Dose-Dependent Effects of Protein Ingestion and Resistance Exercise on Muscle Protein Synthesis in Aging Adults: A Literature Review

Abstract

Introduction: Sarcopenia can lead to physical disability and lower quality of life, but increasing muscle protein synthesis in older adults may reduce its effects. Dose-response curves may be used to determine the optimal protein dose in rested and exercised muscle to elicit maximal muscle protein synthesis.

Methods: A literature review was conducted to explore and summarize the findings on the following topics: the mechanisms of muscle protein synthesis, anabolic resistance, and the dose-responses of muscle protein synthesis to anabolic stimuli in both younger and older individuals.

Results: Reduced phosphorylation in downstream targets of the mammalian target of rapamycin complex 1 pathway is characteristic of muscle protein synthesis in older muscle. Compared to younger muscle, older muscle can elicit a similar maximal muscle protein synthesis response, but is less sensitive to lower doses of protein ingestion. With ingestion of 40g of whey protein, the fractional synthetic rate in older muscle is similar to that of younger muscle with 20g of whey protein ingestion. Marked increases in amino acid oxidation are also observed.

Discussion: Anabolic resistance can be biochemically explained by reduced phosphorylation in the mammalian target of rapamycin complex 1 pathway. Due to this phenomenon, older individuals require greater anabolic stimuli to achieve maximal muscle protein synthesis. However, the most effective protein dose for maximal muscle protein synthesis in older muscle is not well-established.

Conclusion: The muscle protein synthesis dose-response curve for older individuals reveals blunted responses to stimuli due to anabolic resistance. Further research is warranted to determine the optimal protein dose for maximal muscle protein synthesis in older adults.