Physiology

Introduction 

Childhood is a decisive period for overall development throughout the lifespan (WHO, 2009). It is well established that a child’s diet plays an important role in determining growth and health in adult life (Tanner, 1981). Healthy hydration is an important part of a balanced diet, whereas current studies seem to indicate that the fluid intake of children is often suboptimal compared to established reference values (Kant et al., 2010, Kavouras et al., 2011, Stahl et al., 2007, Stookey et al., 2011).

 

In most studies, from birth to adolescence, childhood can be divided into three age ranges: infants (0-2 years), children (3-12 years) and adolescents (13-17 years).

 

The purpose of this document is to present the current scientific evidence on hydration in children. Physiological development during childhood is very complex; hence water physiology and needs vary according to age. This document provides an overview of the current knowledge about hydration physiology and water intake in children and points out main differences compared with adults. It addresses water physiology, the importance of adequate hydration for health, guidelines and fluid consumption in children as well as current practices and efforts to increase water intake when suboptimal. 

 

I. Characteristics of water physiology from infancy to adolescence

Over the lifespan, numerous changes occur in the physiology and metabolism of the human body. Hence, body needs for energy, nutrients and water change from infancy to adulthood, with the highest requirements relative to body mass observed during infancy and adolescence to support growth and physiological development.

 

Water physiology changes rapidly in the first years of life, and then more slowly to progressively reach adult physiology by adolescence. 

I.1. Body water content

Water is the major component of the human body. On average, it represents 60% of an adult’s body weight (Watson et al., 1980).

 

Moreover, due to difference in body composition, children have higher body water content relative to body mass than adults. On average, water represents 75% of the body mass in infants in the first 6 months of life (Altman, 1961). Then it decreases rapidly between 6 months and 2 years and at a slower rate during childhood. It reaches an adult level by the age of puberty (>12 years). It is also after 12 years old that gender differences appear: water as a percentage of total body weight decreases at a faster rate in girls, due to the fact that in general, women have a higher percentage of body fat than men (Figure 1) (Altman, 1961, Novak, 1989). 

 

Figure 1. Mean total body water as a percentage of body weight per age group in males and females.

 (Adapted from Altman, 1961). 

 

Nonetheless, total body water varies, depending on individual body composition: lean body mass is approximately 73% while fat mass has a significantly lower body water (10%) (Péronnet et al., 2012, Altman, 1961). 

I.2. Body water balance

Body water balance is defined as the equilibrium between body water gains and body water losses. 

I.2.1. Main water losses in children

Under normal conditions, body water is mostly lost through urine and skin, and to a lesser extent, from lungs and faeces. 

Urinary water losses

Body metabolism produces waste which has to be eliminated, in major part through urine. The urinary tract, which comprises the kidneys, ureters, bladder and urethra, produces, stores and excretes urine (Figure 2). 

Figure 2. The urinary tract. 

 

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).

egulation of body water balance by ADH and the kidney.

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. 

Characteristics of water physiology from infancy to 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.

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