Hydration for Endurance Athletes Training in the Heat

By Ryan Eckert | May 07, 2018, 3:56 p.m. (ET)

hydration

As the weather begins to heat up across the U.S., managing hydration while exercising in the heat becomes important for triathletes. Regardless of whether you live in a humid or dry climate, the main challenge that triathletes face when the weather heats up is an increase in core body temperature. To understand the importance of hydration and how to properly hydrate, it is important to understand the physiology of exercising in the heat.

Physiology of Exercising in the Heat

It has been well documented that as air temperature increases, endurance performance decreases.3 When we exercise, we convert stored calories (fat and carbohydrates) into energy that is then translated into movement. However, humans are not 100 percent efficient in this conversion, leaving about 70-80 percent of this energy lost as heat production.4 This is the primary reason that we heat up while exercising. With every muscle contraction, we are also generating heat.

This production of heat is typically not a problem when exercising in cooler envrironments as the body is able to maintain a stable core temparature. However this accumulation of heat becomes more difficult for the body to manage when exercising in hot conditions. There are a few different mechanisms through which our body cools itself, however, the major mechanims we are all familiar with is sweating. Sweating works to dissipate heat from the body through evaporation. As our core temperature increases, blood flow to the skin increases, allowing sweat to form and, upon evaporation, heat releases from the body. This process of heat loss is magnified when exercising in hot weather, which is why we sweat more and are at a greater risk of dehydration.

During prolonged endurance exercise in the heat, the importance of maintaining adequate hydration becomes crucial as dehydration of as little as 2 percent has been shown to affect performance.3 Once dehydration begins to set in, heart rate and perceived exertion climb higher for the same given exercise intensity due to more blood flow being directed to the skin and away from the muscles. This impact on the body becomes nearly impossible to counteract. The following are signs of dehydration to be aware of when exercising in the heat.

Signs and symptoms of dehydration:

  • Dizziness, confusion, lightheadedness
  • Dry lips, mouth, skin
  • Physical and mental fatigue
  • Decreased pace and performance
  • Darkened urine (one of the first indicators)
  • Increased body temperature, heart rate and perceived effort

Although exercising in the heat imposes a huge demand on the body, there are ways to manage this challenge. Adequate hydration before, during and after exercise sessions is crucial for maintaining performance in the heat.

Monitoring Hydration Status

The first step in maintaining adequate hydration as a triathlete is to remain hydrated throughout the day. The easiest way of doing so is by checking the color of your urine, ideally first thing each morning. A pale yellow, or lemonade-like color, is indicative of adequate hydration. A darker, strong-smelling urine probably means that you are slightly dehydrated (see the chart below as a guide).1

hydration

Another useful test to perform is determining your sweat rate during exercise, which can be done by following the steps below:

1. Weigh yourself naked before exercise
2. Exercise for 1 hour and drink normally (only water) during exercise (avoid eating)
3. Weigh yourself naked again post-exercise
4. Calculate your sweat rate by subtracting your post-exercise weight from your pre-exercise weight and then adding back in the amount of fluids you consumed

This calculation gives you an estimated hourly sweat rate, which can be useful in determining the amount of  fluids to consume during exercise sessions. This method should be repeated as the seasons change in order to determine sweat rate in both hotter and cooler conditions.

Hydration Recommendations

It is difficult to provide recommendations for fluid intake throughout the day as everyone has different fluid needs based on sweat rate, exercise habits, body weight and environmental conditions.1 However, drinking enough water throughout the day so that your urine is a pale yellow color (see chart in the above section) is a good place to start. Remaining adequately hydrated throughout the day contributes to success in training sessions throughout the day. 

Providing fluid recommendations during exercise is also difficult as fluid needs are dependent on sweat rate and environmental conditions. The table below, however, provides some general guidelines that provide a great place to start. 

“It is also important to consider that sweat contains electrolytes: sodium, potassium, chloride and small amounts of minerals (iron, calcium and magnesium),” explains Registered Dietitian and USA Triathlon Certified Coach Susan Kitchen. “Of all these, sodium takes the biggest hit from sweat loss, thus it’s the most important to replace. Generally speaking, most athletes lose about 500 milligrams sodium per pound of sweat loss, and heavy sweaters can lose in excess of 1,000 mg sodium/hour. Heat acclimation prior to race day helps decrease the salt content of sweat; however, if you are heat acclimatized you actually sweat more, not less. So, it’s best to keep up with the increased loss of fluid.”

Note: It is very useful to know your sweat rate as well as your sweat-sodium concentration when personalizing your hydration plan. However, obtaining your sweat-sodium concentration is much more difficult than obtaining sweat rate and requires special equipment. If you are interested in finding your sweat-sodium concentration, reach out to Race Smart’s Susan Kitchen or look to see if a local registered dietitian offers this test. Knowing both your sweat rate and sodium needs can help in personalizing the recommendations below. 

Hydration Recommendations 1,2

Timing Hydration Strategy 
Pre-Exercise  - Drink sufficiently between exercise sessions and throughout each day so that urine is pale yellow 
During Exercise   
Sessions < 1 hour  - 24-30 oz of plain water
Sessions > 1 hour  - 24-30 oz/hour of plain water or carbohydrate/electrolyte drink (electrolyte/sodium intake should be based off of sodium sweat losses if known)
- Don’t rely on thirst as an indicator to drink fluids; aim to avoid the sensation of thirst in hot conditions
Post-Exercise  - 20-24 oz water or carbohydrate/electrolyte drink — depending on duration of training session — for each lb. of body weight lost during exercise

References
1. NSCA Trainer Tips. (2017). Hydration. Available at: https://www.nsca.com/uploadedFiles/NSCA/Resources/PDF/Education/Tools_and_Resources/TrainerTips_Hydration201601.pdf 
2. Dunford, M., & Doyle, J. A. (2011). Nutrition for sport and exercise. Cengage Learning.
3. Cheuvront, S. N., Kenefick, R. W., Montain, S. J., & Sawka, M. N. (2010). Mechanisms of aerobic performance impairment with heat stress and dehydration. Journal of Applied Physiology, 109(6), 1989-1995.
4. Powers, S. (2014). Exercise physiology: Theory and application to fitness and performance. McGraw-Hill Higher Education.


Ryan Eckert has written for a range of peer-reviewed academic journals and online magazines. He holds a master’s degree in exercise science from Arizona State and is a Certified Strength and Conditioning Specialist® (CSCS®) through the National Strength and Conditioning Association. He is the founder/owner of Peak Endurance Solutions, LLC, a coaching solution for endurance athletes.

The views expressed in this article are the opinion of the author and not necessarily the practices of USA Triathlon. Before starting any new diet or exercise program, you should check with your physician and/or coach.