Beat the Heat!
SUMMARY
Running in the heat and humidity can impair performance and place additional strain on the body.
Smart, progressive training in the heat lets the body adapt and improve performance and thermoregulation.
Several strategies exist to optimize running in the heat including hydration and nutrition, and training and environmental modifications.
Runners with pre-existing conditions or increased stressors may need to limit heat exposure or at the very least be more cautious.
With summer in our midst, and temperatures reaching 90-100 degrees F (at least where I live), let’s talk about running in the heat and how to optimize our runs. The three main questions we will look at are:
How does heat and humidity affect my training and performance?
Should I run in the heat?
What can I do to still run in the heat?
Now before anything else, please be mindful that any guidelines and suggestions are just that - guidelines and suggestions, not medical advice. Working with a qualified healthcare professional one-on-one is important to individualize the recommendations to your needs.
Alright, let’s step in!
How does heat and humidity affect my training and performance?
The most obvious effect we notice from running in the heat and humidity is that we sweat a lot more. This might sound like a no-brainer, but let’s dive into the science a little.
When we run in the heat, we sweat more in an attempt to cool off the body. Studies have shown that endurance athletes exercising in 20 degree C environments (68 degrees F) lose more than 1 liter of liquid (Armstrong, 2021). Imagine how much more we lose when running in 90+ degree F weather!
As we get more dehydrated, we naturally lose weight. Remember, our body is made of water - if we lose water, we lose weight. Losing as little as 2% of our bodyweight in fluids can impair our performance (Armstrong, 2021; Muhamed et al, 2016; Racinais et al, 2015; Casa et al, 2010). This is because our bodies are placed under greater strain to regulate our body temperature.
What exactly does that look like?
First, core temperature rises. If it is too hot, we reach a point where our bodies cannot cool off fast enough. This is especially an issue when humidity is high. As humidity rises, our sweat has a harder time evaporating from our skin. In other words, our primary cooling system is dialed down.
Second, the more we sweat, the more fluid our body loses. As more fluids are lost, our hearts pump faster in an effort to maintain blood flow. For you physiology buffs, this has to do with cardiac output (CO = HR x SV). As stroke volume (amount of blood pumped per heartbeat, or SV) decreases due to less blood being available for pumping, the faster the heart beats to maintain cardiac output (and basic bodily function). You might notice your heart tends to increase faster and and stay high when you run in the heat or for long runs with minimal hydration or increasing fatigue - this is called cardiac drift.
Adding this altogether we have: hearts working harder, blood being shunted from our core to our skin to cool us off, less blood flowing to our muscles and organs - and hence we have decreased performance. This explains at least part of the physiological barriers with running in the heat.
Should I run in the heat?
Well…as we physical therapists love to say, it depends.
You know your body better than anyone. Maybe you already appreciate that you have a low tolerance to heat. Generally, training in the heat and humidity actually improves our thermoregulation. Endurance athletes who train consistently in the heat over 10-14 days can develop adaptations including improved evaporative heat loss, decreased skin temperature, and decreased heart rate (Sekiguchi et al, 2022). This in turn improves blood and oxygen flow, time to exhaustion, VO2max, and even power output (Periard et al, 2024). In other words, our bodies learn how to improve temperature regulation and are under less strain, our muscles get more oxygen, and our bodies become less fatigued.
Now, this has been seen in studies looking at healthy athletes. However, there are certain conditions that are greatly exacerbated by the heat. These include autoimmune conditions like multiple sclerosis, lupus, rheumatoid arthritis, as well as conditions with autonomic dysautonomia including POTs. Unfortunately, not many studies look specifically at endurance training adaptations in the heat for these populations. However, we can appreciate that individuals with these conditions are at risk of being less active in general because of their symptoms and perceptions of exercise. A graded approach for exercise in general is very beneficial, and exercising in the heat should be proceeded with caution.
If you are currently experiencing a flare, it may be wise to hold off on heated training. I will never forget a simple phrase used by an influential running physical therapist and coach Christopher Johnson: “Stress is stress is stress.” If you are already suffering significant stress while training in ambient conditions, adding in another stressor, whether you have a chronic condition or not, may not be ideal at that point in time. This is why it is beneficial to work with a coach or other healthcare professional who can help focus the picture.
What can I do to still run in the heat?
Okay, the part you are probably more interested in. How to optimize running in the heat?
Hydration. The most important factor, in my opinion. In a study looking at performance in dehydrated and euhydrated individuals, dehydrated runners had higher body core temperatures at lower intensity efforts, higher post-run heart rates, and slower race times. In other words, performance suffered and their bodies were working really hard to regulate body temperature. Simple hydration rules according to standard guidelines and research suggest:
Before exercise: 400-800 ml 2-3 hours prior to running. A more accurate guideline is 6ml/kg of bodyweight (Armstrong 2021; Racinais 2015).
