Dietary proteins/essential amino acids (EAAs) are nutrients with anabolic properties that may increase muscle mass or attenuate muscle loss during immobilization and aging via the stimulation of muscle protein synthesis (MPS). An EAA's anabolic threshold, capable to maximize the stimulation of MPS has been hypothesized, but during certain conditions associated with muscle loss, this anabolic threshold seems to increase which reduces the efficacy of dietary EAAs to stimulate MPS. Preliminary studies have demonstrated that acute ingestion of dietary proteins/EAA (with a sufficient amount of leucine) was capable to restore the postprandial MPS during bed rest, immobilization or aging; however, whether these improvements translate into chronic increases (or attenuates loss) of muscle mass is equivocal. For example, although free leucine supplementation acutely increases MPS and muscle mass in some chronic studies, other studies have reported no increases in muscle mass following chronic leucine supplementation. In contrast, chronically increasing leucine intake via the consumption of an overall increase in dietary protein appears to be the most effective dietary intervention toward increasing or attenuating lean mass during aging; however, more research investigating the optimal dose and timing of protein ingestion is necessary. Several studies have demonstrated that decreases in postprandial MPS as a result of increased circulating oxidative and inflammatory are more responsible than muscle protein breakdown for the decreases in muscle mass during disuse and health aging. Therefore, nutritional interventions that reduce oxidation or inflammation in conjunction with higher protein intakes that overcome the anabolic resistance may enhance the MPS response to feeding and either increase muscle mass or attenuate loss. In preliminary studies, antioxidant vitamins and amino acids with antioxidant or anti-inflammatory properties show potential to restore the anabolic response associated with protein ingestion. More research, however, is required to investigate if these nutrients translate to increases in MPS and, ultimately, increased lean mass in aging humans. The purpose of the present review is to discuss the role of protein/EAA intake to enhance postprandial MPS during conditions associated with muscle loss, and bring new perspectives and challenges associated nutritional interventions aimed to optimize the anabolic effects of dietary protein/EAAs ingestion.
Keywords: Amino acids; Atrophy; Betaine; Essential amino acids; Glycine; Hypertrophy; Leucine; Postprandial muscle protein synthesis; Proteins.