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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.

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Hitting the wall -

However, fat is less efficient than glycogen, leading to slower performance and increased fatigue. This is known as "hitting the wall" or "bonking," a common experience for long-distance runners.

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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.

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Muscle healing -

While some muscle damage is temporary and helps with adaptation, excessive strain can lead to overuse injuries like tendonitis or strains. Proper post-race recovery and stretching are crucial for muscle healing.

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Joint stress and impact -

The repeated pounding of feet hitting the pavement during a marathon can place significant strain on joints, particularly the knees, hips, and ankles. This can lead to joint pain, inflammation, and increased wear over time.

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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.

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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.

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Temporary increase in blood pressure -

Over time, this continuous cardiovascular activity strengthens the heart muscle and improves circulation. However, after prolonged exertion, some runners may experience an elevated heart rate and a temporary increase in blood pressure due to the intensity of the activity.

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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.

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Hydration is key -

Dehydration can impair performance and increase the risk of heat exhaustion or heatstroke. Proper hydration before, during, and after the race is crucial to maintain thermoregulation and avoid overheating.

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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.

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The importance of recovery -

Runners are more vulnerable to colds or other illnesses in the days following the race. This is why recovery is key, with proper nutrition, sleep, and hydration being important to help the immune system recover and fight off potential infections.

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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.

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Stomach strain and proper nutrition -

The jarring motion of running also affects the stomach. Eating the wrong foods before a race or not fueling properly during the race can exacerbate these issues. Training your gut to handle food and hydration is essential.

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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.

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All about balance -

The key is to drink fluids in moderation, balancing water and electrolyte replenishment, especially in hot or humid conditions where fluid loss is greater.

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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.

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Endorphin rush and post-race fatigue -

Endorphins, the body's natural painkillers, are released to help runners push through the pain. After the race, runners may experience a "crash" as hormone levels normalize, leading to fatigue and temporary mood changes.

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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.

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Motivation to push through -

Mental exhaustion can cause lapses in form, concentration, and pacing. Runners often rely on mental strategies, such as setting mini-goals or using positive self-talk, to push through moments of mental fatigue and stay focused on the finish line.

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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.

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Irritation in motion -

Additionally, runners may experience skin irritation due to sweat and salt buildup. Properly fitted shoes, moisture-wicking socks, and anti-chafing products can help mitigate these issues.

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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.

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When the cold hits the lungs -

In colder weather, cold air can irritate the respiratory system, leading to discomfort or wheezing. Breathing techniques and training at higher altitudes can help improve lung capacity and endurance for long-distance runners.

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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.

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Beating the acid buildup -

The body works to neutralize this buildup by buffering the acid. But after a marathon, runners may still experience muscle soreness and stiffness due to metabolic acidosis. Recovery strategies like stretching, foam rolling, and hydration help alleviate the effects of lactic acid buildup.

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Neuromuscular coordination -

Running long distances can affect your neuromuscular coordination, as fatigue sets in. Your body's ability to efficiently control movements may decline, causing your gait to alter and increasing the risk of injury.

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Motor control -

The brain and nervous system can struggle to maintain motor control under exhaustion, leading to a decrease in running form and efficiency. With proper training, runners can increase their endurance and maintain better coordination throughout the race.

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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.

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The brain eats itself -

This process, known as “cerebral cannibalism,” is a response to extreme metabolic stress. While this may cause temporary changes in brain function, these effects are reversible with proper recovery. This phenomenon highlights the brain's extraordinary adaptability in extreme endurance events.

Sources: ( Nature Metabolism) (Live Science) (Everyday Health) (Runner's World) 

See also: Running technique—this is how you avoid injuries

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