Water intake and hydration physiology during childhood - Health importance
Physiology I. Characteristics of water physiology from infancy to adolescenceI.1. Body water contentI.2. Body water balanceI.2.1. Main water losses in childrenI.2.2. Water gainsI.3. Body water balance regulationI.3.1. Regulation by kidneysI.3.2. Thirst mechanismI.3.3. ThermoregulationHealth importanceII. Importance of adequate hydration for children's healthII.1. Assessing risk of dehydrationII.2. Hydration status and physical activityII.2.1. Effects of dehydration during physical activityII.2.2. Are children affected differently than adults during exercise?II.2.3. Water consumption and physical activityII.3. Hydration status and cognitive functionII.3.1. What is cognition?II.3.2. The impact of dehydration on cognitive function in childrenII.3.3. Beneficial effect of increasing water intakeConsumptionIII. The Challenge of hydration in childrenIII.1. Total water intakes and fluid consumption in childrenIII.1.1. Guidelines for total water intakeIII.1.2. Fluid consumption in childrenIII.2. Improved water intake: a corner-stone of childhood lifestyle programsIII.2.1. The promisingeffect of water intake on overweight preventionIII.2.2. A priority action for a healthier lifestyle in childrenIII.2.3. Action worldwide: example of a long-term development programAs a conclusionReferencesQuiz
Water is an essential nutrient and the main component of the human body (Peronnet et al., 2012). Therefore it seems consistent that hydration status and water consumption can have consequences for children, particularly when considering physical activity and cognitive function.
Although it has no absolute definition, dehydration is typically defined as either depletion in total body water content due to excessive fluid losses, diminished fluid intake or a combination of both (Begum et al., 2010).
Evaluation of the hydration status of an individual consists of the measurement (direct or indirect) of total body water (Shirreffs, 2003). While no single method is universally agreed-upon to assess hydration status, a combination of several markers seems to be the most appropriate for the general population. More precisely, urinary biomarkers of hydration status such as 24h- volume, specific gravity, osmolality and color have been shown to vary significantly between adults who drink low (<1.2 L/d) or high (>2.0 L/d) daily fluid volumes (Perrier et al., 2012). These results suggest that urinary biomarkers are well suited to detect small differences in hydration status that occur in normal daily life.
Other methods and markers can also be used including blood, salivary indices, dilution techniques, neutron activation analysis and ratings of thirst. Body mass change is often the quickest, simplest, and most accurate technique to use when substantial water losses are expected over a short period of time, such as during an exercise session. Ultimately, the relevance of the method depends on measurement conditions: in field situations, easy-to-use, safe, portable and inexpensive techniques are more appropriate (Armstrong, 2007).
During physical activity, dehydration impairs a range of physiologic functions, increasing thermoregulatory and cardiovascular strain (Murray, 2007). It leads to impairment of performance and increased risk of exertional heat-illness (muscle heat cramps, fainting, heat exhaustion, life-threatening heatstroke), particularly during sustained physical exercise and in warm/hot conditions (Council on Sports Medicine and Fitness and Council on School Health, 2011).
In adult athletes, there is little doubt that dehydration of more than 2% of body mass compromises physiological functions and impairs physical performance (Sawka et al., 1999).
The relationship between hydration status and performance decrement in children have been evaluated to a lesser extent. Different studies show that even 1-2% body mass dehydration reduces aerobic performance in prepubertal boys (Dougherty et al., 2006, Wilk et al., 2002). Moreover, Kavouras et al. (2011) demonstrated that improvement in hydration status through an educational intervention led to significant enhancement in endurance performance in exercising children. Thus, promoting fluid intake in exercising children is certainly important.
From a mechanistic point of view, because pre-pubertal children have lower sweat rates than adults, they may better retain body water during exercise. However, higher insensible water losses relative to body mass implies greater water requirement per unit of body weight compared to adults. Available data about children’s physiological responses and physical performance in the heat are not sufficient to draw definitive conclusions. Furthermore, in contrast to former hypotheses, current research indicates that thermoregulation in the heat may be similar for children and adults despite different dominant thermoregulatory mechanisms. To date, there is no significant evidence that children would be at different risk of dehydration or hyperthermia than adults during physical activity under heat (Rowland, 2008).
