Hydration during exercise

Hydration During Exercise: Staying Hydrated and Performing Strong – a Guest Article by Juli Brüning (Part 2/3)

When we think about fueling during training and competition, our strategy should go beyond just energy intake and “carbohydrates per hour.” A proper hydration strategy is a key component to optimizing performance and minimizing health risks. Fluids are vital for all bodily functions and must be considered especially during physical activity, when we lose a significant amount of fluid through sweat. In addition to electrolyte loss, sweating during exertion mainly leads to a fluid deficit which, if left unaddressed, can quickly result in dehydration. Sweat loss is generally referred to as an individual sweat rate, typically expressed in milliliters (ml) or liters (L) per hour. How high these sweat rates are depends partly on how “sweaty” athletes are, and partly on external factors. Sweat rates can vary greatly among athletes and there is no one-size-fits-all solution.

Key influencing factors at a glance:

Exercise intensity (power, speed)
Ambient temperature, relative humidity, wind speed
Clothing
Heat acclimatization and training status
Genetic factors

Real-world examples:

Large-scale, high-quality studies confirm that sweat rates—regardless of sport—vary greatly from person to person. They show that, under the same conditions, rates can be as low as 0.3 L/h or as high as an extreme 2.5–3 L/h.¹

Figure 1: Individual differences in sweat rate among athletes. Adapted from Asker Jeukendrup mysportscience.

As a general rule:

the more intense the exertion,
the warmer the ambient temperature,
the less suitable the clothing (poor breathability),
the higher the sweat rate.
Additionally, higher humidity limits the body’s ability to cool itself, since only sweat that evaporates from the skin has a cooling effect. In humid conditions, the air is saturated with water vapor, making it difficult for sweat to evaporate—meaning the cooling effect is lost.

On the other hand, the following lead to lower sweat rates:

Cooler conditions,
Lower intensities, and
Stronger wind activity.

Depending on the start time and course profile of a trail or endurance race, conditions can vary (day/night, elevation, weather, intensity changes). Sweat loss and fluid intake should therefore not be seen as fixed numbers, but as a continuum.⁸ It’s advisable to adapt fluid intake flexibly to these dynamic conditions, rather than relying on a fixed amount.²

Optimal Hydration: Avoiding Both Over- and Underhydration

Risks and performance loss due to underhydration

Fatigue at the end of a long race is generally associated with carbohydrate depletion, but it can equally be caused by dehydration. Multiple studies have shown that athletic performance is impaired when an athlete is just 2% dehydrated relative to body weight. A fluid loss of more than 5% can reduce performance capacity by approximately 30%.³ This effect is amplified in the heat. Dehydration can also cause a feeling of “hitting the wall” and significantly reduce well-being.^{3, 4, 5, 6}

Dehydration impairs performance by:

Reducing blood volume (less oxygen/nutrients for muscles)
Impairing cooling mechanisms (reduced skin blood flow & sweat production)
Increasing body temperature (risk of overheating)
Increasing carbohydrate usage (faster fatigue)
Raising the risk of heat-related issues (heat stroke, heat exhaustion, gastrointestinal problems)

Dangers of overhydration

Too little fluid is risky—but too much can be dangerous as well. Excessive intake can lead to exercise-induced hyponatremia—a life-threatening condition where sodium levels in the blood drop too low. In severe cases, cerebral edema, coma, or even death may result (unfortunately, there have been such cases in ultra-endurance sports). 7, 8, 9

In some cases, excessive fluid intake may be almost more dangerous than mild underhydration—so what’s the optimum?

How 'sweaty' are you?

A rough, subjective assessment of your sweating behavior (light, moderate, heavy) can serve as an initial orientation. However, for an individual hydration strategy, a more precise measurement of sweat rate is essential. This method is simple, can be done by yourself, and repeated under various conditions. All you need is a body scale, a towel, and a basic understanding of math to use the results.

Instructions for measuring sweat rate:

Go to the bathroom and weigh yourself before the session: Ideally naked – or account for the clothing weight separately.
Do your workout: 60 minutes, no longer than 90 minutes to avoid measurement errors caused by depleted glycogen stores.
Dry off after the session and weigh yourself again
Weight loss in grams / kg equals sweat loss in ml / L
If you drink fluids during the session: Weigh yourself with the bottle/soft flask before and after to include this amount in the calculation.

Example:

Runner, 17°C, mild conditions
Weight before: 70 kg
Run session: 60 min
Weight after: 69.25 kg
Fluid intake: None
Body weight loss: - 0.75 kg
Sweat rate per hour: 0.75 liters/h

Since sweat rates can vary greatly depending on conditions, it's a good idea to perform measurements under different intensities and weather conditions. For races, the conditions should be mimicked as closely as possible.

How much dehydration is tolerable?

