Running a marathon is one of the most physically demanding challenges a human body can undergo.
The 26.2-mile journey puts your cardiovascular, muscular, immune, and even psychological systems through a grueling test.
Whether you’re training for your first race or you’ve got a medal rack collecting dust, understanding what your body goes through during those hours on the road can make all the difference—before, during, and after race day.
Here are 13 fascinating, and sometimes surprising, things that happen to your body when you run a marathon.
1. You Get Temporarily Shorter
Yes, really.
Studies have shown that marathon runners can lose up to half an inch in height during the race.
This isn’t about bad posture—it’s due to spinal compression.
The intervertebral discs, which act like fluid-filled cushions between your vertebrae, lose fluid during prolonged physical activity. Add in the constant pounding of your feet on pavement and tension in the back muscles, and your spine compresses ever so slightly.
The good news? Your height returns to normal once your body rehydrates and decompresses.
2. You Lose A Shocking amount of (Water) Weight
Marathoners can lose between 2 to 5 kilograms (roughly 4.5 to 11 pounds) over the course of a race—almost entirely in water.
That’s why hydration isn’t just recommended during a marathon; it’s essential.
Dehydration doesn’t just impair performance—it can be dangerous. A drop in fluid levels affects your blood pressure, slows digestion, impairs temperature regulation, and increases the risk of heat exhaustion or even heat stroke.
Studies show that most runners need between 15 and 30 ounces of water or electrolyte solution per hour. Sip steadily, not just when you’re thirsty.
3. Your Nose Will Start Running, Too
It’s not just sweat and tears—your nose might get in on the action as well.
Exercise-induced rhinitis, sometimes called “runner’s hay fever,” affects over half of long-distance runners.
The increased breathing rate during exertion irritates the mucous membranes in your nose, prompting an overproduction of mucus. For many, this means a persistent sniffle throughout the race, often worsened by cold or dry air.
While annoying, it’s harmless—just another strange side effect of endurance.
4. You’ll Burn Thousands of Calories—And Still Not Want to Eat
Depending on your size, pace, and metabolic rate, you’ll burn between 2,600 to 3,500 calories during a marathon—roughly the equivalent of 10 Big Macs. But ironically, many runners find themselves with no appetite after finishing.
This is due to the suppression of ghrelin (the “hunger hormone”) and an increase in peptide YY, a hormone that tells your brain you’re full. Blood flow is also redirected to your muscles during intense activity, so digestion takes a backseat.
If you’re not hungry right away, focus on rehydrating first, then ease into small, carb-rich snacks to kickstart recovery.
5. Your Muscles Begin to Break Down—On a Cellular Level
Throughout the race, your muscles are constantly contracting and absorbing impact forces. As the miles tick by, waste products like lactic acid and ammonia accumulate, and microscopic tears form in your muscle fibers.
This is completely normal—and part of how muscles grow stronger over time—but in the short term, it leads to soreness and fatigue.
In extreme cases, overexertion can lead to a condition called rhabdomyolysis, where damaged muscle tissue breaks down rapidly and releases harmful proteins into the bloodstream, potentially stressing the kidneys.
If your urine turns brown or you experience severe muscle pain post-race, seek medical attention.
6. Cramping Becomes a Real Risk
Muscle cramps are one of the most common—and most frustrating—issues runners face during a marathon.
Although the exact cause isn’t fully understood, it’s likely a combination of dehydration, electrolyte loss, neuromuscular fatigue, and overexertion. Setting off too fast or deviating from your usual training pace can spike your risk.
To prevent cramps, many sports physicians recommend hydrating with electrolytes, practicing race-day nutrition strategies during training, and occasionally altering your stride during the race to stimulate different muscle groups.
7. You Might Hit “The Wall”
“The Wall” is legendary among marathoners.
It typically strikes around mile 18 to 20, when your body’s glycogen stores are nearly depleted.
Glycogen is the stored form of carbohydrates your body uses for quick energy.
Once it’s gone, your body has to turn to slower-burning fat stores, and every step starts to feel like a monumental effort. This physical depletion is often accompanied by mental fatigue, which makes it even harder to push through.
Taking in simple carbohydrates throughout the race—via gels, chews, sports drinks, or bananas—can help stave off the wall.
8. Your Digestive System May Go Rogue
Gastrointestinal distress affects up to 50% of long-distance runners. Symptoms range from stomach cramps to nausea, reflux, and the dreaded mid-race bathroom emergency.
This happens because blood is diverted away from your digestive organs to fuel your muscles, and the jostling motion of running doesn’t help either.
Certain foods, caffeine, or sports gels can exacerbate GI issues, so it’s crucial to test your nutrition plan during training to find what your body tolerates best. Still, even the best-prepared runner isn’t immune to race-day surprises.
9. Your Nipples—And Your Feet—Take a Beating
Prolonged rubbing of sweat-soaked fabric can lead to nipple chafing and even bleeding, especially in men. This is due to the salt in your sweat, which acts like sandpaper against your skin.
Nip guards, tape, or moisture-wicking gear can prevent this unpleasant surprise.
Meanwhile, your feet endure hours of impact, often in slightly swollen conditions. That pressure can cause blisters, bruised toenails, and, in some cases, toenails that turn black and fall off entirely. Shoes that are too tight—or even too loose—can make matters worse.
Investing in proper footwear and moisture-wicking socks can help minimize damage.
10. Your Kidneys and Heart Are Under Serious Strain
During a marathon, your cardiovascular system is working overtime.
Your heart pumps more than twice its usual output to keep muscles oxygenated, and your kidneys—already dealing with dehydration and electrolyte loss—must filter the byproducts of exertion and muscle breakdown.
Studies have shown that marathoners can experience temporary kidney dysfunction on race day.
Fortunately, for healthy individuals, this typically resolves on its own within 24 to 72 hours. Still, it’s a reminder of how much stress endurance racing places on even the most vital organs.
11. You Might Struggle to Sleep That Night
It sounds counterintuitive, but many marathoners report restlessness and insomnia after a race, even when they’re physically exhausted.
This is largely due to elevated levels of adrenaline, cortisol, and endorphins—chemicals associated with stress, excitement, and physical exertion. Your body is buzzing with activity, even though your legs are toast.
Add in dehydration, soreness, and maybe a post-race beer or two, and it’s no wonder sleep can be elusive. Prioritizing hydration, avoiding caffeine late in the day, and giving your body time to wind down can help.
12. Your Immune System Becomes More Vulnerable
Within hours of finishing a marathon, your immune function temporarily dips.
This is known as the “open window” theory—your body has just been through extreme physical stress, and inflammatory markers spike while white blood cell levels drop. This makes you more susceptible to infections, especially respiratory illnesses.
Avoid crowded spaces, wash your hands often, and get plenty of sleep to help protect yourself from the post-race sniffles.
13. You Might Feel Euphoric—Or Oddly Empty
Crossing the finish line often delivers a massive emotional high.
Endorphins, dopamine, serotonin—all the feel-good brain chemicals—surge as your body and mind recognize the achievement. But in the days following, many runners experience what’s known as the “marathon blues.”
After months of training, strict schedules, and anticipation, the sudden stop can create a psychological void. It’s a textbook case of emotional comedown—especially if your sleep is disrupted or your body is still in recovery mode.
The best way to bounce back is to rest, reflect, and start planning your next goal.
