The Short Answer
Acute stress and chronic harm are not the same thing, but they cannot always be separated by reassurance alone. A multiday race can produce large, highly individual changes in laboratory markers and function. Many improve after the event; some acute conditions require treatment, and the cumulative consequences of repeatedly exposing the body to extreme endurance loads are not fully known.
Acute Effects (During and After Race)
| System | Possible acute pattern | When to get checked |
|---|---|---|
| Muscles | Soreness, weakness, and a highly variable creatine-kinase rise | Severe or worsening pain, profound weakness, swelling, or tea-colored urine |
| Heart | Post-race biomarker and pumping-function changes can occur | Chest pain, fainting, palpitations with illness, or unusual breathlessness |
| Kidneys | Creatinine and other kidney-injury markers may rise after long events | Very low or dark urine, persistent vomiting, flank pain, confusion, or ongoing swelling |
| GI system | Nausea, vomiting, diarrhea, abdominal pain, or impaired intake | Blood, severe pain, repeated vomiting, confusion, or inability to keep fluid down |
| Inflammatory and immune markers | Multiple markers shift after prolonged exercise | Fever, worsening illness, or symptoms that do not follow an improving recovery pattern |
Long-Term Evidence
Evidence is not strong enough to declare ultrarunning either harmless or inevitably damaging. The often-cited ULTRA Study baseline report described a generally healthy, self-selected group of active ultrarunners at one point in time; it did not track a randomized comparison for 10–20 years or prove cause and effect. Reviews identify plausible concerns involving cardiac remodeling and arrhythmias, repeated kidney injury, energy availability, and musculoskeletal health, while emphasizing that ultrarunner-specific long-term data are sparse. Read more about multiday running safety.
Where the Real Risk Is
Important acute risks include heat or cold illness, overdrinking-related hyponatremia, inadequate fluid intake in heat, underfueling, poor pacing, exercise while ill, and ignoring warning signs. Nonsteroidal anti-inflammatory drugs such as ibuprofen can add kidney risk during an ultra; do not use them during a race unless a clinician who knows your history has advised you to do so.
Runners with heart disease symptoms, unexplained fainting, severe asthma, kidney disease, or a history of heat illness should get medical advice before attempting long ultras. The goal is not to make ultrarunning seem harmless. The goal is to recognize expected fatigue without dismissing symptoms that need assessment.
What About Joints?
The common claim that running inevitably destroys knees is not supported by evidence from recreational running populations. However, studies differ in exposure and quality, and they do not establish that all levels of competitive ultrarunning are risk-free. Prior injury is an important confounder. Persistent joint swelling, locking, instability, focal bone pain, or pain that changes gait deserves evaluation rather than a generic instruction to keep running.
Sources
- Scheer, V. et al. (2022) — "Potential Long-Term Health Problems Associated with Ultra-Endurance Running: A Narrative Review." Sports Medicine, 52(4), 725–740.
- Hoffman, M.D. & Krishnan, E. (2014) — "Health and Exercise-Related Medical Issues among 1,212 Ultramarathon Runners." PLoS ONE, 9(1), e83867.
- Rojas-Valverde, D. et al. (2021) — "Exertional Rhabdomyolysis and Acute Kidney Injury in Endurance Sports: A Systematic Review." European Journal of Sport Science, 21(2), 261–274.
- Burfield, M. et al. (2023) — "The Association between Running Volume and Knee Osteoarthritis Prevalence: A Systematic Review and Meta-Analysis." Physical Therapy in Sport, 61, 1–10.
- Lipman, G.S. et al. (2017) — "Ibuprofen versus Placebo Effect on Acute Kidney Injury in Ultramarathons: A Randomised Controlled Trial." Emergency Medicine Journal, 34(10), 637–642.