For a long time, it was believed that sweat losses during exercise had to be replaced 1:1, and this was the official guideline in the American College of Sports Medicine position paper until 2007.¹⁰ Today, sports nutrition science understands that a certain level of dehydration is tolerable before performance begins to suffer due to fluid loss. This level is somewhat individual. However, based on a significant number of studies and practical examples, a guideline of 2-4% of body weight has emerged, which applies to most athletes. Since 2007, the revised ACSM recommendation has been to consume fluids during exercise in an amount that prevents dehydration of more than 2% of body weight.¹¹

Let’s take the runner from the earlier example:

Runner, 24°C, warm conditions
Weight before: 70 kg
Run session: 60 min
Weight after: 68.2 kg
Fluid intake: None
Body weight loss: - 1.8 kg
Sweat rate per hour: 1.8 liters/h

With a sweat loss of 1.8 liters per hour, an athlete would lose 6.4% of their body weight during a 2.5-hour race without fluid intake. Since dehydration of more than 2–4% should be avoided, this becomes particularly risky in longer events where fluid deficits can accumulate over several hours (or in multi-stage races, where dehydration can build up over days).

The sweet spot: Drinking to thirst vs. following a plan

In everyday life and during shorter, less intense sessions or races, drinking according to thirst is often sufficient. It's important to listen to your body and manage fluid intake accordingly – even if hydration isn’t perfect, your fluid and electrolyte balance will usually regulate itself within a few hours after the session. Provided that you start the session well hydrated.

For longer sessions or races in more demanding conditions, relying on thirst alone is not enough, as the sensation of thirst only kicks in around 2% dehydration. A proactive approach is needed to offset fluid losses!

Those training or racing over longer distances – especially in heat or high humidity – should absolutely have a well-thought-out hydration strategy based on their sweat sodium concentration and sweat rate.

You’ll find out exactly how to implement this strategy in the upcoming blog on the optimal hydration strategy for training and racing. (Part 3, coming in the next few weeks).

Conclusion:

Proper fluid intake is complex and even a challenge for pros, as shown in Tom Evans’ highly recommended 2024 UTMB debrief. In general, relying solely on thirst can be misleading, as it often sets in when the body is already dehydrated. On the other hand, it’s a myth that “more is always better.” Excessive fluid intake can lead to dangerous hyponatremia. Measuring sweat rate helps provide clearer guidance and supports more informed decisions.

¹ Barnes KA, Anderson ML, Stofan JR, Dalrymple KJ, Reimel AJ, Roberts TJ, Randell RK, Ungaro CT, Baker LB. Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport. J Sports Sci. 2019 Oct;37(20):2356
2366.

² Does dehydration impair exercise performance? Sawka MN, Noakes TD. Med Sci Sports Exerc. 2007 Aug;39(8):1209-17. doi: 10.1249/mss.0b013e318124a664.

³ Pichan G, Gauttam RK, Tomar OS, Bajaj AC. Effect of primary hypohydration on physical work capacity. Int J Biometeorol. 1988 Sep;32(3):176-80. doi: 10.1007/BF01045276. PMID: 3198291.

⁴ Sawka, M.N. and Pandolf, K.B. (1990) Effects of Body Water Loss on Physiological Function and Exercise Performance. In: Gisolfi, C.V. and Lamb, D.R., Eds., Fluid Homeostasis during Exercise, Benchmark Press, Carmel, 1-38.

⁵ Michael N. Sawka, Scott J. Montain, William A. Latzka, Hydration effects on thermoregulation and performance in the heat, Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, Volume 128, Issue 4,2001,Pages 679-690,ISSN 1095-6433.

⁶ Armstrong LE, Costill DL, Fink WJ. Influence of diuretic-induced dehydration on competitive running performance. Med Sci Sports Exerc. 1985 Aug;17(4):456-61. doi: 10.1249/00005768-198508000-00009. PMID: 4033401.

⁷ Hiller WD. Dehydration and hyponatremia during triathlons. Med Sci Sports Exerc. 1989;21(5 Suppl):S219–21.

⁸ Lewis D, Blow A, Tye J, et al. Considering exercise-associated hyponatraemia as a continuum. Case Reports 2018;2018:bcr-2017-222916.

⁹ Hew-Butler T, Loi V, Pani A, Rosner MH. Exercise-Associated Hyponatremia: 2017 Update. Front Med (Lausanne). 2017 Mar 3;4:21. doi: 10.3389/fmed.2017.00021.

¹⁰ Convertino VA, Armstrong LE, Coyle EF, Mack GW, Sawka MN, Senay LC Jr, Sherman WM. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 1996 Jan;28(1):i-vii. doi: 10.1097/00005768-199610000-00045. PMID: 9303999.

¹¹ American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007 Feb;39(2):377-90. doi: 10.1249/mss.0b013e31802ca597. PMID: 17277604.

¹² Berning, Jacqueline. (2008). The Encyclopaedia of Sports Medicine: An IOC Medical Commission Publication, Volume 19.
10.1002/9780470693766.ch33.

¹³ Jeukendrup, A. (JAHR). How much do you sweat and how much sodium do you lose? [Abb. 1]. Mysportsscience. https://www.mysportscience.com/post/how-much-do-you-sweat (zuletzt abgerufen am: 12. 03.2025).

Abbildungsverzeichnis:

Cover Picture: Precision Hydration Ltd. | www.precisionhydration.com.

Figure 1: Individuelle Unterschiede in Schweißrate bei Athlet*innen.

Juli Brüning

Juli holds a Master’s degree in Nutritional Science, is a performance nutritionist, and an ultra trail runner for the Craft Elite Run Team Germany. She supports athletes in all sports and nutrition science-related areas and works as a Sweat Test Center and athlete support for Precision Fuel and Hydration. Her work focuses on metabolic processes in nutrition physiology and female-specific physiology.