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Rare & Remarkable Human Traits

What Running 100 Miles Does to the Human Body

What a 100-mile run does to muscles, kidneys, energy and sleep, using measured ultramarathon studies and noting where the evidence is thin.

By Written Facts StaffOctober 15, 20269 min read

You know the feeling of walking down the stairs the morning after a long hike or a hard fall leaf-raking weekend: stiff thighs, heavy legs, a body that seems to be sending you a bill. A 100-mile run sends a much bigger bill, all at once. Muscle breaks down and leaks enzymes into the blood. Inflammation surges. The kidneys filter less for a while. Many runners go a night or more without sleep, and some see things that aren't there. Most of it clears within days. Here's what researchers have measured so far, and where their data run out.

The short answer

A 100-mile (161 km) race works like a large, mostly controlled dose of stress on the body. Damaged muscle fibers release creatine kinase (CK) into the bloodstream. Inflammation markers climb, sometimes more than a hundredfold. Kidney filtration drops, sometimes far enough to meet the clinical definition of acute kidney injury. Running through the night brings sleep deprivation, and with it falls and hallucinations. Feet can swell, especially in runners who drink a lot.

For most finishers, nearly all of this reverses within a day or a few days. A small number get seriously ill, usually when heat, too much fluid or low blood sodium are involved.

One caveat applies to the whole article. No single study has measured "the whole body at 100 miles." The evidence comes from a mix of 100-mile races and nearby distances, from 67 km to 217 km, so each figure below gives the distance it was measured at. A review in Frontiers in Physiology notes that strain on muscle, liver and kidneys tends to grow with the distance covered.

The energy bill

The first cost is fuel. In a study of a 161-km mountain ultramarathon published in the International Journal of Sports Physiology and Performance (Creer et al., 2025), two male runners each finished in 32.8 hours. Measured with doubly labeled water, a standard method for tracking energy use in people going about their normal activity, they burned about 15,723 and 15,888 kcal.

During the race they took in only about 50% of that. Even with aid stations every few miles, each runner finished roughly 7,800 kcal short. Eating while running for a day and a half is hard. Stomachs rebel, and appetite fades.

The bill also kept coming after the finish. Over the next seven days, both runners' energy use stayed at about 1.8 to 1.9 times their basal metabolic rate, roughly 2,700 to 3,200 kcal a day. Their bodies were still working hard while they were resting, likely busy with repair.

That repair is a reminder of how small the evidence is. This was a study of two people. It gives a careful measurement of what a 100-mile race can cost, not an average for every runner, course or pace. Much of the extra energy those bodies spent afterward probably went into the muscle damage covered next.

Muscles and inflammation

Every downhill step makes the thigh muscles brake against gravity while they lengthen. That kind of contraction is especially hard on muscle fibers, and mountain ultras involve thousands of these steps.

Blood tests show the damage through creatine kinase (CK), an enzyme that normally stays inside muscle cells and leaks out when they're damaged. In a 10-runner study of a 67-km Austrian mountain race with about 4,500 m of climbing, average CK rose from 185.3 U/L before the start to 2,572.9 U/L at the finish, an increase of about 1,289%. It hadn't peaked yet. At 24 hours it was 2,865.4 U/L.

Inflammation follows. In a 2018 study of a 217-km mountain ultramarathon with six runners, CK was already about 30 times higher than the pre-race level by the 84-km mark. C-reactive protein (CRP), a general marker of inflammation, rose from about 0.9 mg/L before the start to 243 times that level by 177 km, and it stayed high through the finish.

Many runners reach for painkillers, and the Western States 100 offered a natural test. A study of 54 athletes at the 2005 race (Nieman et al., 2006) found that ibuprofen did not reduce muscle damage or soreness. The 29 runners who took it had higher inflammation markers. They also showed signs of endotoxemia, meaning bacterial toxins leaking from the gut into the blood. This is a single study, but it found no sign that the pills protected the muscles, and some sign that they made inflammation worse.

