Protein Catabolism The rate of endogenous protein breakdown (catabolism) to supply energy needs normally falls during uncomplicated energy deprivation. After about 10 days of total starvation, the unstressed individual loses about 12–18 g/d protein (equivalent to approximately 2 oz of muscle tissue or 2–3 g of nitrogen). By contrast, in injury and sepsis, protein breakdown accelerates in proportion to the degree of stress, to 30–60 g/d after elective surgery, 60–90 g/d with infection, 100–130 g/d with severe sepsis or skeletal trauma, and 175 g/d with major burns or head injuries. These losses are reflected by proportional increases in the excretion. | Chapter 072. Malnutrition and Nutritional Assessment Part 4 Protein Catabolism The rate of endogenous protein breakdown catabolism to supply energy needs normally falls during uncomplicated energy deprivation. After about 10 days of total starvation the unstressed individual loses about 12-18 g d protein equivalent to approximately 2 oz of muscle tissue or 2-3 g of nitrogen . By contrast in injury and sepsis protein breakdown accelerates in proportion to the degree of stress to 30-60 g d after elective surgery 60-90 g d with infection 100-130 g d with severe sepsis or skeletal trauma and 175 g d with major burns or head injuries. These losses are reflected by proportional increases in the excretion of urea nitrogen the major byproduct of protein breakdown. Gluconeogenesis The major aim of protein catabolism during a state of starvation is to provide the glucogenic amino acids especially alanine and glutamine that serve as substrates for endogenous glucose production gluconeogenesis in the liver. In the hypometabolic starved state protein breakdown for gluconeogenesis is minimized especially as ketones derived from fatty acids become the substrate preferred by certain tissues. In the hypermetabolic stress state gluconeogenesis increases dramatically and in proportion to the degree of the insult to increase the supply of glucose the major fuel of reparation . Glucose is the only fuel that can be utilized by hypoxic tissues anaerobic glycolysis white blood cells and newly generated fibroblasts. Infusions of glucose partially offset a negative energy balance but do not significantly suppress the high rates of gluconeogenesis in the catabolic patient. Hence adequate supplies of protein are needed to replace the amino acids utilized for this metabolic response. In summary the hypometabolic patient is adapted to starvation and conserves body mass by reducing the metabolic rate and using fat as the primary fuel rather than glucose and its precursor amino acids . The .
