In order to discuss hydration requirements with exercise, heat balance must first be outlined. Heat balance or heat storage (HS) is the sum of heat gain and loss (see equation below). Heat balance during exercise occurs through four main processes: evaporation (sweat and respiratory; E), conduction (contact with a solid surface; Cd), convection (contact with a fluid; Cv), and radiation (electromagnetic waves; R). Exercise increases metabolic heat production (M):

While all four mechanisms are important at rest, evaporation is the primary mechanism for heat loss during exercise. Not accounting for changes in the environment, heat loss through evaporation must make up for the increase in metabolic heat production. This is apparent in the equation below as an increase in metabolic heat production increases heat storage (+) and evaporation reduces heat storage (−). The increase in metabolic heat production during exercise occurs because the body is ~25 % efficient at converting stored fuels (fat and carbohydrates) into usable energy for muscle contraction and movement. The remaining ~75 % of the energy released in metabolism accumulates in the body as heat and raises the body’s core temperature. Consequently, higher metabolic demands on the body, or an increase in exercise intensity, will result in greater body heat accumulation and accelerated core temperature. In order to stabilize the rise in body heat, or core temperature, sweat volume increases with an increase in exercise intensity, or metabolic heat production.

Environmental factors affect all the mechanisms responsible for heat storage shown in the above equation. A hot environment can increase the stored body heat by increasing, or reducing the cooling effect of, conduction, convection, and radiation. However, a hot environment can increase the amount of sweat produced and, therefore, the potential for heat loss through evaporation. Relative humidity, or the amount of water in the air relative to the total amount the air can hold, impacts the effectiveness of sweat to maintain core body temperature by directly affecting the rate of evaporation of sweat from the skin. Under extreme environmental conditions, high ambient temperature, and relative humidity, sweat beads on the skin and rolls off before it has the chance to evaporate. In dry or less humid environments, ambient air can hold substantially more moisture, and fluid evaporates from the skin into the surrounding air and, with it, carries substantial heat from the remaining water on the skin. The overall effect is cooler water on the skin and overall heat loss from the body.

Dehydration results from loss of significant amounts of body water during exercise that exceeds 1-h duration depending on the intensity and type of exercise and resultant sweat rate. Larger amounts of water are lost as the result of exercise at greater intensity especially when coupled with hot and humid environments as shown previously. However, even when exercise is conducted in milder environmental conditions, sweat is still produced. Sweat rate is also influenced by the level of acclimation to a hot/humid environment. Acclimatized individuals can achieve sweat rates of around 3 L/h during a bout of intense exercise and may reach a total volume of 12 L in a single day.

The level of dehydration will determine the level of physiologic and performance changes that occur. Dehydration approaching 2 % negatively affects body temperature regulation and cardiovascular functioning. For example, in one study, body fluid deficits of only 1 % body weight increased the core temperature when compared with that seen during equivalent exercise and a normal hydrated state; and for each liter of fluid lost, the heart rate increased approximately 8 beats per minute and there was a 1.0 L/min decline in cardiac output [11]. As dehydration worsens, the plasma volume and peripheral blood flow decline, and thus, the sweat rate declines in the exercising individual, further complicating the body’s ability to maintain a safe core temperature. Specifically, dehydration prior to exercise or competition equal to approximately 5 % of body weight can result in an increase in heart rate and core temperature and a reduction in the sweating rate, VO₂max, and exercise capacity compared to a normal euhydrated state [12]. In a dehydrated state, this is explained by the reduction in blood volume that results in less ventricular filling pressure, which ultimately causes an increased heart rate and an approximately 25–30 % reduction in stroke volume. The consequences are reduced cardiac output and arterial blood pressure, leading to reduced aerobic performance [13]. In one study, physical work capacity and physiological function were shown to decline with fluid loss of around 4–5 % body weight [14]. Dehydration equal to 4.3 % of the body mass reduced walking endurance by 48 % along with a 22 % decrease in VO₂max in this study [14]. The rise in core body temperature is directly related to reduced peripheral blood flow and sweat rate. This becomes most pronounced in hot humid environments in which ambient air is, for the most part, completely saturated.

