How to Train in the Heat

Dr. Julien Périard is the Research Professor and Deputy Director, University of Canberra Research Institute for Sport and Exercise. He has worked with both amateur and professional athletes from various disciplines, along with National and International Federations (FIFA, UCI, World Athletics, and World Triathlon), and was also an elite triathlete.

As summer advances in the Northern Hemisphere, athletes are going to be training consistently in hotter conditions. Is there a different plan of attack for training in dry heat, vs. humid heat?

The plan should always be to monitor hydration status and how one feels during long workouts in the heat, whether dry or humid. Heat stress exacerbates the rise in core temperature and by extension the cardiovascular response.

Humid conditions may lead to:

• 📈 a greater rise in core temperature when exercising at the same workload, leading to a higher heart rate, and perceived exertion.

This is due to the environmental conditions being less favorable for losing heat from the body.

When it’s humid, much of the sweat being produced will not evaporate and cool down the skin. Instead, sweat will drip off the skin and lead to dehydration, without attenuating the rise in body temperature that occurs when it evaporates.

• This evaporation of sweat is the main pathway through which heat is lost in hot environmental conditions.

• When planning to train in humid heat, understanding how the workout might be impacted is important.

Athletes definitely need more overall liquids training in hot conditions. But do they also need more carbs? What about sodium?

Greater fluid intake is recommended during prolonged exercise in hot conditions as the sweat rate will be higher. The greater fluid loss via sweating is also accompanied by a greater volume loss of electrolytes in the sweat, including sodium.

🧂 Individual sodium losses will vary depending on fitness and heat acclimatization status. As such, supplementing fluids with sodium may be warranted for individuals whose sweat sodium concentration is high.

• There are tests available for this, but a simple test is to look at clothing after a long run or ride to see if it’s caked in salt (sodium) or to try a salty drink during exercise. If the drink doesn’t taste too salty, chances are the body is in need of sodium!

🍩 In terms of carbohydrates, when exercise workload or absolute intensity in the heat is matched to that of cooler conditions (for example 200 W on the bike), the relative intensity will progressively increase as body temperature increases.

💓 This will be noted in the drift in heart rate compared to when training in cooler conditions at the same workload. Given that relative exercise intensity will be higher, so will the rate of glycogen (carbohydrate) use.

• So yes, it may be worth ingesting more carbs during long hard workouts in the heat.

Some call training in heat “poor man’s altitude” Why is that? What is happening physiologically in the body while training in the heat?

Heat and altitude are quite different and induce different adaptations.

⛰️ The goal of training at altitude for endurance athletes is to increase hemoglobin content; oxygen-carrying capacity essentially. This typically occurs after 3 weeks of spending about 14 hours a day at altitudes above 2500 m.

🔥 Heat exposure leads to a rapid increase in plasma volume and an increase in sweat rate, among other adaptations.

• These adaptations contribute to lowering heart rate and core temperature during exercise at a given workload, as well as an increasing VO2max in the heat. These adaptations develop over the course of 5-14 days with daily heat exposures of 60-90 min.

The goal of heat training is to increase whole-body temperature and induce sweating, to drive the adaptive response.

There are recent studies that show an increase in both plasma volume and hemoglobin content after very long heat acclimation protocols (5 ½ weeks of 5 x 60 min heat sessions per week). This protocol length is much longer than typical laboratory-based approaches and likely very different from what elite athletes may undertake due to potential difficulties in integrating this many sessions into the training program; although it may depend on the period of the season.

Do you want maximal exposure to heat? I.e. all training sessions in hot conditions? Or Is it better to be selective with exposure?

At the elite level, or any level really, it’s better to be selective. That is because the ability to maintain a given workload will be compromised in the heat during prolonged exposures. So, if the goal is to maintain or hit specific power outputs or running speeds, the relative effort to achieve these will be much harder and reflected in both physiological and perceptual responses.

That’s not to say that hard efforts can’t or shouldn’t be done in the heat, simply that these should be carefully selected.

✔️ Generally speaking, athletes may want to conduct easier sessions in the heat and expose themselves to the stimulus for 60-90 min per session. This can be done based on maintaining a certain heart rate or perceived exertion, such that workload decreases as thermal (body temperature) develops.

• Alternatively, a particular speed or power output can be maintained throughout each session, which will become relatively easier to maintain as acclimation develops.

Thank you Dr. Périard for your incredible insight! You can follow him @DRJPeriard