Hydration vs. dehydration
The volume of water in the body consists of ~60–70 % of total body weight in men and 50–60 % of total body weight in women depending on body composition (1). From this, it is clear that staying hydrated is of critical importance for not only well-being but avoiding fatality.
The amount of water in the body can be influenced by various factors such as the weather (specifically heat and humidity), dietary choices, energy expenditure, sweat rate, health issues such as chronic diarrhoea and vomiting or training at altitude. When the body is well hydrated, it can naturally thermoregulate itself. Thermoregulation is highly important for exercise performance, as it helps to reduce hyperventilation, heart rate and sweat rate during exercise (1).
Fluid balance regulation is made possible by minerals commonly known as electrolytes (sodium, potassium, magnesium, calcium, chloride). By regulating fluid in the body, electrolytes play an essential role in reducing the onset of dehydration. They help transport glucose and other nutrients into cells and remove waste products from the body. The correct balance of these minerals is highly important to avoid cramping, fainting, nausea and light-headedness but are easily lost through sweat during exercise, which on hot days can range from 800 ml to > 3 L per hour (2).
Dehydration occurs when more fluids leave the body than enter it. If dehydration takes place, normal physiological functions are impaired and slow down. The loss of fluid and electrolytes leads to a reduced blood volume thereby decreasing stroke volume, increasing heart rate, lowering blood pressure and reducing the amount of oxygen reaching the tissues. All of the above can be detrimental to performance and place strain on the cardiovascular system (1). Studies have indicated that > 2% body mass loss by dehydration results in a significant performance deficit (2)(3).
How can I monitor my hydration status?
There are many different testing methods to measure the amount of fluid an athlete loses during various sessions (see below), however, an easy way to estimate hydration status on a daily basis is by observing the colour of your urine. Observing the colour of urine throughout the day can give a good idea of whether or not you are drinking enough fluids.
This chart can be used as a guideline:
What are possible signs & symptoms of dehydration?
- Dry mouth
- Low blood pressure
- Muscle weakness
- Unusually high heart rate at low intensities or rest
- Severe cases: heat stroke, seizures, kidney problems, death
What are the most hydrating fluids?
- Still and sparkling water
- Sports drink
- Alcohol-free beer
- Fruit juices (specifically orange juice)
Can I over hydrate?
Yes. This is quite a common problem in endurance events, as athletes are afraid of dehydration and tend to over hydrate. Over hydration can be dangerous, as it can cause kidney failure.
How can I stay hydrated before, during & after training?
Fluid requirements will vary considerably among athletes due to sweat loss. It is recommended that individual athletes adopt their own tailored hydration strategy to ensure optimal hydration.
During training/ competition, athletes should consume both fluids and electrolytes to replace what is lost through sweat and must drink before thirst is sensed. In order to avoid dehydration in an event, aim to increase fluid intake a few days leading up to the competition.
Rehydration requires 1.5 L (150 % of body weight deficit) for every 1 kg lost (1 kg = 1.5 L) with a small amount of salt (sodium). Sodium is needed to better retain the ingested fluid (4). Sports recovery drinks should already contain the electrolytes needed post exercise, otherwise, a pinch of salt or a salty snack will do.
The following calculation can be used to rehydrate after sessions producing high sweat rates:
Body weight (BW) deficit = (Pre-exercise BW) – (post-exercise BW)
Fluid consumption requirement = BW deficit x 150 %
- Harris, P. R., Keen, D. A., Constantopoulos, E., Weninger, S. N., Hines, E., Koppinger, M. P., … Konhilas, J. P. (2019). Fluid type influences acute hydration and muscle performance recovery in human subjects. Journal of the International Society of Sports Nutrition, 16(1), 15.
- James, L. J., Funnell, M. P., James, R. M., & Mears, S. A. (2019). Does Hypohydration Really Impair Endurance Performance? Methodological Considerations for Interpreting Hydration Research. Sports Medicine, 49(2), 103–114.
- Cheuvront, S. N., Carter, R., & Sawka, M. N. (2003). Fluid balance and endurance exercise performance. Current Sports Medicine Reports, 2(4), 202–208.
- Maughan, R. J., Watson, P., Cordery, P. A. A., Walsh, N. P., Oliver, S. J., Dolci, A., … Galloway, S. D. R. (2019). Sucrose and Sodium but not Caffeine Content Influence the Retention of Beverages in Humans Under Euhydrated Conditions. International Journal of Sport Nutrition and Exercise Metabolism, 29(1), 51–60.