1. The mechanism of the purported protein-sparing effects of different postoperative intravenous regimens still remains controversial. We have therefore measured circulating concentrations of metabolites and hormones in blood and urine nitrogen excretion in patients receiving (a) sodium chloride solution (saline), (b) 1.5 g of glucose day−1 kg−1 body weight, (c) 1.5 g of glucose day−1 kg−1 with insulin (0.56 unit day−1 kg−1) or (d) mixed amino acids (1.5 g day−1 kg−1) after abdominal operation.

2. Compared with results for saline-treated patients, glucose infusion resulted in raised glucose and insulin concentrations and lowered ketone body and non-esterified fatty acid concentrations but did not influence protein catabolism.

3. Addition of insulin lowered blood glucose by approximately 1 mmol/l. Total nitrogen excretion during glucose/insulin infusion was significantly less than during saline infusion.

4. Infusion of amino acids, compared with saline infusion, resulted in raised blood glucose, alanine, serum insulin and plasma glucagon concentrations but lower concentrations of plasma non-esterified fatty acids and blood ketone bodies. Insulin concentrations, however, were similar in both amino acid- and glucose-treated groups.

5. Amino acid infusion increased urea and total nitrogen excretion but net nitrogen loss was only 1.9 mmol of nitrogen day−1 kg−1 compared with 12.7 mmol day−1 kg−1 in the saline-treated group and 11.0 mmol day−1 kg−1 in the glucose-treated group.

6. Glucose (and insulin) infusion appeared to inhibit gluconeogenesis, and amino acids to enhance it. The nitrogen-sparing effect of amino acids appears largely related to their mass and is apparently unrelated to changes in ketone bodies and insulin.

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