While late spring and early autumn are the most popular seasons for racing, summer races do occur. More ultra and trail races occur in summer, as the mountains are finally clear from snow. Some races, such as the Boilermaker 15K, distinguish themselves as being races where the weather brings a new level of challenge. For any summer race, the heat will affect performance. However, research strongly indicates that heat acclimation strategies will improve performance in summer races.
Heat acclimation is the process of adapting your body to exercise in hot conditions. Summer running naturally induces a certain degree of heat acclimation. However, if you are preparing to run a race where it may be 80-degrees or warmer, morning runs in the 60s and 70s may not adequately prepare you. For hot weather races, a strategic approach to heat acclimation will prepare you for your best race possible.
When you run in the heat, your body adjusts its physiological responses to maintain a safe body temperature. Blood flow and sweat rates increase to regulate body temperature – but the body works harder. As a result, endurance performance decreases; the same intensity produces a slower pace compared to temperate weather. Dehydration, which occurs more frequently and to greater degrees in the heat, will compound the performance declines. If your body temperature reaches a critical point, you experience hyperthermia.
However, heat acclimation can partially mitigate the effects of running in the heat. Heat acclimation is repeated exposure to training in hot conditions. As a result, the body undergoes various physiological adaptations. As outlined in a 2015 consensus statement in the British Journal of Sports Medicine, increases in plasma volume and sweat response occur. Your body becomes better at maintaining stable cardiac output and an appropriate fluid-electrolyte balance. You even feel more comfortable when exercising in the heat after acclimation.
These physiological changes due to acclimation to the heat result in improved endurance performance. Heat-acclimated runners will outperform non-acclimated athletes of equal fitness. For runners competing in summer races, particularly summer ultramarathons, heat acclimation assumes a critical role in their training.
Heat acclimation occurs over a relatively short period. Within one week of consistent running in the heat, heart rate has lowered during exercise in the heat. Simultaneously, sweat rate increases. By two weeks of heat acclimation, endurance performance has improved in the heat.
Heat acclimation reverses in approximately 2-4 weeks. Therefore, you want to undergo acclimation closer to your race, rather than in early season.
These strategies are only necessary if you are preparing for a hot weather race. If you are not racing in the heat within 14 days, do not undertake these strategies. If you are working with a coach, do not initiate these heat training approaches without discussing it first.
Heat acclimation strategies are a training stressor. As a result, recovery needs increase and you may need to reduce the intensity of training sessions. For athletes not preparing for a hot-weather race, a normal training plan with proper recovery offers more short-term and long-term benefits than intensive heat acclimation strategies.
For optimal results, heat acclimation requires repeated 60-minute sessions over one to two weeks prior to your race. Since heat training reverses within 2-4 weeks, you likely want to do heat training in the two weeks immediately before your race. The 60-minute sessions are relatively low-intensity but should be intense enough that you sweat. For a majority of recreational runners, the desired intensity range is zone 2 (easy effort).
To implement this heat acclimation approach, start your heat acclimation two weeks prior to race day. During these two weeks, complete 8-10 runs of 60-75 minutes, at easy run intensity, during a time of day when the temperature is 80-90 degrees Fahrenheit (27-32 Celsius).. Do not do this on hard workouts or long runs, when the risk of heat stress is increased.
You can also implement the above protocol using the treadmill or layering clothing (or both). This approach works well if your climate is relatively cooler (lower than 80 degrees). You simply add one extra layer than usual during easy runs. Clothing retains temperature, thus elevating your core temperature and sweat rate during exercise. As a result, you receive some heat acclimation, even if it is mild weather outdoors.
The sauna-induced heat acclimation offers a unique strategy for athletes who may want to maintain training intensity during heat acclimation. When using this strategy, the athlete completes their normal training. Within one hour of completing the run, they sit in the sauna for 15 minutes. The sauna session only needs to be completed three times per week. This approach is reserved for advanced athletes who are able to support their training with ample sleep and nutrition.
One difference between male and female runners is their thermoregulatory response. Female athletes experience a higher basal body temperature during the luteal phase of their menstrual cycle. Some experts have theorized that this monthly increase in body temperature could affect athletic performance in the heat.
