The role of water in the body
Body water is critical for many bodily processes such as regulating blood volume and blood pressure, and transporting oxygen and nutrients, (see below for more information).
Another key role is regulating body core temperature, which is important for athletes because during exercise, the working muscles generate heat which causes a rise in body temperature. In response, sweat production occurs and the evaporation of sweat from the surface of the skin cools the body.
This bodily process prevents a sharp rise in body core temperature and in turn, reduces the risk of heat illness and any associated heat-related impairments in exercise performance.
What is fluid balance?
Sweat losses during exercise vary greatly between and within individual due to factors such as:
Daily fluid requirements
Hydration is typically maintained through habitual eating and drinking habits. Both the European Food Safety Authority (EFSA) and the USA’s Institute of Medicine (IOM) have published daily water intake recommendations for females, click below for details:
9 – 13
years
EFSA
1.9 L/day
~64 oz/day
IOM
2.1 L/day
~71 oz/day
14 – 18
years
EFSA
2.0 L/day
~68 oz/day
IOM
2.3 L/day
~78 oz/day
19+
years
EFSA
2.0 L/day
~68 oz/day
IOM
2.7 L/day
~91 oz/day
In the diet, water is predominantly obtained through consumption of drinking water/beverages (80%).
Some water (20%) is also obtained through the consumption of water-containing foods.
Types of common beverages
Important considerations for athletes
Daily fluid requirements may vary day-to-day depending on a variety of factors including how much the athlete sweats during exercise, daily life demands, and environmental conditions. Importantly for athletes, exercise type and intensity can alter fluid needs. Therefore, it is recommended to:
Daily hydration monitoring
Indicators of
hydration status
Take daily body mass measurements (ideally nude) at the same time of day on consecutive days, and ideally before any activity or consumption of food/drinks. If daily body mass loss is > 1%, compared to baseline euhydrated values, it may indicate a likelihood of dehydration.
A cost/benefit analysis should be performed before introducing daily measures of body mass. If implemented, female athletes should be educated on the reasons behind why this measurement is being taken.
Assessing urine volume and color offers a practical and cost-effective method to estimate hydration status. Reduced urine output and/or a darker urine color (indicative of higher concentration) can indicate dehydration. The urine color scale can help to categorize hydration status.
Thirst signals the body’s need for fluids and can be an initial sign of dehydration. However, the absence of thirst doesn’t guarantee the absence of dehydration. Menstrual cycle phase may alter thirst perception, during the luteal phase the rise in estrogen can decrease the osmotic threshold for thirst. The thirst chart below can be used with athletes as an indicator of their thirst status.
Integration methods for monitoring daily hydration status
Body mass (weight), urine and thirst measures in isolation cannot definitively prove if an athlete is hydrated or dehydrated. The weight, urine colour and thirst (WUT) venn diagram provides a practical hydration assessment tool. The presence of any two indicators suggests a potential for dehydration, while the presence of all three strongly indicates the likelihood of dehydration. Consistent tracking is needed to provide a more accurate picture of hydration status.
Pre-exercise
hydration recommendations
Athletes should begin any type of physical activity in a hydrated state, while also ensuring that they do not feel bloated, or the need to urinate frequently.
It is recommended that athletes consume 5-7 milliliters (mL) of fluid per kilogram (kg) of body mass (mL/kg), 4 hours before the start of exercise. If no urine is produced, or if urine is dark in color, advise athletes to drink an additional 3-5 mL/kg in the 2 hours before exercise starts.
A beverage containing electrolytes (i.e., a sports drink) will help the body to retain the fluids consumed.
During exercise
hydration
recommendations
Acute changes in body mass over the course of exercise can be used to measure fluid loss, and in turn the rate of sweat loss per hour (i.e., sweat rate) can be calculated. This information will then inform fluid recommendations during exercise to prevent significant dehydration (> 2% body mass losses). Follow the steps below to calculate sweat rate.
Post-exercise
hydration recommendations
Rehydration is an important part of the post-exercise recovery process. The aim is for athletes to replace any fluid and electrolyte losses that occurred during exercise, before the next exercise bout begins.
In most situations, water and sodium can be replaced with normal eating and drinking practices. Drinking a beverage with sodium, or eating sodium containing foods, will help replace sweat sodium losses, stimulate thirst, and retain the ingested fluids. Athletes are also advised to sip, and not chug, fluids. The replacement of both fluid and sodium should be combined with other recovery priorities (see 
for more information).
