Short-term bed rest impairs amino acid-induced protein anabolism in humans

Abstract

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 nine 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(-1) day(-1). l[1-(13)C]leucine was used as a marker of whole-body protein kinetics in the postabsorptive state and during a 3 h infusion of an amino acid mixture (0.13 g amino acid (kg lean body mass)(-1) h(-1)). In the postabsorptive state, bed rest decreased (P < 0.05) the rate of leucine disposal (R(d)) to protein synthesis and tended to decrease leucine rate of appearance (R(a)) from proteolysis, whereas the rate of leucine oxidation did not change significantly. Amino acid infusion increased leucine R(d) to protein synthesis and oxidation and decreased leucine R(a) from proteolysis in both the bed rest and ambulatory conditions. Changes from basal in leucine R(d) to protein synthesis were lower (P < 0.05) during bed rest than those in the ambulatory period, whereas changes in leucine R(a) from proteolysis and oxidation were not significantly different. During amino acid infusion, net leucine deposition into body protein was 8 +/- 3% lower during bed rest than during the ambulatory phase. In conclusion, short-term bed rest leads to reduced stimulation of whole-body protein synthesis by amino acid administration. Results of this study were, in part, presented at the meeting, Experimental Biology, 2004, Washington DC.

Figures

Figure 1. Plasma tracer/tracee ratios at steady-state

Plasma [13C]KIC tracer/tracee ratios during primed-continuous l[1-13C]leucine infusion in the basal postabsorptive period (min 160, 170 and 180) and at the end of amino acid infusion (min 340, 350 and 360) in the bed rest and ambulatory phases.

Figure 2. Leucine deposition into protein

Rates of net leucine deposition (i.e. Rd to protein synthesis minus Ra from proteolysis) into body protein during amino acid infusion in ambulatory and bed rest conditions. *P < 0.05, bed rest versus ambulatory.