A 2023 meta-analysis and review in Sports Medicine examined the results of 22 studies. The researchers concluded that female athletes do respond positively to short-term, medium-term, and long-term heat acclimation. Exercising core temperature, resting core temperature, sweat rate, skin temperature, and heart rate all responded positively to heat acclimation. Performance also improved after two weeks of heat acclimation, as measured in power output and time to exhaustion. The only noted difference was that female athletes did not experience plasma volume expansion, although the mechanism is not understood. Instead of changes in plasma volume, the most notable physiological marker of heat adaptation was a drop in exercising heart rate by an average of 10 beats per minute.
Similar to male athletes, female athletes best respond to 8-14 days of heat acclimation, with exercise duration lasting ~65 minutes per day. The review highlighted that presently, no effect of menstrual cycle on heat adaptation is clearly understood. Practically, that means that you do not need to adhere to any complicated routine of cycle syncing while training in the heat.
Dehydration does not enhance heat acclimation. Rather, dehydration elevates your risk of experiencing heat stress or heat-related illness during heat training. For any athlete undertaking heat training, adequate hydration is essential. Bring water or sodium-containing sports drinks on all heat acclimation runs. Sip at it regularly throughout the run, aiming for approximately 10-16 oz of fluid per hour.
If you struggle with hydration during runs, you may benefit from completing a sweat test prior to heat training. A sweat test estimates your sweat loss during exercise via changes in body weight. While exact sweat rates vary based on conditions, a sweat test aids in calibrating your hydration strategy.
You weigh yourself before (before getting dressed, after eating/hydrating) your run. During the run, you record any fluid intake. After the run, you undress, towel off any sweat, and then weigh yourself again. The difference between pre- and post-run weight is your total sweat loss for the run. You then divide the total sweat loss by time spent running (in hours) to determine sweat loss per hour. You can also use a sweat rate calculator such as this one from Featherstone Nutrition.
Importantly, some athletes may need to increase sodium intake during heat training. A 2015 randomized controlled trial in Applied Physiology, Nutrition, and Metabolism demonstrated the superiority of hydration using sodium compared to water-only and glycerol-supplemented approaches. If you notice sodium loss in your sweat, struggle with hydration even when drinking enough, or crave salty foods after runs, you may need to ingest more sodium. You can increase sodium through both sports drink or salt tablet consumption during runs and dietary sodium intake throughout the day.
Recovery nutrition supports adaptations to any training stimulus – including heat training. A 2010 controlled trial published in the Journal of Applied Physiology demonstrated enhanced thermoregulatory responses and greater expansion of plasma volume in athletes who consumed a combination of protein and carbohydrate immediately after aerobic training sessions. The athletes also experienced less heat stress compared to the control group.
Recovery nutrition does not have to be complicated. The general guideline is ~20 grams of protein (or roughly 0.2-0.4 grams of protein per kilogram of body weight) and 0.8-1.2 grams of carbohydrate per kilogram of bodyweight. For a 65-kg (143-lb) athlete, such a meal would include 13-26 grams of protein and 52-78 grams of carbohydrate. You also want to replace any fluid and sodium lost during the run.
If appetite is poor after training in the heat, it is encouraged to utilize liquid calories. Smoothies with fruit and yogurt, recovery mixes with protein and carbs, protein powder mixed with high-carb plant milk, chocolate milk, and other easy-to-eat foods can all provide recovery nutrition needs.
Cooling strategies do not inhibit heat adaptations – and they will help heat training feel more tolerable. Pre- and mid-run strategies both are effective. You can try drinking ice water before, putting ice in your bottle during the run, and dumping cold water on your head/neck during exercise. (This article contains more information on effective cooling techniques for heat training.)
Heat training is not the time to prove yourself! Know the signs and symptoms of heat stress and stop your run if you experience any. Heat illness symptoms include nausea/vomiting, confusion, high heart rate, dizziness/lightheadedness, unquenchable thirst, headache, and fainting.
Like any training stressor, doing too much will inhibit heat acclimation – but doing too little will not encourage enough acclimation. So how do you know you are doing just enough to acclimate to the heat?
Within two weeks, you should see:
For more on heat acclimation, listen to episode 15 of the Tread Lightly Podcast!
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