Many sports and athletic events have the potential to cause large fluid deficits if adequate fluid intake is not encouraged. Triathletes and other elite endurance athletes can lose approximately ≥5 L of fluid (~6–10 % of body mass) during competition. For athletes of lower caliber, fluid loss does not usually exceed 500 mL/h, whereas even in temperate climates of 10 °C (50 °F), soccer players can lose an average of 2 L during a 90-min game [15]. Substantial water loss occurs outside typical exercise-related events too. For example, athletes concerned with “making weight” (e.g., boxers, wrestlers, weightlifters) seek out rapid weight loss methods such as saunas, steam rooms, or severe dietary alterations including restricting fluids, diuretics, and purging. These techniques accelerate weight loss but also increase an athlete’s susceptibility to heat-related illness (e.g., muscle cramps, heat exhaustion, heat stroke) [16, 17].

As mentioned in Sect. 6.3, fluid balance is the sum of fluid intake and fluid loss. In addition, we discussed the effect of water losses, particularly through evaporation, during exercise and the requirements for fluid intake according to environmental conditions and the types of activities in Sect. 6.4. Much research has been conducted on optimal hydration strategies to improve fluid balance during exercise. While additional research is needed to optimize and individualize these strategies, solid recommendations for the pre-exercise, during exercise, and post-exercise time frames have been made. In each section, the recommendations from the American College of Sports Medicine position stand on fluid replacement will be outlined first, followed by supporting evidence. Note that many of the strategies attempt to optimize not only fluid balance but also sodium balance recognizing that sodium, the primary extracellular and intravascular cation, has important implications in reducing water lost in urine.

It is generally accepted that a small increase in the amount of salt added to the food in the diet is adequate to replace any sodium lost through sweat. However, sodium balance can be disturbed by increased loses of large amounts of sweat or urine or ingesting large quantities of plain water in conjunction with a diet low in sodium. However, electrolytes and glucose found in sports drinks may hasten rehydration either through reducing urine losses or increasing fluid intake or both [23, 24]. Restoration of water and sodium balance in recovery occurs more rapidly by adding moderate to high amounts of sodium (20–60 mmol/L) to the rehydration drink or combining solid food with appropriate sodium content. Adding a small amount of potassium (2–5 mmol/L) may enhance water retention in the intracellular space and replace lost potassium during exercise [8, 20]. The American College of Sports Medicine (ACSM) also recommends that sports drinks contain 0.5–0.7 g of sodium per liter of fluid consumed during exercise lasting more than 1 h [20].

Restoring fluid balance in the body requires ingesting a fluid volume that exceeds the volume of fluid or water lost through sweat by approximately 25–50 % [25]. When a fluid is ingested, especially dilute beverages like water, the changes in blood plasma and gut distension cause the body to increase urine production and rapidly decrease the perception of thirst. Therefore, keeping a consistently higher plasma sodium concentration by adding sodium to ingested fluid may sustain the thirst mechanism and promote retention of ingested fluids. Typical activity including exercise generally results in only minimal loss of potassium (partially due to the potassium-conserving mechanism in the kidneys), and at higher-intensity exercise levels, losses appear to have no consequences [26]. Even under extreme conditions, potassium needs can be met by ingesting foods that contain this mineral (e.g., bananas and potassium-rich citrus fruits).

Though hydration is important to exercise, excessive fluid consumption during longer events (i.e., marathons and longer triathlons) under certain conditions can result in the serious medical illness hyponatremia, sometimes referred to as “water intoxication” [22]. Hyponatremia occurs when low plasma sodium concentrations are created in the blood due to the over-ingestion of water. In essence, the blood becomes “dilute” of sodium, thus creating an imbalance in osmotic pressure across the blood–brain barrier that initiates rapid water influx to the brain resulting in edema of brain tissue. In addition, the following symptoms, from mild to severe, can also be caused by this condition: confusion, headache, malaise, nausea, cramping, seizures, coma, pulmonary edema, cardiac arrest, and death [22, 27, 28].

During exercise in hot, humid weather, the body produces a sweat rate of more than 1 L an hour, with a sweat sodium concentration ranging between 20 and 100 mEq L⁻¹ [8]. In addition, repeatedly ingesting large volumes of plain water draws sodium from the extracellular fluid compartment into the unabsorbed intestinal water, further diluting serum sodium concentration [29, 30]. Exercise compounds the problem because urine production decreases during exercise due to a significantly reduced renal blood flow. This reduces the body’s ability to excrete excess water [8, 20]. Although hyponatremia is usually associated with competitive athletes, recreational participants and occupational workers should be aware of the dangers of excessive hydration and should ensure that fluid intake does not exceed fluid loss [8, 20]. It has been recommended that the following steps be taken to prevent development of hyponatremia when planning on competing in prolonged endurance activities [8, 20]: