An ultramarathon is any footrace longer than the standard marathon distance of 42.195 kilometers (26.2 miles). In practice, the most common distances are 50 km, 50 miles, 100 km, and 100 miles, though races stretch far beyond that, and some are not measured in distance at all but in hours or days. The term covers an enormous range of events, from a relatively modest 50 km trail run to a 3,100-mile loop around a city block that takes weeks to complete.
Standard Distances and How They Compare
The four distances you will encounter most often in the ultramarathon world are 50 km (31.1 miles), 50 miles (80.5 km), 100 km (62.1 miles), and 100 miles (161 km). A 50 km race is only about 5 miles longer than a marathon and is widely considered the entry point into ultrarunning. The jump from there to 50 miles roughly doubles the time on your feet. A 100 km race sits in the middle ground, and the 100-mile distance is often treated as the marquee ultramarathon challenge, the equivalent of what a marathon represents for road runners. Races beyond 100 miles exist as well, including events at 200 miles and the extreme Badwater Ultramarathon, a nonstop 217-km run across Death Valley, California.1The Journal of Strength & Conditioning Research. Selected Human Physiological Responses During Extreme Heat: The Badwater Ultramarathon
Participation in these events has grown dramatically. The number of finishers in 100 km ultramarathons worldwide increased exponentially between 1998 and 2011, with data covering more than 112,000 athletes from 102 countries.2PubMed. Participation and performance trends in 100-km ultra-marathons worldwide The same exponential growth pattern held for 50-mile and 100-mile events in North America.3PubMed Central. Runners in their forties dominate ultra-marathons from 50 to 3,100 miles
Time-Limited and Multi-Stage Races
Not all ultramarathons are defined by a finish line at a set distance. Time-limited races give runners a fixed clock and see how far they can go. Common formats include 6-hour, 12-hour, 24-hour, 48-hour, 72-hour, 6-day, and even 10-day events.4Nature / Scientific Reports. Race course characteristics are the most important predictors in 48 h ultramarathon running These tend to be run on short looped courses, sometimes a track or a one-mile circuit, which makes logistics simpler for crew support and aid stations. The appeal is different from a point-to-point race: there is no fixed distance to dread, but there is also no finish line to pull you forward. You simply run until the time expires.
Multi-stage ultramarathons add another layer. In these events, runners cover an ultramarathon distance each day for several consecutive days, often in remote or harsh terrain. The Marathon des Sables, for example, spans roughly 250 km across the Sahara over six stages, with runners carrying their own food and gear. These races test not just endurance but recovery, logistics, and the ability to keep going on damaged legs day after day.
Finishing Times and What They Feel Like
One way to grasp the scale of these distances is through finishing times. A competitive 50 km runner might finish in around 3.5 to 4 hours on a road course, while a mid-pack trail runner could take 6 or 7 hours. For 100-mile races, elite finishers typically come in around 12 to 16 hours, while many participants spend 24 to 30 hours on the course. The Ultra-Trail du Mont-Blanc, one of the world’s most famous mountain ultramarathons at 168 km with roughly 10,000 meters of elevation gain, sees completion times ranging from about 20 hours for the fastest runners to more than 40 hours for those at the back of the pack.5PubMed. Combined effects of sleep deprivation and strenuous exercise on cognitive performances during The North Face® Ultra Trail du Mont Blanc® (UTMB®)
That time-on-feet difference matters more than the distance itself for understanding what the body goes through. A 50 km road ultra might be brutal but over in a few hours. A 100-mile mountain race means running through the night, possibly through a second night, dealing with sleep deprivation, wildly fluctuating temperatures, and the psychological weight of still having 40 miles to go after you have already run farther than most people ever will.
