How Much Should We Drink During Exercise?

Although it is a factor that usually goes unnoticed with respect to others such as training planning or energy intake, avoiding dehydration should be a cornerstone supporting the other determinants of performance. While there is controversy regarding what levels of dehydration actually impair performance, dehydration in sport is usually defined as a loss of more than 2% of body weight. In fact, there is strong evidence that these levels of dehydration can negatively affect performance.

For example, a meta-analysis published in the journal Sports Medicine showed that dehydration (especially from a 2.8% body weight loss) reduces performance by 2,4% in a subsequent test, also affecting other physiological indicators such as maximal oxygen consumption or lactate threshold (1). (1).  In addition, as we discussed in a previous article, although we often associate dehydration with extreme temperatures and its negative effects act synergistically with hyperthermia, it does not need to be hot for dehydration to occur. For example, one study found that dehydration impaired performance even when body temperature was not elevated (2). Therefore, although we often forget about it, avoiding excessive dehydration is also important when it is not hot.

But how do you know how much to drink?

To avoid dehydration, it is likely that on some occasions it is enough to drink "ad libitum", that is, marked by the sensation of thirst. However, on many occasions it has been observed that when athletes drink only guided by their feeling of thirst, fluid intake may be insufficient, thus reaching a state of dehydration (3).  On the other hand, we know that excessive drinking can also have detrimental effects on health, and can lead to what is known as "exercise-associated hyponatremia". Therefore, knowing how much we sweat can be a good strategy to ensure that we replenish fluids correctly.

Variability

There is, however, a great variability between people when analyzing the sweat rate. For example, a study of more than 500 athletes showed that on average athletes sweated 1.21 liters per hour, although the variability was such that there were athletes who sweated 0.3 liters per hour and up to 2% of athletes sweated more than 3 liters per hour, some reaching up to 5.7 liters per hour (4). In fact, variables such as the duration of the exercise, the intensity, the environmental conditions, the clothes or how acclimatized we are can make the same person sweat more or less. But how do we know how much we sweat then?

How much do we sweat during a given exercise session?

To estimate how much we sweat in a specific exercise and specific conditions, we can rely on body weight loss, as Dr. Asker Jeukendrup explains in *MySportScience*. First, we should weigh ourselves before exercise with an empty bladder and preferably without clothes. Then, during training, we record the amount of fluid consumed, considering that each liter drunk is equivalent to one kilo. At the end, we weigh ourselves again after emptying the bladder. To calculate sweat loss, we subtract the final weight from the initial weight, add the fluid ingested and subtract the estimated amount of urine (0,3 liters for each time we urinate). This result can be expressed in litres per hour, thus calculating our sweat rate according to the duration of the activity.

Figure 1. Equation to know the sweating rate during exercise. Retrieved from MySportScience.

Thanks to this simple procedure, we can make simulations of a competition and estimate what our sweat rate is, so that we can individualize the hydration rate as accurately as possible. However, it is important to remember that, as we have mentioned, the sweat rate can vary even for the same person depending on numerous factors, especially environmental conditions, but also others such as the intensity of the exercise, the clothes we wear, or how acclimatized we are to that temperature. Therefore, we must be aware that there is a degree of imprecision, and that in case of doing this simulation we must do it with the conditions as close to the test as possible.

A study published in the *Journal of Applied Physiology*

Researchers developed a mathematical model to predict the sweat rate before a physical activity. By analysing 266 treadmill tests and 168 cycle ergometers under different conditions (15-34ºC), the researchers were able to predict the sweat rate using variables such as weight, height, temperature, wind, relative humidity, intensity and duration of exercise. The accuracy obtained was 0,01 liters per hour in cycling and 0,02 liters per hour in running, thus avoiding a weight loss of more than 2% in events of up to 3 hours, which would prevent dehydration. In addition, they created an online calculator that allows any athlete to estimate their sweat rate with this methodology. You can access the tool at [https://sweatratecalculator.com/hydration-calculator](https://sweatratecalculator.com/hydration-calculator) to perform the calculation.

Conclusions

Avoiding excessive dehydration during exercise is important both to preserve health and to avoid a drop in performance. To do this, and taking into account the high variability that exists in the sweat rate according to individual and environmental conditions, it is important to estimate for each athlete their sweat rate in the conditions in which they are going to exercise. In this article we have shown two methods to estimate the sweat rate that can be used to individualize the hydration process during competition in athletes. Of course, we must not forget that the feeling of thirst is often the most accurate indicator of the presence of dehydration, and should take precedence over any other pre-established planning.

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Author

Pedro Valenzuela
Researcher in the Physiology Unit of the University of Alcalá and in the Performance Control Unit at the Sports Medicine Center (AEPSAD, CAR of Madrid).
Web: www.fissac.com

References

1. Deshayes TA, Jeker D, Goulet EDB. Impact of Pre-exercise Hypohydration on Aerobic Exercise Performance, Peak Oxygen Consumption and Oxygen Consumption at Lactate Threshold: A Systematic Review with Meta-analysis. Sport Med [Internet]. 2020;50(3):581–96. Available from: https://doi.org/10.1007/s40279-019-01223-5
2. Nybo L, Jensen T, Nielsen B, González-Alonso J. Effects of marked hyperthermia with and without dehydration on VO2 kinetics during intense exercise. J Appl Physiol. 2001;90(3):1057–64.
3. Funnell MP, Juett LA, Ferrara R, Mears SA, James LJ. Ad-libitum fluid intake was insufficient to achieve euhydration 20 h after intermittent running in male team sports athletes. Physiol Behav [Internet]. 2023;268(April):114227. Available from: https://doi.org/10.1016/j.physbeh.2023.114227
4. Baker LB, Barnes KA, Anderson ML, Passe DH, Stofan JR. Normative data for regional sweat sodium concentration and whole-body sweating rate in athletes. J Sports Sci [Internet]. 2016;34(4):358–68. Available from: http://dx.doi.org/10.1080/02640414.2015.1055291
5. Jay O, Périard J, Hunt L, Ren H, Suh H, Gonzalez R, et al. Whole body sweat rate prediction: indoor treadmill and cycle ergometer exercise. J Appl Physiol. 2024;137(4):1014–20.