Heat training for ultra runners: A science-based guide

By Henry Howard

For ultra and marathon runners looking to gain a competitive edge, heat training has emerged as one of the most effective yet underutilized training methods available. Also known as heat acclimation or heat adaptation, this practice involves deliberately exposing your body to elevated temperatures during training to trigger physiological adaptations that enhance performance in both hot and temperate conditions.

During my training for the Burning River 100 in late July, I experimented with heat training. For me this involved getting on my indoor bike once or twice a week for 30 to 60 minutes during a recovery ride with additional layers.

It’s hard to tell whether this had any impact on performance during Burning River as I had to deal with multiple other issues on my way to my fifth finish at a 100-miler. My crew was instrumental in my finish, including cooling me down at aid stations.

The science behind heat training

When you exercise in hot conditions, your body undergoes remarkable adaptations to cope with thermal stress. Research shows that consistent heat exposure triggers several key physiological changes that directly benefit endurance performance.

Your body increases plasma volume by up to 20%, essentially expanding your blood volume and improving circulation. This enhanced blood volume allows your heart to pump more efficiently, reducing cardiovascular strain during exercise.

Heat training for ultra runners also improves your body's cooling mechanisms. Your sweat rate increases and begins earlier during exercise, while the sodium concentration in your sweat decreases, helping you maintain electrolyte balance. Perhaps most importantly, your core body temperature threshold for fatigue rises, allowing you to maintain higher intensities for longer periods before overheating becomes a limiting factor.

Studies have demonstrated that these adaptations occur relatively quickly. Most physiological improvements begin within the first week of heat exposure, with full adaptation typically achieved within 10 to 14 days of consistent training.

Performance benefits

The benefits of heat training extend far beyond simply preparing for hot weather races. Research consistently shows that heat-adapted athletes experience improved performance even in cool conditions. A landmark study published in the Journal of Applied Physiology found that cyclists who completed heat acclimation training

improved their time trial performance by 6-8% in both hot and temperate conditions.

For runners specifically, heat training can lead to enhanced VO2 max, improved lactate threshold and better overall endurance capacity. The increased plasma volume acts similarly to altitude training's effects on red blood cell production, providing more oxygen-carrying capacity throughout your system. Additionally, the metabolic adaptations from heat stress can improve your body's ability to buffer lactate and maintain energy production during intense efforts.

Many elite endurance athletes now incorporate heat training into their preparation for major competitions, regardless of expected weather conditions, purely for these performance benefits. Among them is Rod Farvard, who used science as his guide for heat training during his buildup to Western States in 2025 as I wrote about earlier. While his race didn’t go as he hoped, it wasn’t due to heat training issues.)

Effective heat training methods

The most straightforward approach to heat training is environmental heat exposure. This can be achieved through heated indoor training environments, saunas, or hot baths. For runners, spending 20 to 30 minutes in a sauna immediately after workouts can provide effective heat stimulus without adding training stress.

Hot bath protocols have gained popularity due to their convenience and research backing. Immersing yourself in water heated to 104-106°F (40-41°C) for 20 to 40 minutes post-workout can trigger similar adaptations to exercising in heat. This method is particularly appealing because it doesn't interfere with your regular training intensity or volume.

For those with access to controlled environments, training in heated rooms (85-95°F with high humidity) while maintaining normal workout intensities provides excellent heat adaptation stimulus. Many athletes use heated yoga studios or specially designed training facilities for this purpose.

Some athletes choose to overdress for the weather, wearing an extra layer or two during runs. As a coach, I wouldn’t recommend this since the additional clothing would limit the effectiveness of the running workout. (Note: While I referenced my heat training on the bike, these were recovery efforts so the extra layers didn't affect any running related adaptations I was seeking.)

Implementation guidelines

When beginning heat training, start with shorter exposures and gradually increase duration and intensity. Begin with 15 to 20 minute sessions and progress to 30 minutes over the course of two weeks. Monitor your core body temperature if possible, aiming to elevate it to 101-102°F (38.3-38.9°C) during sessions.

Hydration becomes even more critical during heat training. Increase your fluid intake before, during, and after heat exposure, and pay attention to electrolyte replacement. I’d recommend drinking a Gatorade, Powerade or similar electrolyte mix the day before and day of such heat training. Watch for signs of heat exhaustion, including excessive fatigue, nausea, dizziness, or cessation of sweating.

Heat training represents a scientifically proven method for enhancing endurance performance that extends well beyond hot weather preparation.

When implemented safely and systematically, it can provide significant competitive advantages for runners of all levels. As with any training intervention, consistency and gradual progression are key to maximizing benefits while minimizing risks.

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