FREE Shipping on all orders over R1000.

Optimal hydration and fluid requirements for endurance athletes

Dispelling the fear of dehydration

For decades a fear of dehydration has been incorrectly perpetuated especially amongst endurance athletes. Recommendations such as “100% fluid replacement” and “drink the maximum amount that can be tolerated” is still believed. This is further amplified by marketers of sports drinks stressing the importance of avoiding dehydration. Only fairly recently, have the dangers of water intoxication and hyponatremia (low blood Na concentration) been recognised. Unfortunately, the widely held belief that dehydration is the greatest danger to endurance athletes persists, despite the scientific evidence to the contrary.

The focus on dehydration can be traced back to a study in the 1960s entitled “Danger of inadequate water intake in marathon runners”. Despite the study not even studying actual marathon runners and despite reporting that the fastest runners were the most dehydrated the erroneous conclusion was reached that fluid restriction leads to progressive dehydration, which in turn leads to heatstroke and potential collapse whilst ignoring the performance of the fastest athletes.

This conclusion is at odds with current scientific literature and our own experience dealing with elite world-class endurance athletes over five decades. Winners of endurance events are amongst the most dehydrated (according to weight loss classification) and hyperthermic. Weight loss of 1-3 % is common amongst elite endurance athletes in shorter events and up to 5 % in elite ultra-endurance athletes. Exercise is only stopped at much higher degrees of dehydration. Any degree of dehydration is not dangerous nor does it impede performance.

During endurance exercise ATP (energy) is predominantly produced from fat & glycogen (stored carbohydrates) via the various metabolic pathways. This leads to “metabolic water” that is lost through sweat but does not require replacement. The non-replacement water can be up to 2 kg over a marathon and higher in longer events.

It is possible to run a marathon without consuming any fluids, obviously with a degree of hydration. However fluid consumption has repeatedly been shown to improve performance. It is true that progressive dehydration leads to increased heart rates (cardiac drift) and core temperature, reduced stroke volume and cardiac output and reduced blood flow – but all in proportion to the degree of dehydration.

Therefore, endurance athletes should avoid overhydration (water toxicity) and hyponatremia at all costs as well as severe dehydration. Slight weight loss of 1- 5% depending on the athlete, duration and intensity of the event is within the normal ranges correlating to peak performance. Slower athletes participating in long events should take cognisance of the fact that they are especially at risk of consuming too much fluids and for developing hyponatraemia.

*Take note that weight gains/losses before and after exercise are not a good measurement of hydration status per se.

Factors influencing fluid loss during exercise

Fluids and electrolytes, predominantly sodium (salt), lost through sweat can vary significantly between athletes, exercise duration/intensity and environmental conditions.  Factors that influence fluid loss include:

  • Genes – Some people innately sweat more than others
  • Body size – The greater the body size, the higher your sweat rate
  • Gender – Female athletes generally have a lower sweat rate compared to men
  • Age – Children have a lower sweat rate than adults. Older athletes have age-related decreased thirst sensitivity that may cause slower voluntary rehydration.
  • Fitness level – Fitter individuals sweat earlier in exercise and in larger volumes
  • Environment – Sweat loss is higher in hot, humid environments
  • Exercise intensity – Sweat loss increases as exercise intensity increases
  • Habitual environment – Athletes who are not used to humid environments may sweat more compared to sensitized “local” athletes.

The effects of carbohydrates and electrolytes on water homeostasis and fluid absorption

Carbohydrates provide energy in the form of glucose to the working muscles. The addition of carbohydrates to fluid promotes water absorption in the small intestine, provided the carbohydrate concentration is not too high.

Furthermore, the addition of electrolytes, and importantly sodium (Na), to water also aids with the absorption of fluid in the small intestine. Additionally, it helps to replace the electrolytes lost in sweat. Notably, Na does not increase fluid absorption in the presence of carbohydrates.

Therefore, the consumption of a sport’s drink with carbohydrates and electrolytes contributes to an athlete’s hydration status. Research also indicates that fluid intake is enhanced when beverages are cool (±15 ºC) and flavoured.

Avoiding overhydration & hyponatremia

Endurance athletes should avoid hyponatremia (water toxicity) at all costs. Hyponatremia is characterised by low sodium concentration in the blood leading some to advocate extra salt intake during exercise to prevent it. However, the primary cause of hyponatremia is not consuming too little sodium but rather too much fluid. Hence athletes should take care not to consume too much fluids.

Salt (sodium) intake during endurance events

During exercise electrolytes, notably sodium (salt) is lost in sweat. Sports drinks containing both carbohydrates and electrolytes (including sodium) improve performance. Typically, sports drinks, such as Octane contain, less sodium than athletes lose in sweat. However, there is still a debate amongst experts whether athletes have to replace all the sodium lost in sweat or not. Who is right? Do athletes really need to replace all the sodium losses during exercise? The current scientific evidence is clear. There is no need to replace all the sodium lost in sweat and a typical sports drink provides sufficient sodium.  Salt tablets and drinks very high in sodium are unnecessary to maintain normal blood sodium levels and no performance benefit is gained.

The benefits of an isotonic or hypotonic drink

Fluid is best absorbed by the body when the solution is hypotonic or isotonic. Isotonic and hypotonic drinks increase gastric emptying.

These terms refer to the osmolality of the sports drink, which is a measure of the concentration of the solution (the number of particles dissolved in a kg of liquid/fluid). Water absorption is driven by the local osmotic gradient that controls free water movement in either direction across the intestinal wall. A drink that has more or less the same osmolality (concentration) as blood plasma is referred to as an isotonic solution. When the drink has a lower osmolality, it is more diluted and the solution is called hypotonic.

A highly concentrated solution is called a hypertonic solution. Hypertonic solutions, such as coke, lead to water movement into the gastrointestinal tract. A hypertonic solution could be dangerous to consume during training or competition, especially in high temperatures, and can cause diarrhoea and dehydration.

Athletes should consume isotonic or hypotonic drinks. Athletes should individualise their fluid and carbohydrate intake according to personal preference (taste) and environmental and performance factors.

How much fluid should you drink?

  • Begin a race/training well hydrated (athletes can use a urine colour chart to evaluate their hydration status prior to races).
  • Drink according to thirst, normally between 400 – 900ml per hour of an iso- or hypotonic sports drink.
  • Rather drink regularly at frequent intervals.
  • Always use cold beverages, if possible.
  • Avoid excessive intake of fluids and sodium (cramp blockers and salt tablets).
  • Take care not to use NSAIDS during racing.
  • Endurance athletes should focus on consuming fluid early in training/racing to replace lost fluid.
  • Replace fluid losses between transition or rest phases when fluid can’t be consumed during physical exertion.
  • Individualise your fluid intake over time.
  • After training alcohol should be avoided before the lost fluid and electrolytes are replaced as it delays rehydration, glycogen replenishment and the repair of soft tissue damage.

Shopping Cart 0

You are almost subscribed!

To complete the subscription process, please verify your email account by clicking on the link in the verification email we’ve just sent you. The verification email may be in your spam folder.


Sign up to our newsletter and receive 10% discount on your first order