How to Best Adjust your Training for Running in the Heat

Summer running can be an exciting time: more daylight hours, the start of fall race training, and finally not having to wear several layers on a run! However, summer running comes with its own challenge: the heat and humidity. Running in the heat can feel really hard! 

It’s not just you – and you are not losing fitness. Running in the heat is physiologically more challenging. This article delves into why running in the heat feels so hard – along with practical tips for how to make running in the heat feel better. 

You can also learn about running in the heat and heat acclimation on episode 15 of the Tread Lightly Podcast!

Why Does Running in the Heat Feel So Hard?

Running in the heat is not a matter of mental toughness. The fact of the matter is that heat, humidity, and direct sun exposure on you all impair aerobic performance. 

Maximum aerobic power declines in the heat. Heart rate goes up and stroke volume goes down – which means overall cardiac output is down despite your heart rate being higher. Your brain and muscles experience reduced blood flow, as your body sends more blood to the skin for cooling. All of these physiological events are compounded by fluid losses via higher sweat rates in the heat. Due to increased fluid losses, your blood volume decreases, which means your heart has to work even harder to send oxygen-rich blood to the working muscles. 

Metabolically, you use more carbohydrates when running in the heat. As a result of increased reliance on glycolysis for energy production, produce more metabolites (such as hydrogen ions), which lead to fatigue sooner.  

Additionally, your nervous system regulates your effort. When running in hot conditions, your muscles send more inhibitory feedback to your brain. Your central nervous system may even reduce muscle contraction as a protective mechanism. 

Your rate of perceived exertion (RPE) spikes as well. So whether you use heart rate or RPE to gauge your training intensity, you will notice that the same paces feel harder and correspond to higher heart rates when running in the heat. 

All of these physiological changes are due to the body’s need for thermoregulation. 

The Benefits of Running in the Heat

You may have heard the adage that “fall PRs are made in summer.” You have to train often through the worst of summer weather for your October or November race. During this time, your body acclimates to the heat and humidity, with changes to hemoglobin mass and blood volume occurring. When you reach your fall race, and the temperature drops, you now can experience a performance boost from more hemoglobin and blood volume – and without the heat stress of summer weather. 

Heat acclimation is all the buzz in the endurance sports world. Unless you are preparing for a strenuous race in the heat, you do not need to do anything special. By simply training outdoors in summer, your body will adapt to the increased heat stress. 

A 2019 study in Frontiers in Physiology examined adaptations in cyclists who did short, easy sessions consistently in the heat.  These cyclists had higher power outputs at lactate threshold and during time trials compared to the controls. The heat-trained cyclists also experienced increases in plasma volume and hemoglobin mass, which may have been the mechanisms behind their performance improvements. 

Notably, those changes all came from short, easy efforts in the heat. The cyclists in the 2019 study were not taking their hardest or longest sessions in very hot conditions. Possibly, doing every training session in the heat of the day could trigger overtraining or increase the athlete’s risk of heat stress. 

(This article provides an in-depth discussion of heat acclimation.) 

The Effects of Dehydration When Running in the Heat

Hydration status plays an integral role in heat tolerance during exercise. If you find that you really struggle with running in the heat, even when you slow down, you may need to pay more attention to your hydration status. 

Hydration is always a concern for runners; however, it can be more difficult to stay hydrated during summer training. Sweat rates typically increase in the heat. As a result, you typically lose more fluids to sweat in the summer than in other seasons. Due to the increased sweat loss, your hydration needs increase. 

In summer, small amounts of dehydration can accumulate over time. You may finish one session slightly dehydrated and then fully rehydrated after. As a result, you start the next session slightly dehydrated – and become more dehydrated as you go. 

Why is dehydration so detrimental? Dehydration impairs your body’s ability to regulate core temperature. When you experience dehydration as minor as ~2%, your body cannot dissipate heat as efficiently. Dehydration causes a decline in blood volume and an increase of plasma osmolarity, which means you cannot sweat effectively to cool down. Your core temperature increases, which causes your body to protectively lower your exercise ceiling. 

While heat will cause a performance decline no matter what, that decline compounds if you are dehydrated. Even 2% of bodyweight loss due to sweating can shorten your time to exhaustion. When you combine that with the lower cardiac output you experience in the heat, your running performance will really suffer. 

Training while dehydrated will not improve your heat adaptations. Your body cannot adapt to dehydration. There are no benefits to training dehydrated; your performance will decline and your risk of heat stress will increase. 

You may also be more prone to gastrointestinal distress when you are dehydrated. Dehydration slows down your gastric emptying rate by as much as 20-25%. This lowered rate of gastric emptying means that fuel and fluids will stay in your stomach and GI tract longer. If you feel like fluids are sloshing around your stomach on a hot day, that means you may actually be dehydrated and need to keep drinking. 

