Hydration is the process by which water is ingested and absorbed into the body. Given the essential role that water plays in so many bodily processes, hydration is crucial to human function at any time. Hydration is of special importance as a part of the preparation, participation, and recovery of every athlete from the stresses of training or competition. The antonym of hydration is dehydration.

In the nomenclature of sport science, water and fluid replacement have similar meanings, with the two words used interchangeably. The other companion terms employed in connection with hydration include pre-hydration, the ingestion of water prior to a training session or competition; over-hydration, a circumstance caused by the ingestion of water when the body's sodium levels are too low, which prevents the proper absorption of the water; and re-hydration, the act of water replacement at the conclusion of the dehydrating events.

Sports science has played a significant role in the advancement of the knowledge available to the athletic community concerning the importance of hydration. In the preseason American football training camps of 50 years ago, water on the sidelines was unknown; many coaches believed that water consumed during competition tended to bloat athletes, and water was handed out very sparingly. Athletes in many endurance activities, where the hydration needs of the body are most acute, would not carry or otherwise have fluids available for their use during events. It is well understood in modern athletic training that water is the key to athletic comfort and performance.

Hydration, by definition, refers to water; in modern practice, hydration is often achieved through the use of a combination of water-based fluids, energy sources such as glucose-based products, and electrolytes such as sodium.

An understanding of the supreme importance of hydration in sports begins with the function of the cardiovascular system. Plasma, the fluid component of blood, is 90% water; the volume of the blood contained within the body is approximately 5 qt (5 l). When the water level in the body is reduced, the blood plasma levels are correspondingly less, which renders the blood less efficient as it is slower and thicker within the blood vessels.

How the blood plasma loses some of its water composition is tied to the function of the body's thermoregulatory system, which is the process by which the body maintains its internal temperatures within relatively narrow operating limits, irrespective of what the temperature outside of the body might be. The thermoregulatory system includes the internal devices by which the body regulates its internal temperature, the most important of which are contained in the brain, liver, and cardiovascular system.

The normal human temperature is 98.6°F (37°C) within the core of the body. Temperatures may vary on the peripheral surfaces like those of the cheeks, if exposed to cold or other factors. During athletic activity, the body generates greater energy to provide muscle power; this metabolic process itself generates heat. In addition to the actual production of energy, the body is often exposed to external heat sources, such as warm outdoor conditions or an overheated gymnasium. To combat the effect of these conditions upon the core temperature, the thermoregulatory system will prompt the cardiovascular system to transport warm blood from the core of the body to the cooler surface skin. Through the action of the tiny blood vessels (capillaries), fluid, in the form of perspiration, is released to the surface of the skin through its pores. Subject to the humidity present in the immediate atmosphere, the perspiration will evaporate from the skin when released. Perspiration is almost entirely composed of water, with some sodium and other minerals included. In this fashion, while the body endeavors to cool itself through perspiration, valuable water is lost from the cardiovascular system, and its function is potentially impaired.

The loss of water through perspiration has other important effects upon other performance systems. The central nervous system (CNS) conveys a ceaseless stream of signals to the working muscles of the body during an athletic activity. When the body sustains reduced levels of water through perspiration, it tends to also create a shortfall of sodium, a mineral of importance to both the monitoring of water levels that occurs in the kidneys, as well as with respect to the transmission of CNS signals. Muscle cramps, an involuntary spasm of the muscle tissue, commonly occur when an athlete is both dehydrated and when the body has sustained a loss of sodium.

There are a number of important hydration steps that every athlete should take, irrespective of either the sport or the athlete's level of ability, for both the preservation of the athlete's ongoing physical health and performance improvement. The first is the regular consumption of water. Water taken regularly throughout the day is essential to the maintenance of a proper hydrated state, sufficient to support the additional stresses of training or competitions. One gallon of water, divided into approximate servings of eight 8 oz glasses each, will satisfy the typical daily requirements of most persons, in addition to what may be consumed through the needs of sport. Water is utilized best by the body if it consumed in equal portions throughout the day.

In the sedentary periods between exercise sessions, the consumption of high carbohydrate meals tends to promote the overall efficiency of water absorption into the body.

All athletic hydration strategies should be based on the premise that dehydration will begin to occur in advance of the activation of the human thirst mechanism. By the time that an athlete actually feels thirsty, the body fluid supply will be as much as 1 qt (1 liter) below its optimal level.

Water or any alternate fluids used to promote hydration should not include either alcohol or caffeine, as each of the substances functions as a diuretic, those substances that promote the production and excretion of urine, a process that dehydrates the body.

For greatest effect during exercise, water or other appropriate fluids should be consumed immediately prior to and throughout the workout, at roughly equal intervals; 15 minute intervals for consumption is a useful standard. After the workout or the conclusion of the competitive event, fluids should be immediately replaced in the body. Water is best absorbed into the body when it is consumed at a temperature of approximately 40-45°F (4-7°C).

Hydration has a number of other specialized considerations. The body becomes dehydrated at night because its cells and organs continue to function; water should be consumed both at the end of the day and upon waking the next morning. The presence of a cold or other illness will generally increase the body demand for water. Children tend to become dehydrated more quickly than do adults; the hydration pattern for young athletes should be adjusted accordingly.

Dehydration is not to be understood in the same terms as an injury or other structural impairment to the musculoskeletal system. Dehydration is a correctable state, a deficiency that, if unchecked, will lead to other consequences. At its most benign, dehydration causes fatigue, muscle cramping, and an overall poor level of physical performance. In its more advanced states, dehydration will be a contributing cause of hyperthermia, which includes one or more of the heat-based illnesses, heat cramps, heat exhaustion, and heat stroke.

Hyponatremia is not a heat illness, in the sense of being directly triggered by the consequences of the body's efforts to cool its core temperature by way of perspiration. Hyponatremia is over-hydration, where

Pure water often provides more immediate usefulness to the human body than sports drinks.
the body has ingested too much water to be processed in the normal digestion and distribution channels. Hyponatremia occurs when the amount of sodium present is too low. The presence of sodium in water level control in the nephron is the body's mechanism to determine how much water should be maintained within the bloodstream and how much should be diverted to the bladder as urine to be excreted from the body. When water is ingested in circumstances of abnormally low sodium levels, the water will usually remain in the stomach, as the body becomes progressively more dehydrated. If unchecked through sodium replacement, hyponatremia can be fatal. It is a condition that occurs most frequently in long distance events, such as the marathon, the Ironman competitions, or long distance cycling events.

The type of substance to be used for water or fluid replacement is subject to a number of variables. Pure water will never be a poor selection, given its immediate usefulness to the human system. Formulated sports drinks are often consumed by athletes in response to high-intensity activities. Sports drinks typically do not become absorbed into the body through the digestive process as quickly as water; however, these products are often formulated to contain electrolytes and other useful minerals, as well as containing small portions of carbohydrate in the form of glucose. Carbohydrate concentrations of less than 8% by volume are the most useful in this regard. Water, taken in conjunction with small amounts of sodium in the form of salt tablets, tends to be absorbed more quickly by the body than does water alone. Sport drinks with larger quantities of carbohydrate tend to be more difficult to digest, especially if the product is consumed during competition.

SEE ALSO Cardiovascular system; Endurance exercise; Hyponatremia; Thermoregulatory system.