As most athletes know, hydration is a crucial factor when it comes to performing at one’s best. This wasn’t always the case. Quite the opposite: a historical review by Murray and Udermann reveals that not too long ago athletes often avoided drinking fluids before and during exercise. From the beginning of the 20th century until the 1970s it was a common believe that drinking water would adversely affect performance even in endurance events. So called "waterlogging" was considered to be disadvantageous as it was believed to interfere with diaphragm movement and induce heat cramps. (Little, Strayhorn, & Miller, 1994). Furthermore, it was considered a sign of weakness to hydrate during exercise and, at the time, depriving oneself of water was common practice to "get tough" mentally and physically. Despite research that contradicted such practices, it took until the 1970s to effectively change such training and racing doctrines.

Two landmark studies by Pugh et al. (1967) and Wyndham & Strydom (1969) changed this widely accepted belief by showing a correlation between success in long distance running and a tolerance to fluid loss. They also found a linear relationship between dehydration and increased body temperature at dehydration levels exceeding 3 percent. Through muscle activity during exercise, increased metabolic rate and other factors, body temperature can increases at a rate of up to 1°C every five minutes (McArdel et al., 1991). In order to keep temperature stable the body releases heat through conduction, radiation, convection and/or evaporation. Although depending on the outside conditions, evaporation of sweat is one of the most important cooling systems of the body. If lost fluids are not replaced, cooling through evaporation will become insufficient and the temperature equilibrium cannot be maintained, resulting in a rise in body temperature. When an individual loses 5% or more in fluids the body ceases to sweat, leading to a constant increase in body temperature if one continues to exercise. Severe dehydration, followed by a gradual increase in core body temperature will not only lead to a severe decrease in performance, but can also lead to heat sickness and even death.

Fortunately most athletes today are well aware of the importance of hydration pre, during, and post training and racing. Some even tend to hydrate excessively, which may lead to hyponatremia, a fluid-electrolyte imbalance that results in an abnormally low plasma sodium concentration (< 135mmol/l). Hypnatremia causes an osmotic imbalance across the blood-brain barrier, which leads to an influx of water into the brain. Brain swelling that leads to neurological responses like confusion, seizure and possibly even coma, can be the result. Not to mention the negative effects on performance.

Since ad libitum, "to the desire" is not an adequate way to prevent dehydration (due to the delay in thirst responds) and excessive water consumption can lead to hyponatremia, what hydration and sodium-replacement strategy should athletes use?

As sweat rates and fluid absorption differs among individuals and are easily affected by outside conditions, hydration protocols should be established for each athlete. In order to establish individual sweat rates athletes can weigh themselves before and after exercise. However, this is often not practical, which makes it necessary to have general guidelines.

According to the American College of Sport Medicine individuals should drink roughly 500ml about 2h before and another 200-500ml 20min before exercise. During exercise 200-300ml of should be ingested every 10-20min. The shorter the intervals of fluid intake the better, as higher levels of gastric volume promote a better fluid absorption rate. Absorption rate is also influenced by the temperature of the fluids. In this case the cooler the better. In order to decrease the chances of developing hyponatremia it is important to not drink more than 1.2l of fluids per hour and use a 0.5-0.7% sodium chloride solution, especially during exercise that lasts multiple hours. If events last longer than one hour a carbohydrate solution of up to 8% should be used before and during exercise. In addition, 30-60 grams per hour of carbohydrates should be ingested during intense exercise that exceeds one hour.