A century of ultramarathon data shows what goes wrong on an 89-kilometer run

South Africa's Comrades Marathon, run annually between Durban and Pietermaritzburg since 1921, is old enough and consistent enough to function as what a review published this month calls a "natural laboratory" for studying what happens to the human body over roughly 89 kilometers of running [s1]. The review pulls together 42 studies of the race to build one of the more complete pictures available of ultra-endurance running's actual medical toll.

What the review covers

Researchers searched EBSCO, PubMed, SciELO, and Web of Science for studies published through December 2025, screening 116 records down to 42 that met eligibility criteria and focused specifically on the Comrades Marathon [s1]. The review synthesizes field studies and case reports spanning decades of the race's history [s1].

Who runs it, and how the field has changed

Women have been eligible to compete since 1975 and represent 4.2% of finishers in the data reviewed [s1]. The growth in finishers during the 1970s was driven primarily by men aged 30 to 59 [s1]. Men have consistently run faster than women across all age groups, though the performance gap between them has narrowed over time as overall performance improved [s1]. Peak performance age was around 29.9 years for men and 36.0 years for women [s1].

What actually sends runners to medical tents

The review's most useful number for anyone training for a similar event is the overall medical encounter rate: up to 20 per 1,000 starters required medical attention [s1]. The breakdown by organ system shows where that burden concentrates — fluid and electrolyte problems accounted for 8.8% of medical issues (95% range 8.3–9.4%), central nervous system issues 4.0% (3.7–4.5%), and gastrointestinal issues 2.9% (2.6–3.2%) [s1]. By specific diagnosis, dehydration was the single most common problem, at 7.5% (7.0–8.1%), followed by exercise-associated muscle cramping at 3.2% (2.9–3.6%) [s1].

Notably, exercise-associated hyponatremia — low blood sodium from overhydration, a well-publicized risk in endurance events — was uncommon, affecting fewer than 2% of the medical cases reviewed [s1]. Hypernatremia, the opposite problem of too little fluid relative to sodium loss, was substantially more prevalent in the data [s1] — a finding that cuts against a common assumption that overhydration is the dominant sodium-balance risk in ultra-distance events.

The more serious, rarer findings

The review documents isolated deaths in the race's early editions, likely due to ischemic heart disease [s1]. Post-race cardiac biomarker elevations and echocardiographic changes were observed in some runners but were transient, resolving rather than persisting [s1]. The race also produced measurable muscle inflammation without major abnormalities on knee MRI [s1]. In the 1970s specifically, some runners experienced acute kidney injury and renal failure serious enough to require dialysis [s1] — the review does not report similar cases becoming common in more recent decades, consistent with improvements in race-day medical support and runner preparation over the intervening fifty years.

One counterintuitive finding: faster, more highly trained runners showed higher rates of post-race upper respiratory tract infection than their slower, less-trained counterparts [s1] — a pattern that has some precedent in exercise-immunology research suggesting very high training loads can transiently suppress immune function around a major competitive effort.

What this means for ultra-endurance runners

The overall picture from decades of Comrades data is one where serious, race-ending medical emergencies are rare, and the bulk of medical attention goes to manageable, well-understood problems — dehydration and cramping chief among them — rather than the catastrophic cardiac events that tend to dominate public perception of ultra-distance running risk. That said, the review is a synthesis of one specific race's accumulated data over a century of changing conditions, training norms, and medical support, and its findings describe this event's documented pattern rather than a guaranteed profile for any other ultramarathon.

Sources

• The Comrades Marathon: a narrative review of physiological responses and health implications in the world's oldest ultra-marathon — Frontiers in Physiology, 2026-05-18