforces the importance of adequate recovery both to restore physical performance for the next race and to preserve overall health.

Long-term benefits and risks

Preparing for a long-distance race requires significant changes in the athlete's body. The heart, lungs, muscles and brain adapt to meet the demands of effort without compromising health. According to the cardiologist at Einstein, the main benefits of this process include increased cardiovascular efficiency, with a reduction in resting heart rate, improved insulin sensitivity and a greater ability to use fat as an energy source.

However, gains can be compromised if there is excess. Therefore, training regularly is essential to improve physical fitness, protect the heart and strengthen the body. “As in any adaptation process, there is a point at which healthy effort can become overload and cause harm,” says Janot.

Furthermore, runners who train intensely for long periods, without adequate recovery intervals, can develop overtraining syndrome. “Some signs indicate that the limit may be being exceeded: persistent fatigue even after rest, prolonged muscle pain, decline in performance, sleep disorders, mood changes and increased resting heart rate,” warns Janot.

Cases of sudden illness, although rare, also stand out among the risks. A study published in JAMA in May of this year analyzed 29.3 million finishers in marathons and half-marathons in the United States between 2010 and 2023, and found 176 cardiac arrests — 0.54 per 100,000 participants.

The incidence remained stable compared to the period 2000 to 2009, but the fatality rate fell from 71% to 34%, with deaths due to arrest decreasing from 0.39 to 0.20 per 100,000. Most arrests were attributed to coronary artery disease, and the study associates the improvement in survival rates with the presence of defibrillators and immediate application of cardiopulmonary resuscitation in races.

The benefits of long-term training are real and numerous, but they require planning, moderation and attention to the body's signals. “An efficient workout is one that stimulates the body to adapt progressively. It causes temporary fatigue, improves performance over time and respects recovery periods,” advises the cardiologist at Einstein.

6 warning signs during a marathon

1. Dehydration and mineral imbalance

During a long run, the body loses large amounts of water and electrolytes (such as sodium and potassium) through sweat. If a runner does not hydrate properly, he or she may experience low blood pressure, dizziness, cramps, nausea, or even mental confusion.

Cause: excessive sweating, insufficient or incorrect hydration.

2. Hyponatremia (too much water and not enough salt)

Less well-known, this condition occurs when a runner drinks too much water but does not replace sodium. This dilutes the blood and can cause swelling, nausea, weakness and, in severe cases, seizures. Hyponatremia is even seen in veteran athletes: a study published in the New England Journal of Medicine analyzed runners in the Boston Marathon and found cases of symptomatic hyponatremia in 13% of participants.

Cause: excessive consumption of pure water, without balanced rehydration.

3. Muscle cramps

These are involuntary and painful contractions, usually in the legs. They can occur due to muscle fatigue, lack of adequate training or electrolyte imbalance.

Cause: exertion above normal, lack of specific conditioning or loss of salts through sweat.

4. Gastrointestinal problems

Abdominal pain, nausea or an urgent need to go to the bathroom are common complaints.

Cause: inadequate nutrition before or during the test, nervousness, use of poorly tested supplements or excess energy gel.

5. Heat exhaustion or sunstroke

The body may have difficulty controlling its temperature, especially in races carried out on hot and humid days.

Cause: accumulation of body heat, insufficient hydration, inappropriate clothing.

6. Arrhythmias and cardiac complications

Although rare, they can occur especially in people with predisposition or undiagnosed heart disease.

Cause: extreme effort without prior medical evaluation, dehydration, high temperature or use of stimulants.