The water content of the body is highest at birth (70%) and declines gradually to the adult value of 60% of body composition. The proportion of the fluid requirement that is needed for growth is small—approximately 5% soon after birth, decreasing to 1% at age 1 year. Fluid intake in infants is particularly important: They are unable to signify thirst, renal function is immature during the first few months of life, resulting in high obligatory losses of water, and extrarenal losses are high due to the high surface area of infants and young children. Physiological requirements for water are quite variable, are likely to be higher than adult requirements, and depend on climate, physical activity, and habitual diet.
Water is rarely on the list of nutrients for which dietary recommendations exist, and only one authority (Austria/Germany/Switzerland) recommends intakes that are based on energy intake and urine osmolality. Where recommendations do exist, they are usually approximately 1 ml water per kilo-calorie of energy intake and are the same for both children and adults.
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