Trainright Guide to Exercise and Performance in Hot Weather

No one is immune to the detrimental effects of exercising in hot weather. Some athletes respond better than others, but high ambient temperatures and high core temperatures diminish endurance performance for everyone. The good news is that everyone competes in the same conditions, and your response to high temperatures is trainable. The first step to improving your performance in the heat is to PREPARE with the following hydration and thermoregulation strategies.

Quick Facts:

• Assess day-to-day hydration with WUT protocol (Weight, Urine, Thirst) upon waking. If two or more indices point to dehydration, take corrective action.
• Increase intake of fluids and high water content foods
• Acclimatization takes up to 14 days.
• Acclimation activities like passive heat exposure and low-intensity training in the heat can be effective preparations when done prior to traveling to hot environments.
• Pre-cooling with ice slurry drinks, an ice vest, and/or cold towels can improve performance by enabling you to start with lower skin and core temperatures. Ice slurry drink is the most practical method.
• Hyper-hydration or preload hydration products should be reserved for extreme conditions and/or very long and strenuous workouts and events.
• Interval workouts should be scheduled at cooler times of day. It can be advantageous to conduct some moderate-intensity endurance workouts during the heat of the day to aid in heat acclimation.

Day-to-Day Hydration

Monitoring day-to-day hydration is important year-round, but never more important than during hot weather or during the transition from cooler weather to hot weather. When you wake in the morning, there are three indices you need to look at: Weight, Urine, and Thirst.

FIGURE 2- ADAPTED FROM CHEUVRONT AND SAWKA 2005.ILLUSTRATION- ABBY HALL

The WUT concept, devised by researchers Cheuvront and Sawka for hydration status assessment, states that when only one of three indices is positive your hydration status is likely good. When two of the three indices are positive, you are likely dehydrated. And when all three indices are positive you are very likely to be dehydrated. The method is easy. When you wake up, assess your level of thirst. Then observe the color of your urine. After you have urinated, weigh yourself without clothing.

Your morning urine should be light in color or relatively clear. The tipping point for the WUT assessment is if your urine is approximately the color of apple juice or darker. Be aware, however, that clear urine does not necessarily mean you’re in the clear (bad pun intended). If you guzzled a full bottle of water before bed, your morning urine may be light in color because you consumed so much water in a short period of time.  Ideally, you also need to evaluate the color of your urine stream, not the diluted urine in the toilet.

Your weight should remain relatively constant from day to day, varying about 1%. Even if you are gradually losing weight as part of a weight management goal, the changes from one day to the next won’t vary more than 1%.

Why WUT Matters

The WUT assessment is important because it can indicate whether your overall daily fluid intake is adequate to replenish all water losses. You are constantly losing fluid, from respiration to keeping yourself cool in a warm office environment. Exercise fluid losses simply add to this daily water loss. In many cases, athletes start workouts already 2% down in bodyweight due to mild dehydration, meaning that a further 2% loss during exercise actually puts you at a 4% deficit during the latter part of your workout or race. Starting your day with a better hydration status means you are optimally prepared for a better and more productive training session or competition.

What to do if the WUT Assessment Indicates Dehydration

When athletes perform the WUT Assessment and it indicates dehydration is likely or very likely, the common response is to guzzle a relatively large volume of water. The problem with this is that it doesn’t address the underlying cause of the problem. The WUT Assessment essentially tells you how well you met your hydration needs over the preceding 24 hours; guzzling fluids overloads your gut and much of the fluid you consume passes right through you and results in a higher urine volume.

A more comprehensive approach to fixing the problem is to increase your fluid consumption across the whole day. The following tips accomplish this goal in a gradual and balanced manner:

• Consume 16-20 ounces of water immediately upon waking.
• Consume 8-20 ounces of water with each meal during the day.
• Carry a water bottle. Out of sight is out of mind, so carrying a water bottle reinforces the imperative to continue consuming fluids throughout the day. It also helps distribute your water intake across the entire day, rather than only focusing it on consuming relatively high volumes of water a few times per day.
• Increase consumption of fruits and vegetables. While there is some variation in the water content of various fruits and vegetables, nearly all of them are at least 80% water by weight. Food typically makes up about 20% of the fluid you consume during the day, and including more fresh fruits and vegetables can help to both boost your water intake and displace low-water content foods. Some of the highest water-content choices include watermelon, strawberries, Romaine lettuce, cantaloupe, tomatoes, pineapple, and bell peppers. Consider cold soups and smoothies.

