Peggy & Marco Lachmann-Anke (Pixabay)

Marathon and ultra-marathon differ not only in the distance, but also and more importantly on the length of the event.

Additionally, many of these events are held in hot weather conditions. Thus, it is important to know how runners can be affected by heat.

But also, the running pace differs, affecting the body thermoregulation in different ways.


Heat production and body temperature

Of all the energy consumed by the muscles, about 75-80% is “wasted” as heat. This metabolic heat must be dissipated to maintain core temperature below certain levels to keep the muscles functioning with maximal efficiency.

Increases in core temperature impair performance due to changes in the circulatory system. Blood flow is diverted to the skin to improve heat dissipation. An ambient temperature above 25⁰C has been shown to cause a 3% decrease in the performance of male marathon runners.

Heat production is mainly affected by exercise intensity. Thus, the moderate intensity of ultra-endurance events makes them easier to tolerate for athletes, without risking a heat stroke.

Among the marathon runners the slowest ones were affected the most by the heat. On the contrary the slowest ultra-marathon runners were affected the least by the heat. A possible explanation could be that marathon races are usually more crowded and heat loss limited.


Running environment

The most popular marathons are held in cities, using streets and avenues that protect against solar radiation. But these streets also impair wind circulation due to the buildings, that affect heat dissipation, and use asphalt roads, that store the heat and may increase thermal stress.

On the contrary, ultra-marathons are usually held in natural environments, that can vary greatly. It is not the same a race in a forest than one in a desert. Additionally, they can take place in altitude or in a changing topography. All these factors have a clear influence on heat dissipation. And sometimes arriving to these races imply long travels, being recommended a period of two week to get a good acclimation.

Main heat stress challenges in marathon runners (from Bouscaren et al., 2020)


Marathon runners tend to wear light textiles (t-shirt, tank tops and shorts) while ultra-marathon runners usually wear more complex combinations and must carry, per regulation, additional equipment.

Clothing has a protective effect reducing radiant heat gain. Thus, marathon runners may have a disadvantage as the minimal clothing may increase their heat load in warm conditions. But also, the evaporation of sweat is facilitated, being this the main way of heat loss.

Ultra-marathon runners may have more protection due to the extra layers of clothing, but they can restrict sweat evaporation.

There is always a personal equilibrium for each person. Some prefer to run feeling colder while others prefer to feel warm.



Body water losses beyond 2% of body mass during exercise impair thermoregulatory function and aerobic exercise performance.

Hydration strategy is where we can find one of the main differences between marathon and ultra-marathon runners. Marathon runners rehydrate at drink stations, while ultra-marathon runners usually carry a hydration pack or belt. They can have more regular access to fluids but carry an extra weight and an accessory that may impair evaporation.

As for the gastrointestinal distress, although it affects runners of both disciplines, is more common in ultra-runners. It affects the feeding and hydration capabilities. If fluid losses are not compensated, they could lead to dehydration and impaired thermoregulation.

Regarding hyponatremia it has been well studied in marathoners, where it affects up to 13% of marathon finishers. In ultra-endurance events results are variable, probably because of the very different nature of the events themselves.

And that was our review of the different ways that heat conditions are different in running both distances. We hope that you found it useful.



Heat Stress Challenges in Marathon vs. Ultra-Endurance Running. Bouscaren N, Millet G, Racinais, S. (2019). Front. Sports Act. Living. doi:10.3389/fspor.2019.00059

Main heat stress challenges in ultra-endurance runners (from Bouscaren et al., 2020)

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