Background
The loss of muscle mass during disuse has been attributed to a decline in basal and post-prandial muscle protein synthesis (MPS) rates. Although leucine supplementation has been suggested to stimulate MPS. Therefore, the present study assessed the capacity of leucine supplementation to attenuate the loss of muscle mass and the decline in daily MPS rates during immobilization.

Methods
24 young (23±4 y) and 24 older (69±4 y) adults (male/female: 1/1) underwent 3 days of unilateral knee immobilization by means of a full leg cast with leucine (LEU) or maltodextrin (PLA) supplementation (5 g, three times daily). Prior to and directly after immobilization, quadriceps muscle cross-sectional area (CSA) was assessed by Computed Tomography and a biopsy from the m. vastus lateralis was obtained. Daily MPS rates over the immobilization period were determined by applying deuterated water. Data were analyzed with a 2-way repeated measures ANOVA, and paired and independent samples t-tests. Data are expressed as means +/- standard deviation.

Results
Quadriceps muscle CSA decreased by ~1%, with no differences between the LEU and PLA group in young (from 6993±1174 to 6910±1185 mm², and from 7330±1147 to 7241±1095 mm² respectively; time*treatment: P=0.92) or older adults (from 5753±1086 to 5679±1073 mm² and from 5864±1146 to 5823±1161 mm² respectively; time*treatment: P=0.50). MPS rates were respectively ~15% and ~23% lower in the immobilized compared to the non-immobilized leg of young (1.28±0.29 vs 1.50±0.26%/d, respectively) and older (1.10±0.16 vs 1.47±0.28%/d) adults. The MPS rates in the immobilized leg did not differ between the LEU and PLA groups in young (1.29±0.24 vs 1.26±0.34%/d, respectively, P=0.83) or older (1.10±0.14 vs 1.10±0.19%/d, respectively; P=0.90) adults. 

Conclusion
Free leucine supplementation does not attenuate the decline in MPS rates or preserve leg muscle mass during short-term immobilization in young or older individuals.