Water, along with air and food, is one of the three cornerstones of the physical element to human existence. In its pure state, water is transparent, tasteless, and colorless. The water molecule is comprised of two hydrogen atoms and one oxygen atom, as described by the chemical equation H2O. Water is the most prominent substance found within the body, and its importance to human structure and function is borne out by the relationship of water relative to the size and the composition of the body as a whole. Water constitutes approximately 92% of the volume of blood plasma, the fluid component of blood, as well as forming 60% of the mass of the erythrocytes (red blood cells) present in the bloodstream. A further 80% of the mass of skeletal muscle tissue is water, and the water molecule constitutes over 95% of the total molecules within the body.

Water is also classified by commercial purposes as "hard" and "soft." Hard water is a fluid substance that naturally contains significant quantities of the minerals, calcium and magnesium; these minerals are absent in soft water. Depending on the geological characteristics of the location where the water originates, water may contain other minerals such as sodium and potassium, as well as compounds such as chlorides and sulphides. Many of these mineral-laden waters are reputed to possess health-giving qualities.

From an athletic perspective, water has been described as "nature's original sports drink." All persons, both sedentary and athletic, must consume water on a daily basis, preferably at regular intervals throughout the day, for the body to function properly. Water is lost from the body through the processes of perspiration, through the discharge of breath from the lungs (the breath when exhaled includes water vapor present in the lungs), and through the elimination of the waste products urine and feces. The recommended daily amount of water to be consumed will be subject to the significant variables of age, body size, level of fitness, environmental conditions, type of exercise, the duration and the level of intensity required in exercise, and other related physiological factors. For persons whose physical exercise in any given day is at a moderate level, defined as running 3 mi (4.8 km) or working out 30 minutes on a cardio machine, they will generally be required to consume approximately 3-5 qt (approximately 4 l) of water daily to maintain efficient overall bodily function.

In sports science, the terms water and fluid are often used interchangeably when issues regarding the proper approach to the hydration of athletes is considered. Fluids are those substances that are primarily water, with other substances added to create an effect on the body that is designed to assist with respect to a particular physiological or chemical function in the body. Water with minerals or other additives, milk, sport drinks, and energy drinks are all examples of fluids commonly used by athletes to maintain fluid levels. The hydration strategy referred to as fluid replacement may include consideration of the relative merits of both pure water as well as any other fluid.

The essential role of water in the function of the body is best illustrated in three systems that are central to athletic performance: the cardiovascular system, the osmoregulatory system, and the thermoregulatory system. It is a commonly stated sports science proposition that, if the body loses as little as 1% to 2% of its fluids through dehydration, athletic performance may decline by as much as 10%.

The all-pervasive power of cardiovascular function within the body is felt in every aspect of physical performance. The bloodstream, through the movement of its erythrocytes, carries oxygen and nutrients to working cells to facilitate the further production of energy. As a general proposition, when the body becomes dehydrated, the blood volumes within the system are reduced, which slows the efficiency of both the transport of oxygen and nutrients, as well as a delay in the corresponding removal of metabolic wastes through the blood. The cardiovascular system cannot function effectively if the system is dehydrated, no matter what other steps may be taken by the athlete to assist in performance in any other respect.

Osmoregulation is the built-in mechanism that seeks to ensure the levels of bodily fluids and the electrolytes throughout the body. When the ratio between water and electrolytes fluctuates too far from optimal levels, the body's ability to maintain overall fluid levels and to transmit nerve impulses into muscular action is affected. The proportion of sodium to water is the most important of the ratios sought to be maintained through osmoregulation. The sodium level directly impacts the organ functions that relate to hydration, such as blood pressure and the production of urine by the kidneys. The kidneys excrete urine in accordance with the hormonal signals triggered by fluctuations in sodium levels; when the sodium level is too low, the kidneys will tend to produce more urine to decrease the proportion of water to sodium in the body, with the converse action of limiting urine production when the sodium level is too high.

Thermoregulation is the body's control of its internal temperature in response to all external environmental forces. The most effective regulator of internal temperature is the cooling of the body through the production and release of perspiration. In warm weather exercise, an athlete may lose up to 32 oz (1 l) of fluid per hour; perspiration is not pure water, as it includes both water and minerals, primarily sodium. The body's thirst mechanism, triggered in the hypothalamus region of the brain, is not activated until the body has lost approximately 1 to 2 qt (1 to 2 l) of fluid. It is for this reason that athletes must hydrate on a regular basis during both training and competition, whether or not they have experienced thirst.

With the importance of fluid replacement paramount for every athlete, the quality and the precise formulation of the replacement fluid is a critical issue. The overall goal of fluid replacement is to replace the amount of fluid lost to perspiration and other bodily functions during athletic activity. While pure water will often provide a solution for dehydration, the consumption of large quantities of water for this purpose creates risks for the athlete. Water consumed alone may dramatically affect the desired proportion of sodium to water within the body, signaling the body to prevent the absorption of further water into the bloodstream, directing the excess instead to the tissues of the body. This condition, hyponatremia, is potentially fatal, as the person will become further dehydrated.

For longer periods of exercise, there is considerable scientific support for the proposition that a sports drink, primarily water, with no more than 8% carbohydrate, that also provides sodium and potassium in the mixture, will adequately address the hydration needs of an athlete. The carbohydrate will provide an athlete with another fuel source during exercise; carbohydrate in excess of 8% is more difficult to absorb into the body and may cause cramping.

Energy drinks are a poor hydration choice. Although most of these beverages are over 90% water, all contain stimulants that are also powerful diuretics, primarily caffeine. Recent studies have illustrated that many energy drinks contain over 120 mg of caffeine per 16 oz (500 ml) serving; the U.S. Food and Drug Administration has limited soda to 65 mg of caffeine per 12 oz (350 ml) serving. If the body has begun to sustain the effects of dehydration, the consumption of energy drinks will likely accelerate the process.

SEE ALSO Blood volume; Cardiovascular system; Hydration; Renal function; Sodium and sodium deficits; Thermoregulation, exercise, and thirst; Water (oxygen enhancement).