Pre-Exercise Hydration

When your day-to-day hydration status is good, you have a better chance of having a great workout or event performance. The next step is to consume 16-20 ounces (a regular sized cycling water bottle is typically about 20 ounces) of cold fluid (water or electrolyte-rich sports drink) in the hour prior to exercising in the heat.

This pre-workout bottle serves three purposes:

1. Fluid intake. This is the obvious one.
2. Spares the fluid you’re carrying. Although it’s a good idea to consume fluids very early in workouts, in the real world a lot of athletes wait at least 20 minutes before reaching for a bottle or hydration tube. Cyclists carrying water bottles automatically start rationing fluid to make it last longer. Consuming a bottle before you leave gets water on board, so to speak, for this early period of your workout.
3. Pre-cooling. We’ll cover this in more detail later in this article, but consuming a cold beverage or slushy drink before exercise in the heat can help blunt the increase in core temperature and help you do more high-quality work before being affected by a high core temperature.

What About Hyper-Hydration or Pre-Load Hydration Products?

Hydration products marketed as hyper-hydrators are designed to increase the amount of water your body stores, and they do so by supplying a huge amount of sodium. The idea is that by starting hot-weather exercise with greater body water, you have a bigger reservoir to absorb heat from working muscles, move it to the skin, and dissipate it as sweat. As a result your core temperature doesn’t start rising as early, and rises more slowly during the first hour of exercise.

Keep in mind, these products are not recommended for everyday use. They contain extremely large amounts of sodium (3.5 grams/liter, or 1700mg/500ml bottle). You don’t need these drinks for short or moderate-intensity workouts. They are purpose-specific drinks designed to for athletes training or competing in extreme conditions or athletes who consistently struggle with significant weight loss (5+%) during exercise, are always craving salt, and finish workouts crusty with salt on their skin and clothing.

If you consume them when you don’t need them, the high sodium content can lead to nausea or elevated blood pressure (if you already have blood pressure issues). When might you use one? Perhaps before a long and/or very intense workout or race in abnormally hot weather. As always, try it in training before utilizing it in competition.

Thermoregulation and Acclimation

The human body has an amazing ability to maintain core temperature within a very tight range (97-100 degrees Fahrenheit) despite wild fluctuations in environmental temperatures. When exposed to a hot environment for a prolonged period of time, and especially when exercising in hot and humid environments, your body needs to become more efficient at dissipating heat. As a result, you:

• Start sweating sooner: To stay ahead of rising core temperature, your body kicks your primary cooling system (sweat) into action earlier than when not heat acclimated.
• Sweat more profusely: Your body learns to open the floodgates to get more fluid onto the skin surface for evaporative cooling.
• Sweat more evenly: You have sweat glands all over, and your body needs to increase evaporative cooling you’ll start sweating from everywhere.
• Change the composition of your sweat: The electrolyte content of your sweat decreases as your body tries to pump out more fluid but retain minerals needed for the nervous system and other critical body functions.
• Increase plasma volume: Your blood is what’s transporting heat from your core to your skin for radiant cooling as well as evaporative cooling. Increased plasma volume increases your capacity for heat transfer, and provides fluid for sweat.

Your heart rate response to exercise also changes in hot environments. Athletes experience higher exercise heart rates at a given workload, compared to cooler environments. This increase is much higher for athletes who are not acclimated to the heat, and as you acclimate your exercise heart rates will return to normal. Both acclimated and non-acclimated athletes also experience ‘cardiac drift’ as an acute response to increased core temperature, in order to facilitate heat transfer from your core to your skin/extremities where it can be dissipated through radiation, convection, conduction, or evaporation.

How Long Does It Take an Athlete to Acclimatize to Heat?

The process starts in the first few days you’re exposed to increased temperatures, as long as you’re exposed to the heat for at least an hour each day over the course of consecutive days. It takes between 10-14 days to be completely acclimated to the increased temperatures. This is why some athletes who travel to warm climates suffer initially. Acclimation takes time, it’s difficult to accelerate the process, and the process is hindered by dehydration and/or exercise at high-intensity levels – which are pretty much impossible to avoid while racing through the desert.

It’s also important to note that some adaptation to heat is better than none. The figure below from Periard et al (2015) shows that the sweet spot for adaptation is about 5-9 days. This is important from a practical standpoint, as you always have to balance the training compromises (lower intensities) that come during proactively adapting to heat.

Adapted from Périard et al 2015

What’s the Difference Between Acclimation and Acclimatization?

Heat acclimatization is a passive process that results from exposure to hot environmental conditions. In other words, acclimatization occurs as you’re going about your normal daily activities. Acclimation is an active process of ‘heat training’, where your activities are designed to improve your performance in hot environments.