What the Body Burns
During multi-day races, some athletes burn six to seven times their basal metabolic rate, roughly 7,000 to 8,000 calories a day. But research on ultra-endurance athletes found that the body cannot sustain that output indefinitely. Over longer periods of 30 to 52 weeks, energy expenditure averaged about 2.4 to 2.5 times basal metabolic rate, suggesting a metabolic ceiling that even the most extreme athletes rarely exceed.6Current Biology. Energy expenditure and metabolic scope in ultra-endurance athletes Briefly spiking above that limit is possible, but sustaining it long-term leads to the body breaking down its own tissue.7Current Biology. Ultra-endurance athletes and the metabolic ceiling
As an ultramarathon progresses, the body shifts its fuel source. During a 24-hour race, researchers observed a progressive shift toward fat burning to maintain blood glucose levels, along with changes in blood chemistry driven by altered breathing patterns over many hours of sustained effort.8PubMed Central. Acute metabolic responses to a 24-h ultra-marathon race in male amateur runners This is why ultramarathon pacing feels fundamentally different from marathon pacing. You are not trying to hold a fast pace as long as possible; you are trying to keep your body’s energy systems ticking over for an absurdly long time.
The Toll on Heart, Muscles, and Kidneys
Running 100 km or more produces measurable damage to several organ systems, most of which resolves on its own but can look alarming in blood work. After a 100 km race, about two-thirds of runners showed elevated cardiac troponin T, a protein that signals heart muscle stress, at levels above the clinical reference range.9PubMed Central. Factors Related to Cardiac Troponin T Increase after Participation in a 100 Km Ultra-Marathon A 118 km mountain race produced similar findings, with half of finishers exceeding the reference limit for cardiac troponin and 87 percent exceeding it for NT-proBNP, another cardiac stress marker.10PubMed Central. Cardiac Damage Biomarkers and Heart Rate Variability Following a 118-Km Mountain Race: Relationship with Performance and Recovery These elevations are generally transient and do not appear to cause lasting damage in healthy runners, but they illustrate the sheer stress the heart absorbs.
Skeletal muscle damage scales with distance. When researchers compared a marathon, a 100 km race, and a 308 km race, markers of muscle breakdown like creatine kinase were significantly higher after each step up in distance. Interestingly, the temporary decline in kidney function was actually worse after the 100 km race than the 308 km race, likely because the 100 km was run at higher intensity.11PubMed Central. Comparison of Changes in Biochemical Markers for Skeletal Muscles, Hepatic Metabolism, and Renal Function after Three Types of Long-distance Running At the extreme end, a case report from a 165 km race documented severe rhabdomyolysis, with muscle damage markers still well above normal six days after the finish.12Sports medicine: research and practice. The severity and dynamics of changes in the serum concentration of biochemical markers of muscle tissue damage in an experienced ultramarathon runner after overcoming a distance of 165 km against the background of continuing physical activity
The immune system also reacts. Research on ultra-endurance runners showed notable changes in T-cell populations during the 48 hours after a race, with an increase in regulatory T-cells and a decrease in effector memory T-cells, which were likely redirected to sites of tissue damage and inflammation throughout the body.13PubMed Central. An ultra-endurance event leads to changes in circulating regulatory T-cells, CD4+ naïve and CD8+ effector memory T-cells in the 48 h post-race recovery period This temporary immune reshuffling is one reason many ultra runners report catching colds in the weeks after a big race.
Gut Problems and Hydration Hazards
Gastrointestinal distress is one of the most common complaints in ultramarathons and one of the leading causes of dropping out. A study of the 161 km Western States Endurance Run surveyed 272 finishers and non-finishers about upper and lower GI symptoms, including nausea, vomiting, bloating, diarrhea, and cramping, experienced across four segments of the race.14PubMed Central. Gastrointestinal distress is common during a 161-km ultramarathon Nausea in particular tends to intensify through the later miles, when blood is shunted away from the gut toward working muscles and the skin for cooling.
Hydration strategy is a genuine safety concern, not just a performance one. Exercise-associated hyponatremia, where blood sodium drops dangerously low from overdrinking, is a well-documented risk. In one multi-stage ultramarathon study, body weight gain correlated with hyponatremia, and 75 percent of non-finishers on one stage had the condition compared with only 5 percent of finishers.15PubMed. Exercise-Associated Hyponatremia, Hypernatremia, and Hydration Status in Multistage Ultramarathons The lesson most experienced ultra runners internalize is to drink to thirst rather than on a fixed schedule, and to include sodium in their fluid intake.
