The Short Answer
An ultra places several demands on the body at once: prolonged energy use, repeated loading, changing temperature and fluid balance, and sometimes a night or more without normal sleep. There is no established sequence in which the gut fails first, muscles next and the brain last. These systems interact, and problems may appear at different points.
This page covers changes during and shortly after a race. For questions about lasting effects, see long-term ultrarunning health. For practical recovery decisions, use the post-ultramarathon recovery guide.
What Studies Have Actually Measured
| System | Observed finding | How to interpret it |
|---|---|---|
| Energy | John et al. (2025): mean deficit 6,797 kcal in a study enrolling 43 runners across 100, 160.9 and 230 km. | Deficit is expenditure minus intake. This is not total energy burned or a fueling target. |
| Muscles | Millet et al. (2011): knee-extensor force fell about 35% in 22 runners after 166 km with extensive climbing and descending. | A small sample of experienced mountain runners; terrain and testing method matter. |
| Gut | Stuempfle and Hoffman (2015): 96% of 272 questionnaire respondents at Western States reported at least one gut symptom. | The definition included mild symptoms such as belching. It does not mean 96% became seriously ill. |
| Heart | Lima et al. (2024): a review of 17 studies found lower post-race measures of pumping and filling function. | Immediate measurements do not establish permanent damage or diagnose an individual runner. |
| Kidneys and sodium | Lecina et al. (2022): 468 of 1,113 runners assessed for acute kidney injury met the included studies’ criteria. | This roughly 42% figure comes from selected ultra-trail studies; definitions and clinical severity require context. |
| Sleep and alertness | Kishi et al. (2024): 80% of 1,154 surveyed mountain-ultra finishers reported symptoms they attributed to sleep deprivation. | Self-reports from two races, not a prediction of symptoms at a particular hour. |
| Feet | Cejka et al. (2012): average foot volume did not change significantly in 76 runners over 100 km. | Individual swelling still occurred; a group average cannot determine your shoe fit. |
Energy and Digestion: Related, but Different Problems
Energy deficit means the body used more energy than the runner consumed. In the John study, the reported deficit ranged from 417 to 18,364 kcal across participants at one event. Combining several distances into an average hides substantial variation. It cannot tell you the food intake you will tolerate or the energy cost of your chosen course.
Gut symptoms can make eating harder. The Western States survey included both finishers and non-finishers, with responses from 71% of starters. Among responding finishers, 43.9% said symptoms affected performance. That is a different measure from having any symptom at all. The survey relied on recall after the race and does not establish the rate at every 161-km event.
Costa et al.’s review describes impaired digestion and changes in gut integrity during strenuous exercise, with heat and running adding to the strain. Its finding around two hours at 60% of maximum oxygen uptake describes the exercise conditions reviewed, not a timer at which every runner’s gut becomes damaged. Use the gut-training guide for rehearsing food and fluid choices within normal training.
Muscles, Heart and Kidneys: What a Marker Can Tell You
Muscle force and blood markers measure different things. The mountain study found wide variation in creatine kinase, a marker of muscle damage. Its average does not describe every runner or explain an individual’s symptoms.
The cardiac meta-analysis found left-ventricular ejection fraction about 3.7 percentage points lower and heart rate about 18 beats per minute higher immediately after racing, on average. Ejection fraction is the proportion of blood pumped out of the left ventricle with each beat. The analysis concerns acute changes; it does not show that each runner sustains permanent heart damage. New symptoms still need assessment.
Kidney-related blood tests can also change. The Lecina review included 2,950 runners overall, but only 1,113 contributed to its kidney-injury estimate. Not everyone was assessed for every outcome. Its sodium findings came from a different subset. These results should not be presented as the chance that any reader will develop kidney failure or low sodium, and an average recovery time should not delay care.
How the Demands Change With Duration
A daytime 50K, an overnight mountain race and a multiday event share some demands but expose runners to different conditions. Consider the duration alongside climbing, descending, heat, altitude and access to food or shelter. Distance alone cannot tell you when a symptom will begin.
Sleep becomes a separate planning problem when the event crosses normal sleeping hours. Kishi’s survey concerned finishers of 111- and 165-km mountain races; 77% reported sleeping during the event. Symptoms attributed to sleep loss included falls and hallucinations. The study describes what runners reported, not proof that those symptoms are harmless or that a particular nap schedule prevents them.
Stage races provide scheduled breaks, while continuous multiday races require runners to choose when to rest. A single-race study cannot simply be multiplied by the number of days. Use the sleep strategy guide to plan rest options, crew checks and safe transport home before fatigue makes those decisions harder.
Recovery: Feeling Better and Performing Better Differ
| When measured | What was observed | Practical limit |
|---|---|---|
| Days 3 and 5 | Hoffman et al.: among 72 finishers after 161 km, maximal 400-m times remained 26% and 12% slower, respectively. Soreness and fatigue returned to baseline by day 5. | Feeling recovered and regaining performance did not happen together. |
| About 6 days | Rojas-Valverde et al.: the review reported muscle- and kidney-injury markers usually returning to baseline after a mean of 5.86 days. | Diagnostic criteria differed across studies. This is not a deadline for individual recovery. |
| Days 9–16 | Millet et al.: most neuromuscular recovery occurred within 9 days; measurements returned to baseline by day 16 in the 11 runners followed after the mountain race. | A small follow-up sample cannot clear every runner to resume hard training. |
Track whether ordinary movement, sleep, appetite and energy are returning toward your usual pattern. Worsening or persistent symptoms deserve assessment rather than waiting for a study’s average recovery day. Consult the recovery guide for a gradual return to activity; the research above explains why one calendar rule is inadequate.
Sources and Evidence Limits
This is an editorial synthesis of published research, not original clinical research or a medical review of your circumstances. Individual race studies, questionnaires and reviews answer different questions. We identify their populations and separate observed measurements from practical interpretation. This article has not been reviewed by a named clinician.
- John et al. (2025): Does Distance Matter? Metabolic and Muscular Challenges of a Non-Stop Ultramarathon with Sub-Analysis Depending on Running Distance — energy balance at the 2024 TorTour de Ruhr.
- Millet et al. (2011): Neuromuscular consequences of an extreme mountain ultra-marathon — pre/post-race testing and a smaller recovery sample.
- Stuempfle and Hoffman (2015): Gastrointestinal distress is common during a 161-km ultramarathon — retrospective symptom questionnaire.
- Costa et al. (2017): Systematic review: exercise-induced gastrointestinal syndrome-implications for health and intestinal disease — gastrointestinal function and integrity.
- Lima et al. (2024): Acute effect of ultramarathon on systolic and diastolic cardiac function: Systematic review and meta-analysis — immediate cardiac measurements across 17 studies.
- Lecina et al. (2022): Acute Kidney Injury and Hyponatremia in Ultra-Trail Racing: A Systematic Review — different participant subsets for kidney and sodium outcomes.
- Kishi et al. (2024): Sleep and Ultramarathon: Exploring Patterns, Strategies, and Repercussions of 1,154 Mountain Ultramarathons Finishers — self-reported sleep experiences among finishers.
- Cejka et al. (2012): An increased fluid intake leads to feet swelling in 100-km ultra-marathoners - an observational field study — observational measurements of foot volume.
- Hoffman et al. (2017): Determinants of recovery from a 161-km ultramarathon — symptoms and performance during recovery.
- Rojas-Valverde et al. (2021): Exertional rhabdomyolysis and acute kidney injury in endurance sports: A systematic review — biomarker recovery and differences in diagnostic criteria.