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. These systems interact, and problems can appear at any point rather than in a fixed order.
This page covers changes during and shortly after a race. For lasting effects, see long-term ultrarunning health; for practical recovery decisions, use the post-ultramarathon recovery guide.
What Studies Have Measured
| System | Finding | How to read it |
|---|---|---|
| Energy | John et al. (2025): estimated expenditure averaged 7,402 kcal at 100 km, 11,420 at 160.9 km and 14,635 at 230 km; the mean deficit was 6,797 kcal among 39 finishers. | Expenditure came from heart-rate data. The deficit is what runners did not replace, not a fueling target. |
| Muscles | Millet et al. (2011): knee-extensor strength fell 32% and creatine kinase rose from 144 to 13,633 IU/L on average in 22 male finishers of the 166-km UTMB. | A mountain race with about 9,500 m of climbing; terrain and testing method change the size of the drop. |
| Gut | Stuempfle and Hoffman (2015): 96% of 272 Western States respondents reported at least one gut symptom; 43.9% of finishers said symptoms affected their race. | Mild symptoms such as belching counted, so this is not 96% seriously ill. |
| Heart | Lima et al. (2024): across 17 studies, ejection fraction was 3.7 percentage points lower and heart rate 18 beats per minute higher right after the race. | Acute changes measured at the finish; they do not show lasting damage. |
| Kidneys | Lecina et al. (2022): 468 of 1,113 runners assessed (42%) met criteria for acute kidney injury. | 392 of the 468 cases were the mildest stage; the one runner who met failure criteria did not need dialysis. |
| Blood sodium | Lecina et al. (2022): 195 of 2,065 runners (9.4%) had exercise-associated hyponatremia. Hoffman et al. (2013): 15.1% of 669 finishers over five years of 161-km races. | All cases in the review were mild or moderate. The yearly rate in the 161-km races ranged from 4.6% to 51.0%. |
| Sleep | Kishi et al. (2024): among 1,154 finishers of 111- and 165-km mountain races, 77% napped and 80% reported at least one symptom they put down to sleep loss, including hallucinations (34%) and falls (14.6%). | Self-reports from two races at the 2018 Grand Raid Réunion. |
| Body weight and feet | Cejka et al. (2012): body mass fell 2.4% and average foot volume did not change significantly in 76 male finishers of a 100-km road race. | Individual feet still swelled by up to 12.5%; an average cannot set your shoe size. |
| Brain (multiday) | Freund et al. (2012): grey-matter volume fell about 6% by around 4,000 km of the 4,487-km TransEurope FootRace in 10 men. | Back to baseline in the 7 rescanned about 8 months later; no new brain lesions were found. |
Hour by Hour: How the Strain Builds
The first hours
Fueling starts to matter early. Costa et al.'s review puts the threshold for significant gut disturbance at about two hours at 60% of maximal oxygen uptake, regardless of fitness, with heat and running itself making it worse. Practise eating from the first hour in training so the race is not the first test.
Through the first day
Energy out exceeds energy in from the start, and the gap widens with distance. At the TorTour de Ruhr, the average deficit was 5,221 kcal at 100 km, 7,066 kcal at 100 miles and 8,562 kcal at 230 km. Pace falls too: in a GPS study of a 173-km trail race, runners slowed over the race on level, uphill and downhill sections alike.
The first night
Sleep pressure and the body-clock low arrive together in the small hours. In Kishi's survey of mountain-ultra finishers, 77% napped during the race, a third reported hallucinations and one in seven reported falls that they put down to sleep loss. The sleep strategy guide sets out when and how long to nap.
At the finish
After a mountain 100-miler, knee-extensor strength is down by about a third and creatine kinase is up roughly a hundredfold. The heart pumps out a slightly smaller share of its blood with each beat, and about four in ten runners meet criteria for acute kidney injury, mostly the mildest stage. About one in ten finishes with low blood sodium.
Day after day
Stage races and continuous multiday races repeat these strains with little recovery between them, and a single-race study cannot simply be multiplied by the number of days. In 7-day desert stage races, 85% of 396 runners needed medical care, mostly for skin problems and blisters. During the TransEurope FootRace, runners lost about 6% of their body weight and grey-matter volume over roughly 4,000 km.
