Water and electrolyte homeostasis is critical for the functioning of all organs and indeed for maintaining health in general. Numerous studies have shown that even moderate water losses of 2% of body weight significantly impair not only endurance exercise, both in laboratories and field studies, but also mental performance in hot environments.1–5
Thermoregulation in Children
The vast majority of the published research on thermoregulation has been performed in well-trained adults exercising in the heat. On the other hand, little information is available concerning children exercising under similar environmental conditions. Noteworthy is the lack of data considering this specific population, despite the fact that it is of great importance because of the significant dehydration observed in many young athletes, along with the regularity with which athletic youth start exercise under hypohydrated state.6–8
Traditionally, children have been observed to exhibit a lower thermoregulatory capacity during exercise, compared with adults, leading to impaired athletic performance and a higher risk for heat injuries. Early studies indicated that children usually exhibit lower maximal aerobic capacity, higher adiposity, and lower sweat rate than adults when exercising in warm environments, because of their greater body surface area-to-mass ratio. This fact, in turn, leads to faster heat storage, especially when ambient temperature exceeds skin temperature.9–12 In addition, the body limits its reliance on evaporative heat loss.13 Another important parameter is the fact that children, like adults, sometimes do not drink enough to adequately replace water losses under warm conditions and exhibit hypohydration even when fluids are provided ad libitum. This phenomenon has been previously described as involuntary dehydration.14–17 However, in contrast to the aforementioned observations, current research fails to indicate thermoregulatory differences between children and adults while exercising in the heat. Recent studies have shown that children and adults, exercising in equal relative intensity workloads and similar environmental conditions, demonstrate similar thermoregulation responses.18–20 These findings show that children, despite certain anatomical and physiological differences, have similar rectal and skin temperatures as well as cardiovascular responses. Similarly, the risk for heat illness is not greater, nor is exercise-tolerance time in hot ambient conditions lower when compared with their adult counterparts.
Hydration Studies in Youth
Most of the earlier work on children’s thermoregulatory and fluid balance responses while exercising in warm conditions have been performed in sedentary nonacclimatized children under controlled environments (ie, climatic chambers).14,16,21,22 There are only limited studies that have investigated the hydration status of children in free-living conditions.23,24 To our knowledge, the hydration status in free-living athletic children has only recently been investigated systematically.
In 2006, Stover et al25 examined the hydration status, assessed by urine specific gravity (USG), of young football players training for 5 consecutive days. The athletes’ pretraining USG values were consistently high each day (>1.022), indicating that they were training in a state of dehydration. However, when an acute drinking strategy was applied (two 591-mL bottles of water or sport drink, one consumed between dinner and sleep and the other before morning training), hydration status was significantly improved, as indicated by USG values.
Similarly, Decher and colleagues6 observed the hydration status and the hydration knowledge of 67 active young athletes during their stay in a 4-day summer sports camp. Throughout their stay at the camp, approximately 55% of the athletes experienced significant to severe dehydration (USG ≥1.025). What is more interesting is that, according to the athletes’ results in their hydration awareness questionnaires, they seemed to recognize when they were doing a good or a bad job hydrating. However, they clearly demonstrated a failure to translate hydration knowledge into successful hydration strategies.6
Recent work by McDermott et al8 showed that the majority of 33 boys who arrived at a 5-day football camp were not only hypohydrated, but also maintained this condition throughout their stay. Despite the fact that they were in negative fluid balance when they entered training, they were further dehydrated during training, and they were also dehydrated through the rest of the day, leading to impaired recovery and impaired subsequent performance.
In 2010, Yeargin et al26 investigated thermoregulatory responses to multiple consecutive days of American football in 25 heat-acclimatized boys (15 ± 1 years). The results showed that the majority of the athletes experienced minimal to moderate dehydration before and after each training session, according to the values of urine osmolality (881 ± 285 and 856 ± 259 mOsmol/kg, respectively) having replaced approximately two-thirds of their sweat losses during training. Furthermore, they rehydrated poorly between exercise bouts, indicating inadequate hydration habits when they were not at practice.
Finally, in a study conducted by Kavouras et al27 in a summer sports camp, more than 90% of the young volleyball and basketball athletes started the training camp inadequately hydrated. The main finding was that a simple intervention program (ie, a lecture on hydration and the use of the urine color chart, improved water availability, and body weighing before and after practice) enhanced hydration status over just a 2-day period in exercising children. Moreover, this improvement in hydration status, through ad libitum water intake, led to a significant improvement in children’s endurance performance.27 However, it is important to point out that, despite the significant reduction in the percentage of dehydrated children after the intervention, almost 60% remained hypohydrated, suggesting that thirst did not lead to successful fluid replenishment.
In agreement with the studies discussed above, it is clear that additional research on fluid requirements for young athletes is required. New data indicated that young athletes appear to experience similar levels of dehydration during exercise as adults. Moreover, recent findings on the hydration status of young athletes indicate that dehydration is very common among young athletes exercising in warm environmental conditions. Furthermore, it is clear that children do not effectively translate hydration knowledge into successful hydration strategies. Therefore, hydration education and improvement of hydration accessibility, as well as simple and realistic hydration strategies alongside continued efforts made by coaches, athletic trainers, camp staff, parents, and athletes, can significantly benefit youths exercising in warm conditions.
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