Protein

Adults Protein calories comprise a significant proportion of the energy requirement of a severely burned patient. Intact protein, rather than amino acids, is associated with better weight maintenance and improved survival. Nitrogen loss must be estimated regularly in a burn patient in order to ensure adequate nitrogen replacement. Total nitrogen loss (TNL) is impossible to measure accurately since 20-30% of nitrogen loss occurs in the exudate from wounds. There is some doubt regarding the use of urinary urea nitrogen (UUN) to estimate total urinary nitrogen (TUN), from which TNL is usually calculated. In healthy, unstressed subjects, urea comprises 80% of the TUN, but ureagenesis is inconsistent after burn injury and varies widely depending on the extent and course of illness. If measurement of TUN is available this will reflect nitrogen loss more accurately:

Total nitrogen loss must then be compared with nitrogen supply (NS) to calculate the nitrogen balance (NB):

The aim is to keep a positive balance, and a suitable starting point would be 2 or 3 g protein/kg lean body weight/day; 6.25 g protein is equivalent to 1 g nitrogen. Urinary excretion of 3-methyl histidine has been used as a measure of skeletal protein catabo-lism. Nitrogen input from blood products is appreciable, accounting for 15% of total nitrogen intake, but is often ignored when calculating nitrogen balance, which therefore underestimates protein intake.

Amino acids play an important role in adaption to burn injury both as gluconeogenic substrates and as substrates for acute phase protein synthesis and wound repair. Arginine flux appears to be increased in burns patients, but plasma levels of arginine and glutamine appear to be greatly reduced following burn injury. These changes have prompted supplementary feeding with particular amino acids. Interest has focused on ornithine a-ketoglu-tarate (OAK) and its metabolites arginine and glu-tamine. OAK is also a precursor for proline, the incorporation of which into collagen is a rate-limiting step in collagen synthesis. Arginine also increases collagen deposition, and in animal models of burn injury increased arginine provision has been associated with increased wound healing and improved immune function. There is evidence of a clinically important reduction in healing time and infectious episodes following OAK-, arginine-, or glutamine-supplemented feeding in human studies. Glutamine is the most abundant amino acid in the body and is the major fuel source for the intestine. Its presence prevents villous atrophy and maintains mucosal integrity as well as stimulates blood flow to the gut. Glutamine supplementation of feed reduces the incidence of gram-negative bacteremia in patients with severe burns. The proposed mechanism is the reduction in bacterial translocation across the gut wall; glutamine has been shown to reduce bacterial translocation in a rat model. However, the evidence for a significant clinical effect of arginine- or glutamine-supplemented enteral feed is still equivocal.

Children Protein needs are often estimated by formulas based on lean body weight. To estimate preburn weight, the 50th centile weight for height should be used. For children younger than 1 year old, 3 or 4g protein/kg lean body weight is suggested to provide adequate nutritional support for graft coverage and healing; this should be reduced to 2.5-3 g protein/kg lean body weight for children 1-3 years of age. In older children, protein requirements are further reduced to 1.5-2.5 g/kg lean body weight. When nitrogen balance is calculated for children, the following formulas have been suggested:

Age 0-4 years: TNL = UUN + 2 Age 4—10 years: TNL = UUN + 3

Protein-enriched diets, containing 25% calories as protein compared to 16% in normal diets, have been associated with improved nitrogen balance, improved immune function, and fewer infective episodes in children with severe burns. Until recently, albumin was a mainstay of fluid requirements in children with severe burns, which contributes to protein provision. However, studies have shown increased morbidity and mortality in critically ill patients given albumin. It seems that the outcome for children with severe burns is no worse if they receive albumin supplementation only when albumin levels decline below 10 g/l (or 15 g/l in the presence of intolerance of enteral feed). Use of albumin should be reviewed regularly.

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