132 2 5 10 Table 1 Progressive symptoms of dehydration during exercise [6] Percent dehydration (%) Typical symptoms Curr Rev Musculoskelet Med (2008) 1:131–136 Thirst, skin ﬂushing, fatigue, weakness, dark-colored urine Decreased sweating, increased core temperature, increased heart and respiratory rate Muscle spasms, confusion, seizures, potential loss of consciousness transfer of energy down a thermal gradient. Convection is the transfer of energy through moving gases or liquids. Evaporation is the heat loss through insensible water loss and sweating. At rest, evaporation accounts for *25% of heat loss, with 2.43 kJ of heat lost for every milliliter of water that evaporates [8]. This percentage signiﬁcantly increases with exercise (Table 1). Despite strong research that dehydration will cause decrements in endurance performance, ﬂuid needs during competition are widely variable. These variables include, but are not limited to the type of sport, competitor experience, and environmental factors [6]. The adequate intake of total water for young men and women is considered to be 2.7–3.7 l daily [9]. However, this reference does not account for water loss through sweat during activity, which has been documented as 1.0–2.5 l/h or greater, and can exceed 10 l daily [10, 11]. In comparison to endurance-related activities, limited research on high-intensity anaerobic activities reveals that performance is not adversely affected by moderate dehy- dration or moderate hyperthermia [12, 13]. The theorized reasoning is that the heat strain associated with these short duration activities is not sufﬁcient to cause performance decrements. Therefore, it is the heat strain with continued activity that at least partially contributes to performance impairments. Numerous hydration supplements are marketed with claims to improve athletic performance. This marketing even solicits water, both plain and ﬂavored. Most athletes have heard of and probably sampled various carbohydrate and electrolyte beverages. Data for hydration aids-con- taining glycerol to improve pre-competition hyperhydration are limited [8]. There are some ‘‘sports drinks’’ that are being marketed because of their protein content and other ergogenic aids, such as various herbs and stimulants. This review will focus on macronutrients and electrolytes associated with hydration supplementation. There is limited research on some of the fringe supple- ments such as stimulants and herbs. Overall, there is little consensus on the optimal ﬂuid replacement beverage. Training or competition of high-intensity sports such as soccer, basketball, and tennis have relatively mild to moderate dehydration levels with voluntary ﬂuid intake [6]. Participants in these events may beneﬁt from sports drinks by optimizing hydration through improved palatability [14, 15]. However, it is important that these drinks do not contain excessive concentrations of carbohydrate, as this could decrease performance secondary to gastrointestinal distress [16]. Although modest, Shi et al. in 2004, revealed that carbohydrate drinks with concentrations as low as 8% resulted in gastrointestinal distress and impaired perfor- mance during high-intensity intermittent exercise [17]. Other than for improved palatability, there are insufﬁcient data to support the consumption of carbohydrate drinks by athletes competing and training less than 1 h, with the underlying assumption that they have optimized hydration during these events and have started these events both eu- hydrated and with optimized meal schedules. Gastric emptying studies have shown that ingested carbohydrate solutions reach peak intestinal absorption in 20–30 min, with longer periods of absorption for greater concentrations of carbohydrates or proteins [18]. The addition of electro- lytes to a carbohydrate-based solution has not been shown to enhance either gastric or intestinal absorption [19]. Gastric emptying is also related to activity intensity, with greater intensity leading to delayed absorption [20]. Given the delayed gastric emptying associated with protein drinks, and the propensity for gastrointestinal distress that can occur at higher concentration solutions and increasing exercise intensities, there are relatively few times when protein-based hydration solutions should be recommended during activity. Basically, from a physiologic perspective, water is the optimal hydration strategy for activities lasting less than 1 h. Post-exercise rehydration is recommended for any ﬂuid deﬁcits that occur during activity. However, in real world situations, the likelihood that athletes will be able to sufﬁciently rehydrate on plain water alone between events is remote given the highly competitive nature of sports such as soccer and athletes’ condensed training schedules [21]. Although data are limited, personal experience reveals that many athletes start off an event or training day at less than optimal hydration status. Therefore, individu- alization for athletes based on exercise intensity, sport, sweat rate, and environmental factors is recommended. Environmental factors ‘‘Water, water, everywhere, nor any drop to drink.’’ This quote by Samuel Taylor Coleridge in The Rime of the Ancient Mariner though written over 200 years ago can be