Energy and Digestion
Running costs roughly 50–70 kcal per km on flat ground; the ISSN position stand cites 47–71 kcal per km. That puts a 100-mile race at about 7,500–11,500 kcal before climbing, in line with the 11,420 kcal the TorTour de Ruhr study estimated for 160.9 km. Few runners eat that much on the move: those 100-mile finishers ended about 7,000 kcal short on average. The ISSN suggests 150–400 kcal an hour, including 30–50 g of carbohydrate. Estimate your own expenditure with the running calorie calculator.
Gut symptoms make eating harder. In the Western States survey, which included finishers and non-finishers and drew responses from 71% of starters, 35.6% of non-finishers named gut symptoms as a reason for dropping out. Train the gut within normal sessions with the gut-training guide.
Muscles, Heart, Kidneys and Sodium
Muscle force and blood markers measure different things. After the UTMB, creatine kinase averaged about 13,600 IU/L with a wide spread, and two runners above 45,000 IU/L had no problems. The same paper describes a runner from the 2008 race whose creatine kinase reached 429,240 IU/L and who needed dialysis after taking anti-inflammatory drugs and having diarrhoea. A high number alone is not a diagnosis; severe pain, weakness or dark urine is the warning sign.
Ejection fraction is the share of blood the left ventricle pumps out with each beat. A drop of 3.7 percentage points is an acute change measured at the finish; the meta-analysis does not show lasting damage, but new chest pain or fainting still needs assessment.
Kidney blood tests often change, mostly within the mildest category. Low blood sodium is the risk you can most directly prevent: the hyponatremia consensus names drinking well beyond thirst as the main preventable cause and warns that sodium in sports drinks does not make overdrinking safe. The hydration and sodium guide has the practical limits.
Recovery: Feeling Better and Performing Better Differ
| When measured | What was observed | Study population |
|---|---|---|
| Days 3 and 5 | 400-m times were 26% and 12% slower; soreness and fatigue were back to baseline by day 5. | 72 Western States finishers (Hoffman et al. 2017) |
| About 6 days | Muscle- and kidney-injury markers usually returned to baseline after about 6 days (5.86). | Review of 43 endurance-sport publications; diagnostic criteria differed (Rojas-Valverde et al. 2021) |
| Days 9–16 | Most neuromuscular recovery happened within 9 days; strength was back to baseline by day 16, though some electrically evoked measures were still depressed. | 11 UTMB finishers followed after the race (Millet et al. 2011) |
| About 8 months | Grey-matter volume was back to baseline. | 7 TransEurope FootRace runners rescanned (Freund et al. 2012) |
Feeling recovered comes before performing recovered. Our recovery guide plans on about three weeks before full training after a 100-mile or 24-hour race, longer after multiday races; worsening or persistent symptoms deserve assessment rather than waiting for a study's average.
Sources and Evidence Limits
This is an editorial synthesis of published research, not clinical advice. Race studies, questionnaires and reviews answer different questions, so each finding above names its population. 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 — strength and creatine kinase after the 2009 UTMB, with a recovery subgroup.
- 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 — separate participant groups for kidney and sodium outcomes.
- Hoffman et al. (2013): Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners — hyponatremia over five years of 161-km races.
- Hew-Butler et al. (2015): Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015 — overdrinking as the main preventable cause of hyponatremia.
- 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 — foot volume and body mass in a 100-km road race.
- Kerhervé et al. (2016): Pacing during an ultramarathon running event in hilly terrain — GPS speeds through a 173-km trail race.
- Freund et al. (2012): Substantial and reversible brain gray matter reduction but no acute brain lesions in ultramarathon runners: experience from the TransEurope-FootRace Project — brain MRI during and after the TransEurope FootRace.
- Krabak et al. (2011): Study of injury and illness rates in multiday ultramarathon runners — medical care in 7-day desert stage races.
- Tiller et al. (2019): International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing — ISSN position stand; energy cost of running and hourly intake ranges.
- 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.