What Happens to Your Body When You Run a Marathon

During a marathon, all you can think about is finishing. However, as many runners know, once you finish a marathon, your body doesn’t exactly feel awesome. What happens to your body when you run a marathon provides an explanation for the soreness, fatigue, and nausea. The marathon causes temporary damage on a cellular level.

The marathon is a unique combination of high workload over a long time. Shorter races may tax the muscles more and produce greater amounts of metabolites, but their duration is relatively short. Some research suggests that you may experience more damage compared to ultra marathons, due to the higher intensity on a harder surface.

Systemic Inflammation Occurs After Running a Marathon

According to a 2021 study in the Journal of Strength and Conditioning Research, your body may take 6-9 days to return to normal on a cellular level after running a marathon. Biomarkers related to systemic inflammation – troponin and C-reactive protein – are elevated for days after completing a marathon. 

• Troponin (acute damage to heart muscles): Peaked a few hours after race, elevated for four days after marathon
• C-reactive protein (acute inflammatory response): Peaked 24 hours after marathon, still elevated 8 days after race

These markers actually indicate lingering fatigue to the heart. Even as muscle soreness dissipates, your heart still needs extra time to recover after a marathon.

Some research points to the fact that women may experience less elevation in their biomarkers. However, the statistics are simply not definitive enough to establish any causal relationship between gender and systemic damage in the marathon.

Your Muscles are Damaged – and Likely Very Sore

After a marathon, biomarkers of muscle damage including serum creatine kinase, myoglobin, and lactate dehydrogenase are temporarily and significantly elevated. These markers remain elevated even after soreness subsides – which means your muscles feeling better does not necessarily indicate full recovery. 

• Lactate dehydrogenase (tissue damage marker): Peaks immediately after marathon and in the following ~48 hrs, then takes 8 days to return to normal
• Creatine kinase (marker of acute muscle damage): Peaks 24 hours after marathon, significantly elevated for 6 days following race, back to normal 7-9 days after race

Notably, high carbohydrate consumption during the race may reduce the amount of muscle damage incurred. In a 2020 randomized controlled trial published in Nutrients, runners who consumed 120 grams of carbs per hour had lower levels of the muscle damage biomarkers. 120 grams of carbohydrate per hour is a lot in a road race. However, given the known damage that glycogen depletion elicits in the muscles, theoretically road marathoners may reduce their muscle damage by aiming for high carbohydrate intake (60g+/hr) during the race. (Plus, this practice prevents bonking in the marathon!).

Faster marathoners sustain greater muscle damage (and still have inflammatory markers). They need to be cautious about return to run and allow time for muscle damage to heal. Slower marathoners typically have higher inflammatory markers (but still have notable muscle damage).

Your Lungs and Diaphragm are Fatigued

Most runners feel a temporary burning in their lungs after a race – and there’s an actual physiological reason. As summarized in a 2019 review in Sports Medicine, the labored breathing for several hours takes its toll on the respiratory system. Respiratory muscles experience mild fatigue (15-25%) and pulmonary function slightly decreases (although not at clinically significant levels in healthy runners). 

Glycogen Depletion May Make You Feel Tired

Even if you fuel well during the marathon, you will finish in an energy deficit. Since basic functions partially rely on carbohydrates, you may feel tired and foggy after the race. These symptoms should subside as you replenish fluids and calories. However, don’t be surprised if you are ready for an early bedtime after your marathon. 

You May Feel Nauseous

During a marathon, your body prioritizes blood flow to your working muscles. However, this diverted blood flow comes at the expense of areas including your gastrointestinal system. Your GI system receives less blood flow for function, which can cause nausea. Dehydration and sodium imbalance from sweating for several hours can also contribute to nausea. 

What This All Means for Recovery

What happens to your body when you run a marathon highlights the importance of proper recovery. Most coaches will advise 7-14 days off of running, which aligns with when biomarkers return to normal levels. Eating lots of carbs will replenish glycogen stores and protein will aid in muscle repair. (Here’s how to recover after running a marathon.)