Average urinary volume ranges from 1 to 2 L/d in adults (Manz et al., 2003), but there is a gradual reduction of urinary volume with decreasing fluid intake. In children, average urinary volume is between 600 and 1300 mL/d and increases with age to progressively reach adult values (Alexy et al., 2012). With age, a decrease in urinary volume relative to body weight has been reported in children from 4 to 14 years old (Ebner et al., 2002).
Skin and lung water losses
Skin water losses occur through both sweating and insensible water loss. Sweating is an active mechanism that relies on excretion of water and other components via the sweat glands. In contrast, insensible water loss is solute-free and refers to evaporation of water from the respiratory tract and from passive skin diffusion. Water loss from sweating is low under moderate ambient temperature and a sedentary state (EFSA, 2010).
Body surface area to body mass ratio is different between children and adults. It is twice as high in young children (1-2 years old) as in adults. The difference levels out by adolescence, when children have almost reached their adult size (Silvaggio et al., 1993). This explains why until adolescence, children lose more water relative to body mass from the skin at rest and under thermoneutral conditions compared to adults.
I.2.2. Water gains
Water gains come from water contained in ingested fluid and food, as well as metabolic water (water produced by the organism during nutrient oxidation) (Shirreffs, 2003, Benelam et al., 2010). This last aspect will be addressed in part III.1.
I.3. Body water balance regulation
I.3.1. Regulation by kidneys
Both in children and adults, kidneys are vital organs responsible for regulating the volume and composition of the extracellular fluid via complex neuroendocrine pathways (Andreoli et al., 2000): they are the main organs for the maintenance of electrolyte balance but also water balance. They function as selective filters eliminating metabolic endproducts, and adjusting levels of excreted substance, electrolytes and water in order to maintain a constant blood composition, thanks to reabsorption and secretion mechanisms.
Kidneys are able to reabsorb or eliminate water depending on body needs. To achieve this function, they are influenced by several hormones, especially antidiuretic hormone (ADH), but also aldosterone and natriuretic factors. ADH (antidiuretic hormone, also known as vasopressin) is a water-conserving hormone: it increases water permeability and thereby facilitates the reabsorption of water into the kidney capillaries. ADH is also involved in the mechanism that induces the sensation of thirst (Groff et al., 1995) (Figure 3).
Figure 3. Regulation of body water balance by ADH and the kidney. ↑
As a consequence, renal fluid output can vary widely to maintain total body water depending mainly on salt and water load. However there are limits to both conservation and excretion: Kidneys can regulate urine osmolality within a large range: from 50 mosm/L to 900-1400 mosm/L in adults (Isaacson, 1959, Mertz, 1963).
In children, renal function reaches maturity at about 2 years old, with comparable glomerular filtration rate and urine concentrating and diluting capacity to adults. (Brenner et al., 2008, Gearhart et al., 2009). However, voiding volume and frequency are only fully mature by adolescence (Ellsworth et al., 2005, Gearhart et al., 2009).
I.3.2. Thirst mechanism
When water losses exceed water intake, the body enters a state of hypohydration. Besides the kidneys, thirst also plays a role in restoring water balance by stimulating the desire to drink. Thirst is a sensation influenced by both physiological (cellular tonicity, extracellular volume, stomach distension, ADH blood volume...) and behavioral factors (meals, preferences, availability of beverage...). However, as for adults, thirst and voluntary drinking do not always guarantee adequate hydration (Rowland, 2008, Rowland, 2011). Indeed, some trials have reported that, when dehydrated and left to their own choice, children and adolescents do not drink enough to ensure the complete replacement of water loss (Bar-Or et al., 1980, Rivera-Brown et al., 1999, Santana et al., 1995).
I.3.3. Thermoregulation
Water plays an essential role in the body’s process of temperature control. Evaporation of sweat from the skin is an effective cooling system and represents the main route of heat loss in adults (Delamarche et al., 1990).
In contrast to adults, children lose more heat from convection (dry heat loss) than evaporation (Delamarche et al., 1990). This can be explained by two major physiological differences. First, children have a larger body surface area to body mass ratio and dry heat exchange depends on surface area (Sinclair et al., 2007). It also results from immature sweating mechanisms (Rowland, 2008). Until puberty, children have considerably lower sweating rates than adults, especially boys compared to men (Rowland, 2011).
Despite these differences, equivalent thermoregulation outcomes in children and adults exercising in the heat have been observed when adequately hydrated (Inbar et al., 2004, Rivera-Brown et al., 2006, Rowland et al., 2008). However, in similar situations with water deprivation, limited data suggest that body temperature rises more quickly in children than in adults (Bar-Or et al., 1980).
Thus, pre-pubertal children lose less water than adults during exercise and/or in the heat but their body temperature regulation may be more sensitive to dehydration than adults.
In summary
Overall, physiological differences in infants, children and adolescents in comparison to adults relate to body water content, insensible water losses,sweat rate and kidney function for children below two years old (Figure 4). They are attenuated throughout childhood and become similar to adults by adolescence.
Figure 4. Characteristics of water physiology from infancy to adolescence.↑
Take home messages
Physiological characteristics of hydration in children
Water physiology changes rapidly in the first years of life, and then more slowly to progressively reach an adult model.
Children and particularly infants, have higher body water content relative to body mass than adults.
Until they reach their adult size, children lose more water from the skin at rest and under thermoneutral conditions compared to adults.
Until adolescence, children and particularly infants have a higher water turnover relative to body mass than adults, especially during the first few weeks of life.
Children have higher water requirements per unit of body weight compared to adults, in part due to greater insensible water losses from skin diffusion.
Kidneys reach maturity at about two years old, with urine concentrating and diluting capacity equivalent to adults.
Water intake and hydration physiology during childhood
Physiology
I. Characteristics of water physiology from infancy to adolescence
I.1. Body water content
I.2. Body water balance
I.2.1. Main water losses in children
I.2.2. Water gains
I.3. Body water balance regulation
I.3.1. Regulation by kidneys
I.3.2. Thirst mechanism
I.3.3. Thermoregulation
Health importance
II. Importance of adequate hydration for children's health
II.1. Assessing risk of dehydration
II.2. Hydration status and physical activity
II.2.1. Effects of dehydration during physical activity
II.2.2. Are children affected differently than adults during exercise?
II.2.3. Water consumption and physical activity
II.3. Hydration status and cognitive function
II.3.1. What is cognition?
II.3.2. The impact of dehydration on cognitive function in children
II.3.3. Beneficial effect of increasing water intake
Consumption
III. The Challenge of hydration in children
III.1. Total water intakes and fluid consumption in children
III.1.1. Guidelines for total water intake
III.1.2. Fluid consumption in children
III.2. Improved water intake: a corner-stone of childhood lifestyle programs
III.2.1. The promisingeffect of water intake on overweight prevention
III.2.2. A priority action for a healthier lifestyle in children
III.2.3. Action worldwide: example of a long-term development program
As a conclusion
References
Quiz
