Declines in muscle protein synthesis are a primary metabolic feature of post-operative catabolism: A systematic review and meta-analysis

BackgroundThough it is well appreciated that insulin plays an important role in the regulation of muscle protein metabolism, there is much discrepancy in the literature on the capacity of exogenous insulin administration to increase muscle protein synthesis ratesin vivoin humans.ObjectiveTo assess whether exogenous insulin administration increases muscle protein synthesis rates in young and older adults.DesignA systematic review of clinical trials was performed and the presence or absence of an increase in muscle protein synthesis rate was reported for each individual study arm. In a stepwise manner, multiple models were constructed that excluded study arms based on the following conditions: model 1, concurrent hyperaminoacidemia; model 2, insulin-induced hypoaminoacidemia; model 3, supraphysiological insulin concentrations; and model 4, older, more insulin resistant, subjects.ConclusionsFrom the presented data in the current systematic review, we conclude that: i) exogenous insulin and amino acid administration effectively increase muscle protein synthesis, but this effect is attributed to the hyperaminoacidemia; ii) exogenous insulin administered systemically induces hypoaminoacidemia which obviates any insulin-stimulatory effect on muscle protein synthesis; iii) exogenous insulin resulting in supraphysiological insulin levels exceeding 50 000 pmol/l may effectively augment muscle protein synthesis; iv) exogenous insulin may have a diminished effect on muscle protein synthesis in older adults due to age-related anabolic resistance; and v) exogenous insulin administered systemically does not increase muscle protein synthesis in healthy, young adults.

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Evaluating the Leucine Trigger Hypothesis to Explain the Post-prandial Regulation of Muscle Protein Synthesis in Young and Older Adults: A Systematic Review

Frontiers in Nutrition ◽

10.3389/fnut.2021.685165 ◽

2021 ◽

Vol 8 ◽

Author(s):

Gabriele Zaromskyte ◽

Konstantinos Prokopidis ◽

Theofilos Ioannidis ◽

Kevin D. Tipton ◽

Oliver C. Witard

Keyword(s):

Systematic Review ◽

Older Adults ◽

Protein Synthesis ◽

Muscle Protein ◽

Muscle Protein Synthesis ◽

Sufficient Evidence ◽

Concentration Profiles ◽

Post Exercise ◽

Protein Sources ◽

Protein Rich

Background: The “leucine trigger” hypothesis was originally conceived to explain the post-prandial regulation of muscle protein synthesis (MPS). This hypothesis implicates the magnitude (amplitude and rate) of post-prandial increase in blood leucine concentrations for regulation of the magnitude of MPS response to an ingested protein source. Recent evidence from experimental studies has challenged this theory, with reports of a disconnect between blood leucine concentration profiles and post-prandial rates of MPS in response to protein ingestion.Aim: The primary aim of this systematic review was to qualitatively evaluate the leucine trigger hypothesis to explain the post-prandial regulation of MPS in response to ingested protein at rest and post-exercise in young and older adults. We hypothesized that experimental support for the leucine trigger hypothesis will depend on age, exercise status (rest vs. post-exercise), and type of ingested protein (i.e., isolated proteins vs. protein-rich whole food sources).Methods: This qualitative systematic review extracted data from studies that combined measurements of post-prandial blood leucine concentrations and rates of MPS following ingested protein at rest and following exercise in young and older adults. Data relating to blood leucine concentration profiles and post-prandial MPS rates were extracted from all studies, and reported as providing sufficient or insufficient evidence for the leucine trigger hypothesis.Results: Overall, 16 of the 29 eligible studies provided sufficient evidence to support the leucine trigger hypothesis for explaining divergent post-prandial rates of MPS in response to different ingested protein sources. Of these 16 studies, 13 were conducted in older adults (eight of which conducted measurements post-exercise) and 14 studies included the administration of isolated proteins.Conclusion: This systematic review underscores the merits of the leucine trigger hypothesis for the explanation of the regulation of MPS. However, our data indicate that the leucine trigger hypothesis confers most application in regulating the post-prandial response of MPS to ingested proteins in older adults. Consistent with our hypothesis, we provide data to support the idea that the leucine trigger hypothesis is more relevant within the context of ingesting isolated protein sources rather than protein-rich whole foods. Future mechanistic studies are warranted to understand the complex series of modulatory factors beyond blood leucine concentration profiles within a food matrix that regulate post-prandial rates of MPS.

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Animal Protein versus Plant Protein in Supporting Lean Mass and Muscle Strength: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Nutrients ◽

10.3390/nu13020661 ◽

2021 ◽

Vol 13 (2) ◽

pp. 661 ◽

Cited By ~ 1

Author(s):

Meng Thiam Lim ◽

Bernice Jiaqi Pan ◽

Darel Wee Kiat Toh ◽

Clarinda Nataria Sutanto ◽

Jung Eun Kim

Keyword(s):

Systematic Review ◽

Muscle Strength ◽

Lean Mass ◽

Muscle Protein ◽

Muscle Protein Synthesis ◽

Meta Analysis ◽

Animal Protein ◽

Protein Source ◽

Plant Protein ◽

Younger Adults

Although animal protein is usually considered to be a more potent stimulator of muscle protein synthesis than plant protein, the effect of protein source on lean mass and muscle strength needs to be systematically reviewed. This study aimed to examine potential differences in the effect of animal vs. plant protein on lean mass and muscle strength, and the possible influence of resistance exercise training (RET) and age. The following databases were searched: PubMed, Embase, Scopus and CINAHL Plus with Full Text, and 3081 articles were screened. A total of 18 articles were selected for systematic review, of which, 16 were used for meta-analysis. Total protein intakes were generally above the recommended dietary allowance at the baseline and end of intervention. Results from the meta-analyses demonstrated that protein source did not affect changes in absolute lean mass or muscle strength. However, there was a favoring effect of animal protein on percent lean mass. RET had no influence on the results, while younger adults (<50 years) were found to gain absolute and percent lean mass with animal protein intake (weighted mean difference (WMD), 0.41 kg; 95% confidence interval (CI) 0.08 to 0.74; WMD 0.50%; 95% CI 0.00 to 1.01). Collectively, animal protein tends to be more beneficial for lean mass than plant protein, especially in younger adults.

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