1. Whole-body protein metabolism was determined by a primed constant-rate infusion of l-[l-14C]leucine in patients before and after elective surgery, the nutritional intake being carefully controlled and the surgical stress in individuals being assessed.

2. Pre-operatively, whole-body protein flux (P < 0.05) and synthesis (P < 0.05), along with amino acid oxidation (P < 0.01), increased with nutritional intake whereas protein breakdown remained unaltered. Whole-body protein balance also correlated with intake (P = 0.001).

3. Postoperatively, whole-body protein metabolism was determined with patients either fasted (group 1) or fed (group 2) and the change in metabolism in each individual from a pre-operative study, carried out in the fed state, was calculated. Whole-body protein breakdown increased in both groups (group 1, + 0.91 ±0.74 g day−1 kg−1; mean ± sd, n = 7: group 2, +0.47, +0.63 and +1.01 g day−1 kg−1n = 3), the change being significant in those fasted after surgery (P<0.05). However, the pattern of change in whole-body protein synthesis was entirely different in each group, rising in those fed throughout (+ 0.32, + 0.41 and + 0.66 g day−1kg−1n = 3) but falling in those fasted after surgery (−0.38, −0.80 and −1.33 g day−1kg−1n = 3). The changes in metabolism appeared more marked in those undergoing greatest surgical stress.

4. Some of the factors involved in the calculations are discussed and their effects on the overall conclusions are considered.

5. A concept of whole-body protein metabolism in the metabolic response to trauma is advocated whereby protein breakdown is largely obligatory to the response, whereas synthesis responds to substrate availability.

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