USA Surgery Today

The water content of infants is higher than that of adults (75% of body weight versus 65%). Fluids provide the principal source of water; however, some is provided via oxidation of food and body tissues. Water requirements are related to caloric consumption; therefore, infants must consume much larger amounts of water per unit of body weight than adults. In general, calorie requirements (kcal per kg per day) are matched to the amount of fluid needs (mL per kg per day). The daily consumption of fluid by healthy infants is equivalent to 10% to 15% of their body weight, in contrast to only 2% to 4% by adults. In addition, the natural food of infants and children is much higher in water content than that of adults; the fruits and vegetables consumed by infants and children contain about 90% water. Only 0.5% to 3% of total fluid intake is retained by infants and children. About 50% is excreted through the kidneys, 3% to 10% is lost through the gastrointestinal tract, and 40% to 50% is insensible loss.

The requirement for protein in infants is based on the combined needs of growth and maintenance (Table 7-1). Two percent of the infant’s body weight, compared with 3% of the adult’s body weight, consists of nitrogen. Most of the increase in body nitrogen occurs during the first year of life, which explains the major protein requirements of infancy. The nutritional value of protein is based not only on the amount of nitrogen available, but also on the amino acid composition of the protein (5). Protein provides 4 kcal per gram of energy, and should generally be included in estimates of caloric delivery. Twenty amino acids have been identified, of which nine are essential in infants (Table 7-2).