How Does Heat Acclimation Work?

The training athletes do when they first arrive in hot environments is an example of acclimation. Exercising in the heat 60-90 minutes a day for 5-10 days will initiate the physiological changes necessary to perform optimally in the heat. A reduction in intensity and volume is necessary during this time. Reduce your intensity by 60-70% during the first 3-4 days (ride easy for 60-90min), and then gradually increase the intensity and duration of your workouts over the next 3-5 days. Not only does this give your body time to modify your sweat response, but it also gives you time to adapt to consuming and processing an increased amount of fluid. Be sure to drink plenty of fluids and allow your body time to adapt, and you’ll soon be able to increase your exercise intensity in the heat.

Athletes can also perform heat acclimation training prior to traveling to a hot environment. The US Olympic Training Center in Colorado Springs, CO is home to the High Altitude Training Center, a sealed room that can set up to replicate the elevation, temperature, and humidity of a wide range of competitive environments, like the sea level oxygen level and high heat and humidity of Rio de Janeiro. While living at 6000 feet at the OTC, athletes could train in the conditions they would experience at the Olympic Games. Athletes who don’t have access to the HATC, including ultrarunners preparing for the Badwater 135, use simpler methods of acclimatization training include wearing additional layers of clothing, sitting passively in hot sauna (heat exposure) or training in a hot room.

Additional heat acclimation resources are available here:

• Heat Acclimation: When to start and how long it takes
• Heat Acclimation Cheat Sheet

Pre-Cooling

The goal of pre-cooling is to blunt the rapid increase in core and skin temperature associated with exercise in the heat. It will not prevent you from reaching high skin or core temperatures during a long workout or event, but it will give you more time to perform high-quality work before core temperature hurts your performance.

Lots of pre-cooling methods have been studied, and most are effective but impractical. You can immerse your body in cold water or take a prolonged cold shower, but that’s inconvenient at home and not feasible at most events. Cold air exposure (much colder than normal air conditioning) can be effective, but it requires a long time to work.

A cooling vest is the most convenient method for pre-cooling, and if you are regularly competing in hot temperatures it may be worth the investment. Wear the vest for 30-60 minutes leading up to your event, including during your warmup. If you don’t have a cooling vest, you can use cold towels. For best results, you want to cool a large portion of the body, which is why the vests are more effective than cooling just hands, arms, or your neck. However, in a pinch, an ice collar or ice sock around the back of your neck is better than nothing.

Another pre-cooling technique is the ingestion of cold liquid or an ice slurry. If you have a choice, go with an ice slurry drink before hot weather training or competition, but cold water during exercise. There are some additive benefits to combining external and internal pre-cooling, so if you can do both, do it.

Thermoregulation for Specific Training

While some pre-cooling is easy (ice slurry drink), an ice vest, cold towel, or ice sock is probably not practical for most athletes on a day-to-day basis. Prior to a goal event, however, pre-cooling is worth the investment of time and effort. The most important time to pre-cool is prior to high-intensity work, meaning before interval workouts or short and explosive competitions.

Starting with lower core and skin temperatures increases your heat storage capacity (you store heat internally even when you sweat), meaning you can accumulate more heat in your body before reaching a temperature that hinders performance. This means a higher power output, faster pace, greater work capacity, and lower thermal strain during the period before you reach high core temperature.

The simplest way to stay cooler during exercise is to schedule training during the morning, before the heat of the day. However, this is not always possible for people with busy schedules. Interval workouts are most important workouts to schedule at cooler times of the day because achieving your goal intensities is crucial to the quality of the training session. Moderate-intensity endurance training can, and sometimes should, be scheduled during hotter portions of the day. These training sessions will not be as detrimentally affected by heat because the intensity is lower, and the exposure to heat will aid in acclimatization.

Getting the Best Bang for Your Buck

There are always more things you could do than things you actually can or will do. And that’s OK, because there are always compromises when you add more complexity to training. It’s best to keep it simple by choosing a few modalities you can achieve easily and consistently. For time-crunched athletes who have jobs, families, and limited training time, the best bang for your buck will likely come from:

• Increasing daily fluid intake
• Consuming cold water or an ice slurry beverage before exercise in the heat.
• Scheduling interval workouts in cooler portions of the day (mornings)
• Completing moderate intensity endurance workouts in the heat of the day.
• Scheduling recovery activities in cooler portions of the day
• Schedule specific heat acclimation activities 2-3 weeks before traveling to hot weather environment.