In a 70kg man there are about 10 to 11kg of protein. Daily protein turnover is 250 to 300g. After digestion, all amino acids are absorbed, save 1g of nitrogen which is excreted in the stool. Proteolysis accounts for another 50 to 70g of amino acids. In total, ingested amino acids contribute only 25g to the free amino acid pool, whereas 250 g is provided by endogenous breakdown. Protein turnover decreases with age, but as lean body mass increases, total body turnover remains approximately the same.

Calorie supply is important. Carbohydrate increases muscle protein synthesis under the influence of insulin, fat increases hepatic and other visceral protein synthesis. The average normal requirement of a patient is 0.8g of protein/kg/day. Any kind of trauma, including surgical one, increases this requirement.

The increase in resting energy expenditure that have been shown to occur during catabolic phase of trauma situations (injury and infection), when compared with the decrease that develop during partial starvation.