During exercise: This varies across guidelines. Working with a professional sports dietitian or coach may be helpful. The biggest takeaway seems to be to consume less than 700mL/h to reduce the risk of exertional hyponatremia, a condition where our cells get packed with fluid and can lead to heat cramps and exhaustion, with warning signs being lightheadedness or dizziness, headaches, nausea, shortness of breath (Armstrong 2021).
After exercise: Ideally, hydrate enough to replace 100-150% of the weight lost in running (Armstrong 2021, Pryor et al, 2019). Remember, we lose water (and weight) when running. There are a few ways to calculate this, and one calculator is linked here. It calculates your sweat rate so you know how much fluid you lose during your runs. Simpler methods include:
Weighing yourself before and after your run. The difference is roughly how much fluid you lost. Remember to also consider any fluid you drank or expelled in urine! So if you lost 2 lbs, you lost about 32 oz of liquid. Try to drink that much before your next training session.
If you GAINED weight, you drank too much during your run and will want to decrease it next time.
If you weigh the same, congrats! You hydrated well during your run.
Electrolytes: Daily consumption guidlelines are around 1500mg of sodium.
Before: Consuming 300-600mg/L of sodium/fluid 2-3 hours prior to exercise, along with carbs, may be more helpful in delaying dehydration (Veniamakis et al, 2023).
During: If exercise lasts for more than an hour, or you are particularly prone to “salty sweat” or cramping, you may want to shoot for 500-1000 mg/L, the latter being for extreme conditions (exercise more than 90-120 minutes, hot and humid, prone to cramping and sweating, poor hydration before) (Tiller et al, 2019).
Examples from lower to higher sodium content electrolyte brands: Nuun (300mg), Liquid IV (500mg), Tailwind (630mg) LMNT (1000mg).
After: If you notice salt cravings and have intense or long duration workouts for several days in the heat, you may need to add some salty snacks throughout the day.
Gradual exposure - whether you are in an area that is consistently hot, or you have a race and know it will be hot/humid, a smart program focusing on heat acclimatization can help. There is no one right way of doing it, but a review of heat acclimization guidelines (Pryor et al, 2019) propose the following:
Progressive training over the course of 10-14 days. If you are more sensitive to the heat, stretch it out to about a month while alternating hot and ambient training sessions more often.
Exercise to the point of sweating and feeling warm. You need to train to the point your body is challenged to thermoregulate in order to improve that ability.
Modify intensity and duration. Exercise prescriptions indicate that exercising for 60 minutes at about 60% HRmax (Zone 2) or 30-35 minutes at about 80% HRmax (borderline Zone 3/4) achieve similar results. Professional athletes are trained to tolerate up to 2 hours of lower intensity. If you have a marathon or ultramarathon in hot weather, it may be helpful to work toward that goal, but remember to start with what you can tolerate considering all other stressors!
If you don’t have to train for a hot race and just want to add some outdoor runs in the summer, here are some tips:
Plan accordingly - check the weather. Generally early morning will be cooler, but plan ahead of time!
Run on roads or trails with adequate shade or with cooler pockets.
You can try some cooling devices like headbands or vests, though they might only offer minimal help.
Alternate some outdoor runs with indoor treadmill or track running.
HYDRATE!
Hopefully you feel more prepared about running in the heat. And if you choose to go indoors, I don’t blame you!
References:
Armstrong LE. Rehydration during endurance exercise: Challenges, research, options, methods. Nutrients. 2021;13(3):887. doi:10.3390/nu13030887
Casa DJ, Stearns RL, Lopez RM, et al. Influence of hydration on physiological function and performance during trail running in the heat. Journal of Athletic Training. 2010;45(2):147-156. doi:10.4085/1062-6050-45.2.147
Pryor JL, Johnson EC, Roberts WO, Pryor RR. Application of evidence-based recommendations for heat acclimation: Individual and team sport perspectives. Temperature. 2018;6(1):37-49. doi:10.1080/23328940.2018.1516537
Racinais S, Alonso J-M, Coutts AJ, et al. Consensus recommendations on training and competing in the heat. Sports Medicine. 2015;45(7):925-938. doi:10.1007/s40279-015-0343-6
Sekiguchi Y, Benjamin C, Lee E, et al. Effects of heat acclimation following heat acclimatization on whole body heat exchange in trained endurance athletes. International Journal of Environmental Research and Public Health. 2022;19(11):6412. doi:10.3390/ijerph19116412
Veniamakis E, Kaplanis G, Voulgaris P, Nikolaidis PT. Effects of sodium intake on health and performance in endurance and ultra-endurance sports. International Journal of Environmental Research and Public Health. 2022;19(6):3651. doi:10.3390/ijerph19063651
Tiller N.B, Roberts J.D, Beasley L, et al. International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultramarathon trianing and racing. Journal of International Sports Nutrition. 2019;16(1):50. doi: 10.1186/s12970-019-0312-9.