Water is essential for the body. It plays many important roles in a wide variety of physiological processes including, maintaining blood and cell fluid volumes, as a solute for chemical reaction of cells and maintaining body temperature.
During exercise, our body has to work harder to regulate its temperature. It achieves this through sweating.
Exercise at intensity and/or in hot temperatures induces a high sweat rate and we may not be able to consume fluid at a rate which replenishes the lost fluid. This results in a fluid deficit and can lead to dehydration.
Dehydration has been demonstrated to significantly affect exercise performance and a fluid loss greater than 2% decrease in body weight will result in gastrointestinal symptoms such as nausea and vomiting. It is desirable to limit this deficit during exercise by consuming some fluid, however, this may not always be practically achievable or well tolerated by the athlete depending on timing, type and intensity of the sport being played.
Overhydration is not as common as under hydration in sport however it can have a significant impact on performance especially through discomfort and bloating. Furthermore, hyponatremia or relative decline in sodium levels which occurs in overhydration can have very serious health consequences.
It is therefore important to try to avoid both overhydration and to limit dehydration whilst exercising. Athletes may employ a number of strategies to achieve this. Some elite athletes will weigh themselves immediately before and after exercise. Any change in weight will be due to fluid loss. Over a period of time athletes will be able to become familiar with their sweat rates in different conditions and be able to plan accordingly. To recover after a session or competition, in the 6 hours after exercising, athletes will aim to consume between 1.2 and 1.5 litres of fluid for every kilogram of body weight lost. Given that 1 litre of water weighs 1kg, this constitutes overcompensation. The extra 0.2-0.5litres per kg lost is to compensate for the continued losses in the recovery phase through sweating and urination.