written by Meg Garvey, PhD
Fluid needs during endurance exercise can be challenging to estimate as a wide range of factors - including weather, diet, exercise intensity, and physical readiness, among others - can change an individual’s sweat rate from day to day. Adding to the difficulty is the lack of practical field-based solutions for athletes to accurately measure their fluid requirements and account for the impacting factors. While some methods can provide an approximation for sweat losses and fluid needs, they are impractical for use in the field and, in some cases, are simply inaccurate. The consequences for failing to meet an individual’s fluid needs are high, with significant physical performance impairment resulting at even a 2% dehydration level. This paper explores current methods of hydration management and identifies their respective challenges.
Advice From Others
In 2017, almost 60 million people were involved in U.S.-based running, jogging, or walking events (1), including just under 510,000 finishers of U.S.-based marathons (2). Many of these individuals are self-coached, which increases the need for precise hydration information that is actionable to benefit their overall training, performance, and well-being. A 2013 study by Del Coso et al. showed that most runners reported hydration-related incidents during past races, yet very few used specific hydration plans for their next race (3). Most runners heavily relied on the advice of their peers when it came to hydration strategies (3). Of the athletes surveyed, 74% indicated that they do not use any method of hydration monitoring, despite 45% indicating that they had experienced performance issues due to improper hydration and that hydration was of utmost importance (3). While it has been found that 95% of college endurance athletes and 94% of college endurance-sport coaches agreed that ingesting fluids was necessary pre, during, and post-exercise; however, only 22% indicated the knowledge that electrolyte drinks were superior at rehydrating than water for exercise lasting longer than an hour (4), suggesting that appropriate hydration information is either not being adequately disseminated or implemented.
Even when science-backed hydration guidelines from reputable sources are disseminated, they often need to be more specific to the individual athlete. Many athletes rely on guidelines from expert sources, for example, the American College of Sports Medicine’s position statement (5). However, with significant variability in sweat rates, the American College of Sports Medicine (ACSM) has acknowledged that their guidelines - which recommend anywhere from 0.3 to 2.3 liters per hour - are not specific enough to keep athletes safe and adequately hydrated. The ACSM modified its recommendations to include quantifying sweat losses to create a proper hydration strategy during exercise (6).
Drinking to Thirst
Replenishing fluids ad libitum (or drinking to thirst) remains the most popular method of assessing fluid needs. However, this method of hydration management needs to be more accurate and poses significant risks. Seminal research has demonstrated that athletes drinking ad libitum (drinking to thirst) consume only half of the fluids lost during exercise in both hot and cool environments (7), and the ACSM advises athletes to avoid relying on thirst alone to gauge their fluid replacement needs (6). Cheuvront et al. determined that drinking to thirst contributed to excessive dehydration, defined as a body mass loss of more than 2% (8), by analyzing group means from 14 marathon studies over various athlete abilities and ambient temperatures.
High-performing athletes sometimes use urine tests, such as urine specific gravity (USG), to estimate hydration status and urine concentration to measure dehydration and electrolyte loss (9).Not only is this method inconvenient and cannot be implemented during activity, but it has also been shown to cause an overestimation of sweat loss by 16% to 37% in warm and cool environments, respectively (10).
The standard scientific method to assess fluid losses via sweat is by taking an accurate body mass in the nude before and after physical exertion while refraining from – or correcting for – any activity that imposes a change in body mass (urination, defecation, eating, and/or drinking). The measured change in body mass can be attributed to body water losses and divided by the duration of activity to achieve a sweat rate. While this is the gold standard for measuring body water losses, several independent factors impact sweat rate even for the same individual from one day to the next.
Real-Time Sweat Sensing
Personalized, real-time sweat testing is a valuable and actionable way to curtail the adverse effects of improper hydration while providing a convenient, accurate, and actionable solution during exercise. Utilizing a solution such as this longitudinally and across a range of environmental conditions and workout intensity levels can assist the athlete in understanding their personal thermoregulatory response to changes in a range of factors that impact sweat rate - essentially, in understanding their sweat profile. This personal database can be leveraged in specific hydration planning for a particular day in the future based on forecasted environmental factors, the type of activity, and the anticipated intensity level.
Various independent factors impact sweat rate. Relying on thirst alone will likely lead to misestimation of fluid needs and increase the probability of dehydration. This risk is significantly elevated for prolonged physical exertion, performed at a higher intensity, or served in warmer/more humid conditions. Weigh-ins and urine tests are inconvenient for use in the field; weigh-ins only capture today’s sweat rate, and urine tests can be quite inaccurate. While the consequences of improper hydration can be severe – including death – mild cases occur far more frequently and carry significant physical and cognitive performance impairments. Real-time personalized sweat sensing could be a powerful tool that empowers athletes to optimize their hydration in the field while elevating safety and performance.
- Knechtle, B., Di Gangi, S., Rüst, C. A., & Nikolaidis, P. T. (2020). Performance differences between the sexes in the Boston marathon from 1972 to 2017. The Journal of Strength & Conditioning Research, 34(2), 566-576.
- O'Neal, E. K., Wingo, J. E., Richardson, M. T., Leeper, J. D., Neggers, Y. H., & Bishop, P. A. (2011). Half-marathon and full-marathon runners' hydration practices and perceptions. Journal of Athletic Training, 46(6), 581-591.
- Del Coso, J., Salinero, J. J., Abián-Vicen, J., González-Millán, C., Garde, S., Vega, P., & Pérez-González, B. (2013). Influence of body mass loss and myoglobinuria on the development of muscle fatigue after a marathon in a warm environment. Applied physiology, nutrition, and metabolism, 38(3), 286-291.
- Baron, S., Courbebaisse, M., Lepicard, E. M., & Friedlander, G. (2015). Assessment of hydration status in a large population. British Journal of Nutrition, 113(1), 147-158.
- Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance. Med Sci Sports Exerc. 2016;48(3):543-68.
- ACSM's Health & Fitness Journal 8(3):p 2, May 2004.
- Greenleaf, J. E., & Sargent, F. (1965). Voluntary dehydration in man. Journal of Applied Physiology, 20(4), 719-724.
- Cheuvront SN, Montain SJ, Sawka MN. Fluid replacement and performance during the marathon. Sports Med. 2007;37:353–7.
- Osterberg, K, Horswill, C, Baker, L (2009). Pregame Urine Specific Gravity and Fluid Intake by National Basketball Association Players During Competition. Journal of Athletic Training, 44(1), 53-57.
- Baker, L. (2017). Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra-Interindividual Variability. Sports Medicine, 47, 111-12.