Sleep Deprivation and Hallucinations
Any ultramarathon that takes more than about 20 hours introduces a variable that shorter races never have to deal with: sleep deprivation. At the Ultra-Trail du Mont Blanc, runners completing the course in 27 to 44 hours averaged only about 12 minutes of rest during the entire race. The combined effects of acute sleep loss and prolonged exertion produced marked declines in cognitive function, from slowed reaction times to outright visual hallucinations.16PubMed. Combined effects of sleep deprivation and strenuous exercise on cognitive performances during The North Face® Ultra Trail du Mont Blanc® (UTMB®)
Hallucinations are common enough in 100-mile and longer races that experienced runners talk about them matter-of-factly. Research on a 246 km mountain ultra confirmed that visual hallucinations are associated with the combination of extreme physical exertion and sleep loss, consistent with findings from other long-distance events.17Chinese Journal of Physiology. Visual Hallucinations in 246-Km Mountain Ultra-Marathoners: An Observational Study Runners commonly report seeing animals, people, or objects that are not there, particularly in the second night of a race. Most learn to recognize these as harmless artifacts of fatigue, but they can become dangerous on technical mountain terrain.
Who Runs Ultramarathons and When They Peak
The demographics of ultrarunning challenge assumptions about athletic prime. In 100 km races, peak performance for all finishers lands at about 41 years old in women and 45 in men. Even when looking at the fastest runners, peak age is still around 39 for women and 41 for men.18The Journal of Strength & Conditioning Research. Performance in 100-km Ultramarathoners—At Which Age, It Reaches Its Peak? This is much later than in the marathon, where elites typically peak in their late twenties or early thirties. The difference likely reflects the value of accumulated training volume, race experience, pacing wisdom, and the mental toughness that comes with years of long-distance running.
Sex differences in performance also behave differently than in shorter races. The traditional view is that men are roughly 10 to 12 percent faster than women in endurance events. In ultramarathons, however, the gap appears to narrow as distances increase. One analysis found that in races where men and women compete in comparable numbers, the performance gap shrinks to about 1 to 3 percent.19PubMed. Sex differences in ultramarathon performance in races with comparable numbers of males and females Across 50-mile events specifically, the gap between the fastest men and women narrowed over several decades of data, from about 15 percent down to 9 percent.20PubMed Central. Performance differences between sexes in 50-mile to 3,100-mile ultramarathons Whether this reflects a biological advantage women have at very long distances, a selection effect from who enters these events, or simply the sport becoming more accessible to women is still debated.
Fueling During a Race
Eating while running for 12, 24, or 30 hours is an entirely different challenge from the gels-and-water approach of a marathon. The International Society of Sports Nutrition recommends that ultra runners aim for 150 to 400 calories per hour during racing, with about 30 to 50 grams of carbohydrate and 5 to 10 grams of protein per hour, drawn from a variety of calorie-dense foods.21PubMed Central. International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing The emphasis on variety is practical: after many hours, sweet foods often become repulsive, and runners increasingly crave savory options like broth, potatoes, grilled cheese sandwiches, and even pizza. Aid stations at major ultras look more like buffets than hydration stops.
Even with aggressive eating, runners almost always finish in a significant calorie deficit. The metabolic demands simply outstrip what the stomach can process, especially when gut function deteriorates in the later stages. This is where pacing and nutrition strategy become intertwined. Researchers have modeled optimal carbohydrate intake strategies that account for how quickly carbohydrates can pass through the stomach, be processed by the liver, and reach working muscles, and the picture that emerges is that fueling needs to be consistent from early in the race, not reactive once you already feel depleted.22PubMed. Optimal pacing and carbohydrate intake strategies for ultramarathons
Running in Extreme Heat and Cold
Many iconic ultramarathons take place in brutally hot environments, and thermoregulation becomes as important as fitness. At the Badwater Ultramarathon in Death Valley, where daytime temperatures reach about 47°C (117°F), researchers found that the fastest finisher maintained a lower overall core body temperature than the group average. This is the opposite of what you might expect: it suggests that in very long races, success depends more on staying cool than on tolerating heat.23The Journal of Strength & Conditioning Research. Selected Human Physiological Responses During Extreme Heat: The Badwater Ultramarathon
Tropical environments present an additional challenge because high humidity limits sweat evaporation, the body’s primary cooling mechanism.24PubMed. Ultra-endurance events in tropical environments and countermeasures to optimize performances and health Runners in these conditions often use ice bandanas, cold sponges, and frequent dousing with water to maintain a manageable core temperature. During the 2025 Western States Endurance Run, detailed monitoring of a world-class athlete showed that core temperature averaged 37.1°C but peaked at 39.4°C during the hottest sections.25PubMed. Physiological, nutritional, and thermoregulatory responses of a world-class mountain-ultramarathon athlete during the 2025 Western States Endurance Run 100 That 2.3-degree swing, happening while running at race pace for over 14 hours, gives a sense of the thermal tightrope ultra runners walk.