If dehydration is severe (> 5% body mass loses), or if rapid rehydration is needed (i.e., < 24 h before next exercise bout), the recommendation is for athletes to drink 1.2 - 1.5 L of fluid for each 1 kg of body mass loss.
Hydration considerations
Due to the large variability in sweat rate between and within athletes, it is advised to measure sweat rates in different conditions to inform personalized fluid recommendations. Having personalized hydration plans will help prevent dehydration. However, it is always important for athletes and practitioners to be able to recognize the signs and symptoms of dehydration.
Recognizing dehydration
To increase fluid intake during exercise, schedule regular fluid breaks (if possible),
especially in sessions that are > 60 minutes, of high-intensity, and/or in hot and humid conditions.
High sweat sodium concentrations
When sweating occurs, electrolytes are also lost from the body. One electrolyte of interest is sodium because the concentration of sodium in sweat also varies between individuals. Athletes with high sweat sodium concentrations can often be identified if white salt stains appear on clothing during exercise, or they can be identified through measuring sweat sodium concentrations, however this requires specialist equipment. For athletes with high sweat sodium concentrations, consuming a beverage that contains sodium (such as a sports drink) will help to replace the sodium lost in sweat and it will also stimulate drinking. Sports drinks also contain carbohydrates which can help the uptake of fluid in the gut, and support physical and cognitive performance.
The effect of both sex and menstrual cycle phase on sweating rate and composition
Sweating
rate
Sex differences
Women have a lower maximal sweating capacity (i.e., at very high workloads and in hot, dry environments). Otherwise, lower sweating rates observed in women can usually be attributed to lower body mass and absolute workloads.
Women have lower output per gland and higher heat-activated sweat gland density. This translates to greater sweating efficiency, which may lead to less wasted sweat (drippage) in humid environments.
Menstrual cycle
During the luteal phase there is an increase in the threshold (body core temperature set point) for the onset of sweating and/or decreased sweating sensitivity.
However, there are no differences in whole body sweat rates during exercise across menstrual cycle phases.
Sweating
composition
Sex differences
Women tend to have slightly lower sweat [Na+] and [Cl-] as a population, but no differences when accounting for absolute workload and/or sweating rate.
Menstrual cycle
None
While there are some observed differences in sweating rate and composition between women and men, these variations are not significant enough to warrant different recommendations based on sex.
Practical advice
Optimal hydration is important for both health and performance. It is important to monitor the hydration status of athletes, incorporate personalized hydration plans, and enable strategic fluid intake pre-, during and post-exercise.
Fluid intake can come from a variety of food and beverages. The information below provides practical advice which can be used with athletes on when to ingest different types of beverages. Finally, it is essential that practitioners and athletes recognize the signs of symptoms of dehydration.
Practical tips for using everyday beverages
Hydration
References
Cheuvront, S. N., Carter, R., Castellani, J. W., & Sawka, M. N. (2005). Hypohydration impairs endurance exercise performance in temperate but not cold air. Journal of Applied Physiology (Bethesda, Md.: 1985), 99(5), 1972–1976.
Greenleaf, J. (1992). Problem: Thirst, drinking behavior, and involuntary dehydration. Medicine and Science in Sports and Exercise, 24(6).
Kenefick, R. W., & Cheuvront, S. N. (2012). Hydration for recreational sport and physical activity. Nutrition Reviews, 70 Suppl 2, S137-142.
Rollo, I., Randell, R. K., Baker, L., Leyes, J. Y., Medina Leal, D., Lizarraga, A., Mesalles, J., Jeukendrup, A. E., James, L. J., & Carter, J. M. (2021). Fluid Balance, Sweat Na+ Losses, and Carbohydrate Intake of Elite Male Soccer Players in Response to Low and High Training Intensities in Cool and Hot Environments. Nutrients, 13(2), 401.
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 and Science in Sports and Exercise, 39(2), 377–390.
Surapongchai, J., Saengsirisuwan, V., Rollo, I., Randell, R. K., Nithitsuttibuta, K., Sainiyom, P., Leow, C. H. W., & Lee, J. K. W. (2021). Hydration Status, Fluid Intake, Sweat Rate, and Sweat Sodium Concentration in Recreational Tropical Native Runners. Nutrients, 13(4), 1374.
Tarnowski, C. A., Rollo, I., Carter, J. M., Lizarraga-Dallo, M. A., Oliva, M. P., Clifford, T., James, L. J., & Randell, R. K. (2022). Fluid Balance and Carbohydrate Intake of Elite Female Soccer Players during Training and Competition. Nutrients, 14(15), 3188.
Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance. Medicine and Science in Sports and Exercise, 48(3), 543–568.
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