The damaged muscle matters for more than soreness. Everything it releases into the blood has to be cleared, and that work falls largely on the kidneys.

Kidneys under strain

The kidneys deal with a lot during a long race: less blood flow while the muscles take priority, fluid loss through sweat, and a flood of muscle breakdown products.

The same 67-km Austrian study tracked them closely. Creatinine, a waste product the kidneys clear, rose from 0.90 to 1.54 mg/dL. Estimated filtration rate (eGFR), a standard measure of kidney function, fell from 106.2 to 58.5, a drop of 45%, right after the finish. Of the 8 finishers, 5 met the criteria for stage I acute kidney injury and 2 for stage II. Recovery was quick. After 24 hours, average eGFR was back to 93.2, and only 2 runners still met stage I.

At 100 miles the pattern is similar. In an analysis of 627 Western States finishes with known creatinine values (Hoffman & Weiss, 2016), 36.2% met the "risk" or "injury" level of a standard kidney-injury scale. Those runners tended to finish faster, lose more body weight and have higher CK. Faster runners and more muscle damage meant more kidney strain.

Real kidney failure is rare. A paper in Wilderness & Environmental Medicine found that over a three-year span, 4 of 1,053 Western States starters (0.38%) needed dialysis, for 10 to 42 days. Those races were run in extreme heat, with highs of 36.6 to 38.3°C (about 98 to 101°F). The four runners' average blood sodium was 127 mmol/L, well below normal. The same paper's review of published case reports found only 27 marathon or ultramarathon runners with severe kidney injury, and 15 of them needed dialysis.

The kidneys usually bounce back. What takes a runner from a temporary dip to the hospital tends to be heat combined with problems in fluid and sodium balance. Those same factors show up again below.

Sleep loss, hallucinations and swollen feet

Most 100-mile races run through at least one night, and the slower half of the field often runs through two. A 2024 survey in Sports Medicine – Open asked 1,154 runners from a 165-km race and a 111-km race about sleep. About 77% slept at some point during the race, for an average total of 76 minutes in the 165-km event and 27 minutes in the 111-km event.

That isn't much sleep for a day and a half or more of hard effort. About 80% reported symptoms they attributed to sleep deprivation, including falls and hallucinations. Runners commonly describe trees turning into people or rocks into animals. The good news from the same survey is that most runners recovered normal wakefulness within two days.

A paper in an Italian psychiatry journal (Rivista di Psichiatria) concludes that these hallucinations can happen in ultra-trail runners with no psychiatric or neurological disorder. They come from the conditions of the race, not from an underlying illness.

Feet are the other part of the body that visibly changes. In a field study of 76 runners at a 100-km race, the change in foot volume tracked with how much runners drank (correlation r = 0.54), and drinking more was also tied to lower blood sodium after the race. Across the whole group, average foot volume didn't change. Swelling showed up mainly in the heavy drinkers.

Drinking too much matters even in the cold. In the 100-mile Iditasport race in Alaska, a study published in 2002 found that 7 of 16 participants (44%), a mix of runners and cyclists, developed hyponatremia, or low blood sodium. Those affected drank more per hour (0.5 vs. 0.4 L) and took in less sodium per hour (235 vs. 298 mg) than those who weren't.

Recovery, and when it goes wrong

For most finishers, the body resets surprisingly fast. In the 67-km study, kidney filtration was mostly back within 24 hours. Most runners in the sleep survey felt normally awake within two days. Muscle enzymes can keep rising for a day after the finish, but the overall picture is temporary stress followed by repair, during which the body keeps burning extra energy for about a week.

The serious cases share a few causes: extreme heat, drinking more than the body needs, and low blood sodium, often with heavy muscle breakdown on top. Those are the conditions behind the rare Western States dialysis cases.

Much of this evidence is thin. The 100-mile data come from small studies, sometimes only two or six runners, and several key figures were measured at shorter or longer distances. Researchers can describe what happens inside a 100-mile runner, but they can't yet say how typical any single number is.

Sources (13)

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