When there’s a fluid deficit in the body, exercise at any intensity feels harder than it actually is. Almost every athlete has experienced this.

Sweating is one of the most important cooling mechanisms which prevents our body from overheating. Intense and prolonged endurance sessions, paired with high temperatures, can lead to high sweat rates. The amount of sweat lost per hour depends upon exercise intensity, environmental conditions, fitness level, body weight (as well as other factors), and can vary from less than 0.5 to over 2 litres.

A fluid or water deficit (dehydration) is one of the main reasons for symptoms of fatigue and decreases in performance during longer periods of endurance exercise. Even a fluid deficit of 3% of body weight (in relation to pre-exercise body weight) during exercise can reduce physical and mental performance, especially in warm conditions.

The only way to avoid excessive dehydration is through fluid intake. Before, during and after exercise the correct level of fluid consumption plays a vital role in maintaining fluid levels in the body. Here are three basic rules that endurance athletes can easily follow:

  1. Before exercise ensure you are well hydrated
  2. During exercise you should drink at regular intervals
  3. After exercise make sure you drink enough to compensate for any fluid deficits

What is the optimal amount of fluid intake during exercise?

Fluid replacement recommendations have drastically changed over the years, and ranged from ‘drink nothing’ (early 70s) to ‘drink the maximal amount you can tolerate‘ (1996).

The actual fluid demand during training or competition depends on many different factors, such as e.g. climatic conditions, individual sweat rate, duration and intensity of exercise. The optimal level of fluid intake is therefore highly different for every athlete, and earlier recommendations have since been disproven. Too much fluid, as well as too little, can negatively affect exercise performance!

For athletes without any individual hydration strategy or newbies, general hydration recommendations can provide a reasonable starting point for activities lasting > 60 minutes: drink approx. 400-800ml of fluid per hour, regularly in small sips (e.g. every 15 minutes approx. 150ml). In cooler conditions, and for lower intensities, the hourly requirements are more at the lower end of the scale, whereas warm conditions or high intensity sessions demand a higher fluid intake. However, one shouldn’t forget that there are races where there are only limited opportunities for fluid consumption. As an athlete it is therefore recommended to not only consider the climatic conditions, but also about where the organizers have put feed stops, and/or where you can position your own supplies along the route as well as how much you can carry with you personally. An individual fluid intake strategy should then be practiced and tried out beforehand during training sessions simulating the event.

There is no easy method that allows you to determine exactly your individual fluid demand during exercise. A fairly practical formula often use by pro athletes is as follows:


When you’re not training the colour of your urine can act as an easy indicator of fluid balance: If the colour of urine is light yellow/straw coloured, the fluid balance of your body is good. Slightly darker urine (similar to the colour of beer or apple juice) is a typical sign of inadequate fluid intake, e.g. a fluid deficit. However, it must be taken into account that certain foods (e.g. beetroot), multivitamins or antibiotics can affect the colour of your urine. If this is the case then the colour of urine should no longer be used as a guide.


It’s not just important ‘how much’, but also ‘what’ you drink

For short endurance sessions, water or calorie-free beverages are the ideal choice. During longer sessions many athletes like to quench their thirst with so-called isotonic sports drinks. An isotonic drink has the same osmolality (270-330 mosmol/kg water) as human blood, or more specifically blood plasma, and as a result is absorbed very quickly by the body. ‘Osmolality’ refers to the concentration of osmotically active particles that are defined, among other things, through their amount of sugar and electrolyte particles. Isotonic drinks (like for example PowerBar ISOACTIVE Isotonic Sports Drink, or a homemade special mix of juice, water and salt) are therefore specific carbohydrate-electrolyte solutions. They improve the uptake of fluid during exercise, and as such contribute to the maintenance of exercise performance during prolonged endurance exercise.

Tips on how to choose the right sports drink

  • The exercise intensity and duration is crucial! During short sessions, or to stimulate fat metabolism, it makes sense to choose drinks with no calories. In contrast, for longer and/or more intense sessions, carbohydrate-electrolyte drinks are a good choice. The electrolyte sodium (part of table salt) is lost in the largest quantity through sweat, and is therefore an important ingredient of sports drinks. Sodium is essential for water retention in the body, a well-regulated fluid balance, and for muscle and nerve function.
  • It needs to be well tolerated! Therefore try out beforehand what suits you best personally.
  • It has to taste good! Select a drink that tastes good. If it doesn’t it is more likely that you will not drink sufficient amounts of it.
  • Prevent monotony! Drinks with different flavours add to variety.

© Corinne Mäder, Senior EU Sport Nutrition Manager PowerBar. International Olympic Committee post-graduate Diploma in Sports Nutrition


  • American College of Sports Medicine et al. (2007). American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc, 39(2):377-90.
  • Baker, L.B., & Jeukendrup, A.E. (2014). Optimal composition of fluid-replacement beverages. Compr Physiol, 4(2):575-620.
  • EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA); Scientific Opinion on th esubstantiation of health claims related to carbohydrate-electrolyte solutions and reduction in rated perceived exertion/effort during exercise, enhancement of water absorption during exercise, and maintenance of endurance performance. EFSA Journal, 2011, 9(6):2211.