Hydration, the process by which water is ingested and absorbed into the body, is of paramount-
Successful hydration requires a measure of planning. The development of a hydration strategy will depend on the nature of the event, the anticipated weather conditions, the outcome of any acclimatization to either heat or altitude that the athlete has been subjected as part of training, and the physical attributes of the athlete.
Distance running includes the events known as the middle distances, ranging from 1,500 m to 10,000 m, long distance races, which include the 10,000 m, the half marathon (13.1 mi; 21.1 km) through the marathon (26.2 mi; 42.2 km), and longer ultra-marathon races, and cross-country running, where the race distances vary from approximately 3 mi to 6 mi (6 km to 10 km). Similar hydration strategies will be employed in endurance sports such as cross-country skiing and cycling.
Hydration strategies in distance running are founded upon two distinct physiological considerations: the function of the human thermoregulatory system, combined with the performance of the cardiovascular system during exercise. The thermoregulatory system is responsible for the maintenance of the core body temperature within an optimal range, whether the external air temperature is very hot or very cold. The cardiovascular system, in its myriad of tasks in support of human functions, transports warm blood from the core to the cooler skin where, through the action of the capillaries, water in the form of perspiration is released. This process reduces the amount of blood in the cardiovascular system, because water constitutes 90% of its fluid volume. Reduced blood volumes translate into reduced capabilities in the cardiovascular system to transport oxygen, a critical aspect of energy generation in sport.
Sensible hydration strategies assist the athlete in maintaining fluid volumes, which in turn preserve cardiovascular capabilities and maintain optimal body temperature. The best strategies will have the following components:
The objective of all distance runners is to consume enough water during training sessions or competition to maintain 100% replacement of fluids lost through perspiration. This approach has replaced the older theory that it was important for a distance athlete to drink all that they can. In many countries, supervisory bodies responsible for the organization and sanction of distance races have published guidelines to assist athletes in their race day planning, as to how fluids should be consumed. The United States Track and Field Association (USTAF) is one such body; the USTAF has access to current sports science research to assist in the preparation of its guidelines.
Sanctioned races will also provide explicit rules as to where on a race course an athlete is permitted to obtain fluids. Known as "feeding stations," an athlete is typically permitted to take on water or other electrolytic fluids that the organizer will provide; the athlete may also have a supply of personal fluid replacement choices at a feeding station.
Sports science research conducted over a variety of sports contested in hot weather confirm that, when the athlete loses even as little as 2% of fluids, the performance may decline by as much as 10%. Individual race hydration strategy will be determined by how the athlete performs in practice sessions; as an example, a runner who weighs 160 lb (73 kg) prior to a 15 mi (25 km) run, who weighs 157 lb at the end of the training session, will plan his future hydration strategy with the knowledge that 3 lb (1.5 kg) of fluid were lost as perspiration during the training simulation.