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Animal and human studies have focused on the potential mechanism of the decreased skeletal muscle sensitivity to amino acids in older individuals. A defect in branched chain amino acid (BCAA) pathway activation may be responsible for this alteration [8]. Consequently, the alteration of muscle protein synthesis response to anabolic signals may be counteracted by nutritional strategies aiming at improving BCAA availability. For example, in vitro or in vivo high leucine administration is able to stimulate muscle protein synthesis rate in aged rodents [21, 80, 81]. In these models, leucine acts as an actual mediator able to drive specific intracellular pathways linked with the stimulation of protein translation [82]. Interestingly, when given to old rats for 10 days, the beneficial effect of leucine supplementation persisted, indicating that a long‐term utilization of leucine‐enriched diets may limit muscle wasting in aged individuals [83]. In addition, these data suggest that nutritional manipulations increasing the availability of leucine into skeletal muscle, such as the utilization of a leucine‐rich fast protein, i.e. whey protein, could be beneficial to improve protein retention during aging. The beneficial effect of such a diet on muscle protein synthesis in aged humans has been determined in several studies [68, 84–86]. It is concluded that leucine is able to stimulate muscle protein synthesis in the short term but may not on a longer term [87].

Special attention has also been focused on the impact of citrulline on muscle protein synthesis. This AA is not incorporated in the protein matrix during protein synthesis but it has been shown to stimulate muscle protein synthesis in malnourished animals [88]. Other amino acids, like glutamine, may be helpful to preserve skeletal muscle in older persons, especially in stressed conditions [89].