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Received April 12, 2004
Revised April 27, 2004
Accepted after revision April 29, 2004
1 University of Trieste, Italy
2 German Sport University, Cologne, Germany
3 DLR-Institute of Aerospace Medicine, Cologne
* To whom correspondence should be addressed. E-mail: biolo{at}units.it.
Diminished muscular activity is associated with alterations of protein metabolism. The aim of this study was to evaluate the effect of short-term muscle inactivity on regulation of whole body protein deposition during amino acid infusion to simulate an experimental postprandial state. We studied 9 healthy young volunteers at the end of 14-day periods of strict bed rest and of controlled ambulation using a cross-over design. Subjects received a weight maintaining diet containing 1 g protein/kg per day. L[1-13C]leucine was used as a marker of whole body protein kinetics in the postabsorptive state and during 3 hour infusion of an amino acid mixture (0.13 g amino acid/kg lean body mass per hour). In the postabsorptive state, bed rest decreased (P<0.05) the rate of leucine disposal (Rd) to protein synthesis and tended to decrease leucine rate of appearance (Ra) from proteolysis, whereas the rate of leucine oxidation did not change significantly. Amino acid infusion increased leucine Rd to protein synthesis and oxidation and decreased leucine Ra from proteolysis in both the bed rest and ambulatory conditions. Changes from basal of leucine Rd to protein synthesis were lower (P<0.05) during bed rest than those in the ambulatory period, whereas changes of leucine Ra from proteolysis and oxidation were not significantly different. During amino acid infusion, net leucine deposition into body protein was 8±3% lower during the bed rest than the ambulatory phase. In conclusion, short-term bed rest leads to reduced stimulation of whole-body protein synthesis by amino acid administration.
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