Gut mucosal protein synthesis in fed and fasted humans

Fractional protein synthesis rate (FSR) of duodenal mucosa was measured in two groups of six healthy young men, either in the fed state at the end of a 10-day standardized diet or after a 36-h fast. Protein synthesis rate was measured during a 9-h intravenous infusion of [13C]leucine and [2H5]phenylalanine. The fed group also received an intragastric tracer, [2H3]leucine, mixed with the liquid diet. At the end of the tracer infusion, an endoscopy was performed to take duodenal mucosal biopsies. The major results were that 1) duodenal mucosal protein synthesis was high, 48.0 +/- 8.5% (SE)/day by use of intravenous leucine tracer and intracellular leucine enrichment; 2) it was not affected by feeding whatever the tracer or the precursor pool used for the calculations; 3) the two intravenous tracers gave different FSR values; and 4) with the intragastric tracer, FSR was 25-220% of the rate calculated with the intravenous tracer, depending on the precursor pool used for the calculation. Thus absolute values of FSR should be taken with caution, because they depend on the precursor pool chosen, the route of tracer administration, and the tracer itself. However, gut mucosal protein synthesis as assessed by an intravenous tracer is not affected by feeding in humans.