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Heat index and Hydration Strategies for Athletes

July 25, 2019
Image of marsh in Charleston

By Ethan Konoza SCAT, ATC, PES
Athletic Trainer

Living in Charleston, SC we are all familiar with how hot and humid it can get. Climate is an important factor to consider when discussing nutritional and hydration needs of athletes. As heat index increases, sweat rate would also increase as a part of normal thermoregulatory processes. Evaporation is the predominant heat-loss mechanism during exercise in warm conditions. To combat the increased sweat rate due to increased heat index, athletes would have to take in more fluids to maintain euhydration status (normal state of hydration) and ultimately performance. Athletes and coaches need to consider the climate in which exercise is being done as well as the mode, duration, and intensity. Monitoring weight pre and post exercise is needed to calculate sweat rate. Monitoring this will help athletes and coaches follow appropriate fluid replacement guidelines and recommendations to ensure safety as well as optimal performance. NATA’s position stand on fluid replacement for the physically active can be used to calculate sweat rate; sweat loss must be calculated first:

Sweat loss = body mass before exercise (kg) – body mass after exercise (kg) + (volume of fluid consumed during exercise) – (urine volume, if any [L])

From the previous equation’s results we can than find sweat rate:

Sweat rate (Liters/hour) = sweat loss (L) / exercise duration (h)

Prior to exercise it is recommended that athletes start exercise bouts well hydrated and in a state of euhydration. Athletes should drink 16 to 20 fluid ounces of water within 2 to 3 hours prior to exercise; 10 to 20 minutes prior to exercise it is recommended athletes consume another 7 to 10 fluid ounces of water. During exercise, the goal for athletes is to maintain their hydration status and not allow more than 2% of body mass loss, which in general requires 7 to 10 fluid ounces every 10 to 20 minutes. It has been shown that exercise performance is impaired when a person becomes dehydrated by as little as 2% of body weight and losses of 5% of body weight can decrease the capacity for work by about 30%. Athletes should be careful to not gain weight from fluid during exercise but rather try and maintain a status of euhydration during activity. It is suggested that individuals know their personal sweat rate (see equations above) to develop hydration strategy that is based on their individual needs.

Rehydration post exercise should focus on rapidly replacing fluids to restore euhydration status. This can be monitored through pre and post exercise weight and will allow athletes to determine hydration status prior to their next session of exercise. This improves recovery, reduced hypohydration symptoms, and decreases post exercise fatigue. Additionally, it is recommended that up to 150% of the estimated fluid deficit be consumed post exercise to effectively replace fluid losses over a shorter recovery period (>4 hours). This will be particularly important for many as often times athletes will be partaking in multiple training sessions a day. This increased consumption of fluids is used to compensate for the post exercise diuresis induced by the large fluid load that will ultimately affect hydration status. Fluids lost during exercise are reflected in pre and post activity weight. Any decrease of weight due to fluid loss should be replaced prior to the next activity with combination of normal diet and special consideration to fluid intake.

References

Armstrong, L. E. (2000). Performing in extreme environments. Champaign, IL: Human Kinetics.

Jeukendrup, A., & Gleeson, M. (2010). Sport Nutrition: an introduction to energy production and performance (2nd ed.). Champagne, IL: Human Kinetics.

Kenny, G. P. & Jay, O. (2013). Thermometry, calorimetry, and mean body temperature during heat stress. Comprehensive Physiology, 3(4), 1689-1719.

Maughan, R. J. & Shirreffs, S. M. (2010). Development of hydration strategies to optimize performance for athletes in high-intensity sports and in sports with repeated intense efforts. Scandinavian Journal of Medicine & Science in Sports, 2, 59-69.

McDermott, B. P., Anderson, S. A., Armstrong L. E., Casa, D. J., Cheuvront, S. N., Cooper, L. . . . Roberts, W. O. (2017). National athletic trainers’ association position statement: fluid replacement. The Journal of Athletic training, 52(9), 877-895.

Muhamed, A. M. C., Atkins, K., Stannard, S. R., Mundel, T., & Thompson, M. W. (2016). The effects of a systematic increase in relative humidity on thermoregulatory and circulatory responses during prolonged running exercise in the heat. Temperature (Austin, Tex.), 3(3), 455-464.

Roberts, W. O. (2012). International federation of sports medicine position statement: fluid replacements for sport safety and performance. International Sports Medicine Journal, 13(2), 39-42.

Sawka, M. N., Burke, L. M., Eichner, E. R., Maughan, R. J., Montain, S. J., Stachenfeld, N. S. (2007). American college of sports medicine position stand: exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377-390.