1 Rates of protein synthesis have been measured from the incorporation of 57 mg of l-[1-13C]leucine/kg for 90 min into muscle tissue and colorectal tumours removed at surgery from cancer patients.

2. For the 20 h preceding surgery and during the measurement of protein synthesis, the patients received intravenous saline, conventional intravenous nutrition (0.2 g of N and 103 non-protein kJ/kg body weight) or intravenous nutrition enriched with the branched-chain amino acids leucine, isoleucine and valine (0.2 g of N with 30% from branched-chain amino acids and 103 non-protein kJ/kg body weight).

3. Conventional intravenous nutrition resulted in a significant stimulation of the rate of protein synthesis in both muscle tissue (2.64 ± 0.75%/day versus 1.78 ± 0.51%/day in saline control, means ± SD) and tumour tissue (43.9 ± 10.3%/day versus 22.6 ± 5.6%/day in saline control).

4. Pre-operative nutrition enriched with branched-chain amino acids was less effective than conventional intravenous nutrition in stimulating protein synthesis in both muscle and tumour. The rates of protein synthesis were 2.12 ± 0.41%/day in muscle and 33.7 ± 5.3%/day in the tumours.

5. The expression of proliferating cell nuclear antigen in sections of the tumours showed changes with intravenous feeding of the two different amino acid mixtures that were similar to the changes in protein synthesis, and these two variables were significantly correlated. This is evidence that feeding with conventional mixtures and mixtures enriched with branched-chain amino acids stimulates tumour growth.

6. In this study the mixture enriched with branched-chain amino acids provided no clear advantage for cancer patients, since a smaller response to branched-chain amino acids was observed in both tumours and host muscle tissue.

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