Summer is officially here across the US, meaning adequately hydrating is now a major challenge for all athletes, but particularly for endurance athletes. Endurance athletes demand a lot of their bodies, and the hours upon hours of intense exercise in the heat places a higher demand on our system than we would otherwise experience in cooler weather. Regardless of whether you live in a humid or dry climate, the main challenge that the body faces when the weather heats up is an increase in core body temperature. In order to truly understand the importance of hydration and how to do hydrate properly, it is important to first understand the physiology of exercising in hot environments. This Training Topic will focus on the physiology first and the practical applications of this knowledge second.
Physiology of Exercising in the Heat
In the research literature, it has been very well documented that as the ambient temperature of the air increases endurance performance decreases (3), and any endurance athlete that has exercised in both the cold and the heat can identify with this statement. When we exercise, we convert foodstuff (i.e., stored calories in the form of fat or carbohydrate) into energy that is then translated into mechanical movement. However, humans are not 100% efficient in the conversion of this readily available energy into movement, leaving about 70%-80% of this energy lost as heat production (4). This is the primary reason that we get hot while exercising, because with every muscle contraction, we are also generating heat.
This production of heat is normally not a problem while exercising in cooler envrironments as the body is easily able to maintain a stable core temprature through its cooling mechanisms. However, in the heat, this accumulation of heat becomes more and more difficult for the body to manage as now there is external heat that is imposiong an additional demand upon the body. There are a few different mechanisms through which our body cools itself, however, the main one that we are all familiar with is sweating. Sweating works to dissipate heat from the body through evaporation. As our core temperature increases, bloodflow to the skin increases, allowing sweat to form and, upon evaporation, heat release 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.
If you have exercised in both hot and cold environments, you will have likely noticed that your heart rate is higher in hotter conditions for the same given exercise intensity. Either you slow down your pace to get your heart rate under control, or your heart rate will likely be a bit higher than if you were exercising at that same intensity in cooler conditions. As more and more bloodflow is directed to the skin for the purposes of cooling, this means less bloodflow is being directed to our muscles. This can cause an almost immediate effect on your performance as now your perceived effort for any submaximal exercise intensity will be elevated.
During prolonged endurance exercise in the heat, the importance of maintaining adequate hydration becomes crucial as dehydration of as little as 2% has been shown to affect performance (3). Once dehydration begins to set in, heart rate can and perceived exertion can spike even further out of control as now the heart has to work on overdrive to both keep the body cool and to continue to support exercise performance.
Although exercising in the heat imposes a huge demand on the body, there are ways to manage this challenge. Adequate hydration not only during exercise, but before and after exercise sessions is crucial for remaining well hydrated and for lessening the effects of heat on performance. The next section provides a simple way of monitoring your hydration status as well as general recommendations for hydrating before, during, and after exercise sessions.
Monitoring Hydration Status
The first step in maintaining adequate hydration as an athlete is to remain hydrated throughout the day when you are not exercising. In order to do this, it is helpful to have a simple method of checking hydration status. 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, whereas a darker, strong-smelling urine probably means that you are slightly dehydrated. The following chart can be a useful guide when monitoring your hydration status (1).
Another useful strategy to aid in the maintenance adequate hydration status is to determine your sweat rate, which can be done by following the steps outlined below:
Weight yourself naked before exercise
Exercise for 1 hour and drink normally during exercise (avoid eating)
Weigh yourself naked again post-exercise
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 estiamted hourly sweat rate, which can be useful in determining how much 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.
It is incredibly 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 will put you in a good position to succeed during your exercise sessions. As described above, remaining hydrated, or at least avoiding huge losses of body water that lead to dehydration, will help in maintaining core body temperature and will lessesn the negative effects of heat stress on performance.
Providing fluid recommendations during exercise is just as difficult as providing fluid recommendations throughout the day as fluid needs are dependent on sweat rate and environmental conditions. The table below, however, provides some general guidelines which can be a great place to start.
Note: It is very useful to know your sweat rate (see method in the above section) as well as your sweat sodium concentration (the main electrolyte that is lost in sweat) 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, look to see if a local Registered Dietician or Sports Performance Center offers this test. Knowing both your sweat rate and sodium needs can help in personlizing the recommendations below.
NSCA Trainer Tips. (2017). Hydration. Available at: https://www.nsca.com/uploadedFiles/NSCA/Resources/PDF/Education/Tools_and_Resources/TrainerTips_Hydration201601.pdf
Dunford, M., & Doyle, J. A. (2011). Nutrition for sport and exercise. Cengage Learning.
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.
Powers, S. (2014). Exercise physiology: Theory and application to fitness and performance. McGraw-Hill Higher Education.