How Running Form Breaks Down Over the Course
You can spot an ultra runner approaching the end of a race from a long way off. The shuffling gait, shortened stride, and slightly higher cadence are not just signs of tiredness; they are measurable biomechanical adaptations to fatigue. After a mountain ultramarathon, researchers found that runners increased their stride frequency and shortened their stride length across all running conditions, including flat, uphill, and downhill. The most striking change was in downhill running, where energy cost increased by about 13 percent post-race, even though no significant change occurred on flat or uphill terrain.26PubMed. Energy cost and kinematics of level, uphill and downhill running: fatigue-induced changes after a mountain ultramarathon This makes sense: downhill running demands eccentric muscle contractions that are especially punishing to already-damaged quadriceps.
A separate study of a 110 km ultramarathon confirmed the pattern, finding that stride frequency increased by about 3 percent while ankle range of motion decreased.27PLoS ONE. Effect of the Fatigue Induced by a 110-km Ultramarathon on Tibial Impact Acceleration and Lower Leg Kinematics In practical terms, ultra runners gradually lose the ability to extend their legs fully and compensate by taking quicker, choppier steps. This is partly protective: shorter, faster steps reduce the impact force on each stride. It is also partly involuntary, a consequence of muscles that can no longer produce the same range of motion they could 80 km earlier.
The Psychology of Keeping Going
Finishing an ultramarathon is often described as being more mental than physical, and research supports this to a degree. A review of psychological predictors in ultrarunning found that finishers scored higher in self-efficacy and, perhaps surprisingly, were more willing to seek social support during the race. Runners who tended to avoid asking for help were more likely to drop out. Problem-solving and leaning on others turned out to be the most prominent coping strategies among successful ultra runners.28Annals of Medical and Health Sciences Research. The Psychological Indicators of Success in Ultrarunning-A Review of the Current Psychological Predictors in Ultrarunning
There is also a fascinating quirk in how pain perception works during these events. After a 100-mile race, the fastest runners showed a modest temporary reduction in pressure pain perception, a kind of exercise-induced analgesia. But slower runners did not show the same effect, possibly because the extreme duration of their effort exhausted the pain-suppression systems that the faster runners still had functioning.29PubMed. Pain perception after running a 100-mile ultramarathon The implication is counterintuitive: if you are fast enough to finish a 100-miler relatively quickly, your body may still have some pain-dampening capacity left. If you are grinding through for 30-plus hours, even your internal painkillers may have given out.
An Evolutionary Footnote
Humans are unusual among mammals in two respects that make ultramarathon running possible at all: our locomotor muscles are dominated by fatigue-resistant fiber types, and we can shed metabolic heat through sweating far more effectively than most animals. These traits may have evolved for persistence hunting, where early humans chased prey over long distances until the animal overheated and collapsed.30PubMed. Ethnography and ethnohistory support the efficiency of hunting through endurance running in humans A 2024 study drawing on ethnographic and ethnohistorical records found broad support for this hypothesis across diverse cultures. Whether or not persistence hunting was the primary driver, the hardware is real: humans are built for sustained low-intensity locomotion over very long distances, which is exactly what an ultramarathon demands. The modern sport, in a sense, is just a formalized version of something human bodies have been doing for a very long time.