SUMMARYThis Position Stand provides guidance on fluid replacement to sustain appropriate hydration of individuals performing physical activity. The goal of prehydrating is to start the activity euhydrated and with normal plasma electrolyte levels. Prehydrating with beverages, in addition to normal meals and fluid intake, should be initiated when needed at least several hours before the activity to enable fluid absorption and allow urine output to return to normal levels. The goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance. Because there is considerable variability in sweating rates and sweat electrolyte content between individuals, customized fluid replacement programs are recommended. Individual sweat rates can be estimated by measuring body weight before and after exercise. During exercise, consuming beverages containing electrolytes and carbohydrates can provide benefits over water alone under certain circumstances. After exercise, the goal is to replace any fluid electrolyte deficit. The speed with which rehydration is needed and the magnitude of fluid electrolyte deficits will determine if an aggressive replacement program is merited.INTRODUCTIONPeople perform physical activity throughout a range of environmental conditions (temperature, humidity, sun, wind exposure). Depending upon the metabolic rate, environmental conditions and clothing worn, exercise can induce significant elevations in body (core and skin) temperatures. Body temperature elevations elicit heat loss responses of increased skin blood flow and increased sweat secretion (120,121). Sweat evaporation provides the primary avenue of heat loss during vigorous exercise in warm hot weather; therefore sweat losses can be substantial. Besides containing water, sweat contains electrolytes that are lost. If not appropriately replaced, water and electrolytes imbalances (dehydration and hyponatremia) can develop and adversely impact on the individuals exercise performance and perhaps health (27,72).This Position Stand summarizes current knowledge regarding exercise with respect to fluid electrolyte needs and the impact of their imbalances on exercise performance and health. This position statement replaces the prior Position Stand on exercise and fluid replacement published in 1996 (39). The new Position Stand includes a Strength of Recommendation Taxonomy (SORT) to document the strength of evidence for each conclusion and recommendation (50). Table 1 provides a description of strength of evidence category employed, based on the quality, quantity and consistency of the evidence for each statement. Occasionally review papers have been cited, to reduce the number of references, which provide extensive documentation regarding supporting studies. Recommendations are provided for practical hydration assessment techniques and rehydration strategies for before, during and after exercise. It is recognized that considerable variability exists between individuals, different physical activities and environmental conditions regarding water electrolyte losses so that each person will need to customize these recommendations. Importantly, it is emphasized that during exercise individuals should avoid drinking more fluid than the amount needed to replace their sweat losses.Throughout this Position Stand, the term "euhydration" refers to "normal" body water content, while the terms "hypohydration" and "hyperhydration" refer to body water content deficits and excesses beyond the normal fluctuation in body water content, respectively. The term "dehydration" refers to the loss of body water. The hypohydration that occurs during exercise is usually characterized as hyperosmotic hypovolemia (because sweat is hypotonic to plasma), although iso osmotic hypovolemia can occur when taking some medications (e.g., diuretics) or exposure to cold and hypoxia. For simplicity, the term dehydration will be used to describe both the process of body water loss and hypohydration in this position statement, unless stated otherwise.