Substrate oxidation and the contribution of protein oxidation to energy expenditure after severe head injury.

The 'flow' phase response to head injury is characterized by hypermetabolism and catabolism of lean body mass. In order to measure the contribution of protein oxidation (CPO) to resting metabolic expenditure (RME), 11 severely head injured patients (AIS 5) were studied. All patients had 24 h urine collections for at least 10 days after injury and RME was determined at intervals by indirect calorimetry. No patient received exogenous steroids. Peak urinary nitrogen excretion was 11.63 +/- 1.28 g/m2/day occurring between days 6 and 9 after injury. Fat oxidation was the greatest component of the RME at all times after head injury and the CPO to RME was 26.4 +/- 2.9 per cent during days 1-2, 31.8 +/- 3.3 per cent during days 3-5, 28.6 +/- 3.4 per cent during days 6-9 and 23.3 +/- 3.8 per cent during days 10-20 after injury. These figures are higher than those previously reported for burns, musculoskeletal injury or sepsis. The mechanism for the increased CPO is unclear. It may be related to such conditions of management as paralysis and fasting, but more likely it is an idiosyncratic feature of the metabolic response to head injury.