This is what happens to your body during a marathon

Running a marathon, which spans an intense 26.2 miles (42.2 kilometers), is one of the ultimate tests of human endurance, pushing both body and mind to their limits. This extreme effort triggers a number of physical responses, from muscle breakdown and dehydration to hormone surges. In fact, new research published in Nature Metabolism shows that running a marathon can cause the brain to even start eating itself.

Click on to explore what really happens inside your body during a marathon, and why preparation and recovery are absolutely essential.

Depletion of glycogen stores

During a marathon, your body relies on glycogen (stored sugar) as the primary energy source. As you run, these glycogen stores get depleted. Once that happens, your body starts burning fat for energy.

Muscle fatigue and micro tears

Running a marathon subjects your muscles to repetitive stress, causing them to fatigue. Micro tears form in the muscle fibers, which can lead to soreness and stiffness after the race.

Joint health and injury prevention

The impact can also exacerbate preexisting joint issues like arthritis. Wearing proper shoes and cross-training with low-impact activities can help reduce the risk of joint injuries during long-distance running.

Increasing heart rate

As you run, your heart rate increases to meet the body’s growing demand for oxygen. During a marathon, the heart pumps more blood, and blood vessels dilate to accommodate the increased flow.

Thermoregulation challenges

During a marathon, your body generates heat from the muscles working overtime. To counteract this, your body sweats to cool down. However, sweating causes a loss of fluids and electrolytes, which can lead to dehydration if not properly managed.

Immune system suppression

After running a marathon, the body's immune system is temporarily suppressed. Intense exercise can elevate cortisol (a stress hormone), which reduces the body's ability to fight infections.

Gastrointestinal distress

Marathon running can cause gastrointestinal issues, including nausea, cramps, and diarrhea. When running long distances, blood flow is diverted from the digestive system to the muscles, slowing down digestion and causing discomfort.

Hydration imbalance

Maintaining proper hydration during a marathon is critical but challenging. Dehydration can cause fatigue, dizziness, and muscle cramps. Drinking too much water without replenishing electrolytes can lead to hyponatremia, a potentially dangerous condition where sodium levels in the blood drop too low.

Hormonal fluctuations

Marathon running leads to significant hormonal changes. Cortisol, adrenaline, and other stress-related hormones increase to support energy expenditure and alertness. However, prolonged elevated cortisol levels can lead to muscle breakdown and weaken the immune system.

Mental fatigue and focus

As the race progresses, mental fatigue sets in. The body may be physically drained, but the brain needs to keep you focused and motivated to continue.

Foot issues

During a marathon, your feet undergo significant stress due to the constant pounding of the pavement. This can lead to blisters, calluses, and chafing. The friction between shoes and skin, especially in areas like the toes or heels, can cause painful blisters that slow runners down.

Respiratory demands

The body requires more oxygen during a marathon, which leads to an increase in breathing rate. The lungs work overtime to supply oxygen to muscles, and some runners may feel out of breath or develop a side stitch.

Metabolic acidosis

During long-distance running, muscles produce lactic acid as a byproduct of anaerobic energy production when glycogen stores are low. This accumulation of lactic acid causes muscle fatigue and soreness.

Brain's energy utilization

During a marathon, when glycogen runs low, the brain may start burning its own myelin (a fatty substance that insulates nerve fibers) for energy, as suggested by a 2025 Nature Metabolism study, potentially compromising nerve insulation under stress.