Water composes more than half the human body and is essential in the function of our cells. Temperature maintenance and circulation are but a few vital processes dependent on this important fluid. Thus, an appropriate fluid balance is important to our health and well-being.

When we exercise, our body uses various fuels (carbohydrate, fat, and protein) to provide energy for our muscles to work. The breakdown of these fuel substrates and the resulting muscular contraction produce heat that increases core temperature. To prevent excessive heat buildup, our body responds by increasing skin blood flow and sweat secretion. Heat exchange between the skin and environment, especially via sweat evaporation, is the primary mechanism for dissipating the excess heat. Although necessary, this crucial process also results in a loss of body water and electrolytes.

Exercise intensity, duration, and environmental conditions can affect sweat rates, water loss, and cooling ability. On hot days, the difference between skin and air temperature narrows, challenging our ability to effectively transfer heat to the environment. The addition of humidity further compromises the sweat evaporation cooling process as the ambient air is already highly saturated with water vapor, reducing evaporative heat exchange. In addition, individual characteristics such as economy of movement, body weight, genetics, clothing, and heat acclimatization also influence sweat rates and associated cooling.

Increased sweat rates, particularly those stimulated by hot weather, can result in substantial water and electrolyte losses. Thus, appropriate fluid intake before, during, and after exercise is important. Inappropriate fluid intake, especially over hydration, can have serious medical consequences, including death.

Before the 1970s, athletes were advised not to drink during competitive events. During the following two decades, research was published indicating that dehydration, defined as an exercise weight loss greater than 2%, could impair performance. Thus, drinking guidelines were developed that emphasized the importance of early and regular drinking to stay ahead of thirst, minimize weight loss, and maintain performance.

Unfortunately, these early recommendations led some people to overhydrate, resulting in gastric discomfort, altered performance, and, in rare and severe cases, the development of water intoxication or hyponatremia. Exercise-associated hyponatremia is a serious medicalcondition that can result in death. Risk factorsassociated with hyponatremia include preexercise overhydration, excessive drinking (>1.5 L per hour) during exercise, low body weight, female sex, extreme environmental temperatures, exercise pace, and extended exercise duration.

It is not plausible to recommend a standardized fluid replacement protocol for all people because of the numerous variables involved. However, the following general principles will guide you in developing an individualized fluid replacement strategy.

Physical exercise, especially in hot conditions, can elicit high sweat rates and associated water loss. Although significant dehydration can affect performance, overdrinking can lead to serious hyponatremia. Thus, fluid replacement strategies should be individualized and takeinto consideration environmental conditions, exercise intensity and duration, pre-post body weight, body size, and other individual characteristics.