Hydration for Running in the Heat

Hydration needs are continual. When you are running in summer weather, you need to think about your hydration before, during, and after runs – and in fact, all day long. 

Hydration prior to exercise is very important. If you notice you cannot get enough fluids during a run, if you feel bad from the start, or if you have poor heat tolerance, you may be starting your runs dehydrated. While every runner’s hydration needs will vary, a general recommendation is 12-16 oz of water or sports drink within 2 hours of starting a run. 

Hydration needs during a run will also vary. Some runners sweat lightly, others are heavy sweaters. As outlined in a 2017 review in Sports Medicine, individual factors such as age, gender, heat acclimation, exercise duration, exercise intensity, and even medication usage can impact sweat rates. Sweat rates can range from 0.5 to 2.5 liters per hour – a five-fold difference. 

You can determine your individual sweat rate through a sweat test. Featherstone Nutrition has a calculator on her website that provides instructions for how to perform this test. For reliable results, do a sweat test in summer conditions. 

It is worth noting that, even if you conduct a sweat test, you will experience variability of sweat loss between sessions. A sweat test helps you calibrate your individual hydration needs, but you will still need to adjust based on your thirst during each session. Once you know your sweat rate, you generally aim to drink back one-half to two-thirds of your losses. So if you lose 20 ounces of sweat per hour, your hydration goal is approximately 10-15 ounces per hour.

Finally, hydrate after your run! Have 8-16 oz of sports drink or water once you finish a run. Then, focus on staying well hydrated throughout the rest of the day. 

How to Adjust for Running in the Heat

You can thermoregulate through behavioral changes – which helps prevent you from getting to the point in a run where you feel like you can barely keep moving due to the heat. 

Wear Light Colored Clothing

Dark-colored clothing absorbs heat – which is not what you want on a summer run. Light-colored hats, shirts, and sports bras will help you stay cooler on hot weather days. I even have a white hydration vest for summer long runs! 

If Possible, Run Early in the Day or Run in the Shade 

Solar radiation will make you feel warm, even in relatively mild temperatures. The sun beating down on you raises skin temperature. If you are able, run early in the day, late in the day, or along a shaded route. Less direct sun exposure will help you feel cooler on the run! 

Carry Fluids 

As outlined above, dehydration makes your body more susceptible to heat stress. You may be able to run for an hour in winter without drinking anything. However, you will benefit from carrying sports drink or water on summer runs. (If carrying plain water, consider using salt tablets to replace sodium lost in sweat.) 

A safe guideline is to bring water on any run lasting longer than 50-60 minutes. Pregnant runners, those on medications such as anticholinergics (including GI meds such as Bentyl) and antidepressants, and those with certain medical conditions may need to bring fluids on shorter runs. 

You can use handheld water bottles, hydration belts, or hydration vests to carry fluids. Choose what is most comfortable for you – and what allows you to bring enough for the duration of your run. 

Practice Cooling Techniques, Especially During Long or Hard Sessions

Here’s the cool thing (pun intended) about cooling techniques: so long as your primary working muscles (legs) are warm, you will gain heat adaptations. Cooling techniques that target the head, neck, and core regions will aid in thermoregulation without interfering with heat acclimation. 

A 2015 meta-analysis found that pre-cooling (drinking cold fluids before, using cold towels) and percooling (using ice during the run, dumping cold water on yourself) resulted in an overall 6.7% improvement in performance in the heat. These techniques are especially beneficial during long runs, hard workouts, and races in the heat. 

Focus on Effort, Not Pace

For all the reasons listed above, you will be slower at the same RPE or heart rate when running in the heat. Do not force paces in summer! Use effort for easy runs and hard workouts alike. You may find you even need to scale back the intensity of hard workouts; this article guides through how to scale workouts for the heat and humidity.

Can Running in the Heat Be Dangerous?

There is a point where running in the heat can be dangerous. There is no exact threshold. Your training status, exercise intensity, temperature, humidity, and sun exposure all contribute to heat-related illness. 

Signs and symptoms of heat-related illness include:

• Nausea and vomiting
• Confusion and altered mental state
• High heart rate
• Low blood pressure
• Lightheadedness and dizziness
• Unquenchable thirst
• Muscle twitching 
• Fainting
• Headache
• Hot skin and high body temperature

If you experience any of those, stop running! You may need to seek additional medical care depending on the severity of your symptoms (especially if you lose consciousness.)

If you run with a dog, remember that they are less tolerant of the heat than you are. The same applies to a child in a jogging stroller. If you run with your pet or child, be cautious on hot days! 

Additional References

Jeukendrup & Gleeson. (2016) Sport Nutrition, 3rd ed. Human Kinetics. 

PMID: 33829868