In adequately hydrated children, no increased risk of exertional heat-illness has been observed compared to adults (Inbar et al., 2004, Rivera-Brown et al., 2006, Rowland et al., 2008, Shibasaki et al., 1997). But this risk has not been studied in a situation of water deprivation.
During physical activity, water needs can increase rapidly: an increase in water consumption is therefore recommended during and after exercise to replace the additional water lost in sweat (American Academy of Pediatrics Committee on Nutrition and Council on Sports Medicine and Fitness, 2011).
According to the clinical report of the American Academy of Pediatrics (AAP) Committee on Nutrition and Council on Sports Medicine and Fitness (2011, pp 1182); “with children and adolescents, careful consideration is necessary when selecting a beverage to hydrate before, during, or after exercise”. Generally, for children involved in regular physical activity, consuming water is sufficient to replenish the water lost through sweating during exercise. Water does not increase caloric intake, or kidney solute load. Thus, its consumption is appropriate as part of a healthy lifestyle combining balanced diet with moderate physical activity.
The AAP report also states, “Children and adolescents should be taught to drink water routinely as an initial beverage of choice as long as daily dietary caloric and other nutrient (e.g., calcium, vitamins) needs are being met. Water is also generally the appropriate first choice for hydration before, during, and after most exercise regimens. Children should have free access to water, particularly during school hours”.
Additionally, the Institute of Medicine published in 2007 a report in which it recommends healthier eating behaviors for children and adolescents at school. Recommendations include:
- Restrict foods and beverages that are high in added sugar;
- Encourage plain water availability at no cost for students;
- Promote consumption of water without flavoring, additives, or carbonation;
- Limit sports drinks consumption to the use of young athletes engaged in vigourous physical activity.
Defining cognitive function is difficult and complex. Cognition deals with every aspect of how humans perceive, remember, think, speak and solve problems (Feist et al., 2009). Thus, cognitive function includes attention, learning, memory, and reasoning, as well as seeing, hearing, and the ability to act. It also relates to state of mood, as every psychological phenomenon is a cognitive phenomenon (Neisser, 1967).
Dehydration has been recognized as having a negative effect on cognitive performance, including memory, attention, motor skills and mood in adults (Armstrong, 2012, Edmonds, 2012, Ganio et al., 2011, Pross et al., 2012).
Although data in children are limited, impaired memory has been observed with mild levels of dehydration (1-2% body weight) (Bar-David et al., 2005, Fadda et al., 2008). Similarly, a recent study suggested that brain structure and function could be affected acutely by dehydration in adolescents in situation of dehydration (Kempton et al., 2011).
Consistent results show that 7 to 9-year-old children performed better on tasks of visual attention and visual memory when drinking 200-400 mL of additional water before test (Benton et al., 2009, Edmonds et al., 2009a, Edmonds et al., 2009b). However, the initial hydration status of the children was not assessed. Further research is needed to confirm these first results showing that increased water intake improves cognitive function in children.
As children spend the majority of their day at school, often staying for afterschool activities, encouraging water availability during and after school can contribute to higher water consumption.
Overall, the importance of adequate hydration is clear: staying well-hydrated, by maintaining proper water balance within the body, is an important issue for physically active children and to support their cognitive functions (Figure 5).
Figure 5. Preliminary evidence suggests two important consequences of low drinking water consumption.
Take home messages
Healthy hydration for physical activity
During physical activity, dehydration beyond 1-2% impairs a range of physiologic functions, increasing thermoregulatory stress and cardiovascular strain.
Dehydration has a negative effect on effort tolerance and performance during physical activity.
For children involved in regular physical activity, consuming water is sufficient to replenish the water lost through sweating.
Water is generally the adequate first drink for hydration before, during, and after exercise (American Academy of Pediatrics Committee on Nutrition and Council on Sports Medicine and Fitness, 2011).
Healthy hydration for cognitive function
Mild levels of dehydration (1-2% body weight) impair cognitive function.
Increasing plain water consumption in children may support visual attention and memory.