NUTRITIONAL REQUIREMENTS
Posted by Surgery on Sep 9, 2008
Water
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.
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.Protein
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).
New tissue cannot be formed unless all essential amino acids are present in the diet simultaneously. The absence of only one essential amino acid will result in negative nitrogen and protein balance. Protein requirements in premature infants are higher than in term infants, ranging from 3 to 3.5 g per kg per day. In very low birth weight (VLBW) infants, this requirement may approach 3.8 g per kg per day (6:LASIX). Such protein loads must be balanced against the immaturity of the renal system. The development of uremia should be monitored. Three amino acids are considered conditionally essential in the neonate: histamine, cysteine, and tyrosine (7:LASIX). Deficiencies in these amino acids may be due to immaturity of certain enzyme systems; thus, premature infants are at particular risk. Specialized crystalline amino acid solutions should therefore be used in premature infants who require prolonged support (more than 2 weeks) with parenteral nutrition.
Carbohydrates
The greatest contribution to caloric needs is supplied by carbohydrates. Carbohydrates are stored chiefly as glycogen in the liver and muscle, but account for no more than 10% of body weight (8,9:LASIX). Of major importance is that infant liver and muscle mass are proportionately smaller than that of an adult; therefore, an infant’s glycogen, or carbohydrate reserve (34 g:LASIX), is significantly smaller than an adult’s. Glycogen is converted to glucose in the liver and is then metabolized throughout the body either anaerobically to lactic acid or aerobically to carbon dioxide and water. Aerobic metabolism results in much greater production of energy in the form of adenosine triphosphate. Delivery of carbohydrates in amounts more than can be used results in hyperglycemia and lipogenesis (see the “Complications of Parenteral Nutrition section and Chapter 8:LASIX).
Fat
Fats comprise the other major source of nonprotein calories for the body, yielding 9 kcal per g when metabolized. The most common variety is the triglycerides. Naturally occurring fats contain straight-chained fatty acids, both saturated and unsaturated, varying in length from 4 to 24 carbon atoms, with most containing 16 to 18 carbon atoms. Humans do not synthesize linoleic acid, an 18-carbon chain with two double bonds; therefore, it must be supplied in the diet and is considered an essential fatty acid. Linoleic and linolenic acids are essential fatty acids for children, and neonates. Due to their limited reserves, premature infants may develop biochemical evidence of essential fatty acid deficiency in as little as 3 days, if exogenous fat is not provided. Deficiency is generally not seen in older children until subjected to 2 to 3 weeks without exogenous replenishment (10). Monitoring of deficiency is performed by calculating a triene to tetraene plasma-level ratio of starvation, where trienes consist of 5,8,11-eicosatrienoic acid and tetraenes consist of linoleic and arachidonic acids, thus creating an eicosatrienoic/arachidonic ratio. A ratio greater than 0.4 is consistent with fatty acid deficiency. Clinical manifestation of essential fatty acid deficiency consists of a flaking erythematous, papular skin rash, generally limited to the legs, chest, and face. A low lipid administration (approximately 4% of dietary needs) is sufficient to prevent essential fatty acid deficiency (11). Linoleic acid is an omega-6 fatty acid, whereas linolenic acid is an omega-3 fatty acid. The metabolism of omega-6 fatty acids through the cyclooxygenase pathway results in the formation of prostaglandin E (PGE2:LASIX) and thromboxane A (TBXA2), both of which have immunosuppressive properties along with procoagulant activities. Provision of a balanced combination of exogenous lipids results in lower levels of PGE2 and TBXA2 production, and possibly less host immunosuppression.
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