GUT MUCOSAL PROTEIN-SYNTHESIS IN FED AND FASTED HUMANS

Fractional protein synthesis rate (FSR) of duodenal mucosa was measure d in two groups of six healthy young men, either in the fed state at t he end of a 10-day standardized diet or after a 36-h fast. Protein syn thesis rate was measured during a 9-h intravenous infusion of [C-13]le ucine and [H-2(5)]phenylalanine. The fed group also received an intrag astric tracer, [H-2(3)]leucine, mixed with the liquid diet. At the end of the tracer infusion, an endoscopy was performed to take duodenal m ucosal biopsies. The major results were that 1) duodenal mucosal prote in synthesis was high, 48.0 +/- 8.5% (SE)/day by use of intravenous le ucine tracer and intracellular leucine enrichment; 2) it was not affec ted 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 cal culated with the intravenous tracer, depending on the precursor pool u sed for the calculation Thus absolute values of FSR should be taken wi th caution, because they depend on the precursor pool chosen, the rout e of tracer administration, and the tracer itself. However, gut mucosa l protein synthesis as assessed by an intravenous tracer is not affect ed by feeding in humans.