Making Sense of Muscle Protein Synthesis: A Focus on Muscle Growth During Resistance Training

The Anabolic Response to Protein Ingestion During Recovery From Exercise Has No Upper Limit in Magnitude and Duration In Vivo in Humans: A Commentary

A comprehensive recent study by Trommelen et al. demonstrated that muscle tissue exhibits a greater capacity to incorporate exogenous exogenous protein-derived amino acids into bound muscle protein than was previously appreciated, at least when measured in "anabolically sensitive," recreationally active (but not resistance-trained), young men following resistance exercise. Moreover, this study demonstrated that the duration of the postprandial period is modulated by the dose of ingested protein contained within a meal, that is, the postexercise muscle protein synthesis response to protein ingestion was more prolonged in 100PRO than 25PRO. Both observations represent important scientific advances in the field of protein metabolism. However, we respectfully caution that the practical implications of these findings may have been misinterpreted, at least in terms of dismissing the concept of protein meal distribution as an important factor in optimizing muscle tissue anabolism and/or metabolic health. Moreover, based on emerging evidence, this idea that the anabolic response to protein ingestion has no upper limit does not appear to translate to resistance-trained young women.

Exercise-specific adaptations in human skeletal muscle: Molecular mechanisms of making muscles fit and mighty

The mechanisms leading to a predominantly hypertrophied phenotype versus a predominantly oxidative phenotype, the hallmarks of resistance training (RT) or aerobic training (AT), respectively, are being unraveled. In humans, exposure of naïve persons to either AT or RT results in their skeletal muscle exhibiting generic 'exercise stress-related' signaling, transcription, and translation responses. However, with increasing engagement in AT or RT, the responses become refined, and the phenotype typically associated with each form of exercise emerges. Here, we review some of the mechanisms underpinning the adaptations of how muscles become, through AT, 'fit' and RT, 'mighty.' Much of our understanding of molecular exercise physiology has arisen from targeted analysis of post-translational modifications and measures of protein synthesis. Phosphorylation of specific residue sites has been a dominant focus, with canonical signaling pathways (AMPK and mTOR) studied extensively in the context of AT and RT, respectively. These alone, along with protein synthesis, have only begun to elucidate key differences in AT and RT signaling. Still, key yet uncharacterized differences exist in signaling and regulation of protein synthesis that drive unique adaptation to AT and RT. Omic studies are required to better understand the divergent relationship between exercise and phenotypic outcomes of training.

Mitigating disuse-induced skeletal muscle atrophy in ageing: Resistance exercise as a critical countermeasure

The gradual deterioration of physiological systems with ageing makes it difficult to maintain skeletal muscle mass (sarcopenia), at least partly due to the presence of 'anabolic resistance', resulting in muscle loss. Sarcopenia can be transiently but markedly accelerated through periods of muscle disuse-induced (i.e., unloading) atrophy due to reduced physical activity, sickness, immobilisation or hospitalisation. Periods of disuse are detrimental to older adults' overall quality of life and substantially increase their risk of falls, physical and social dependence, and early mortality. Disuse events induce skeletal muscle atrophy through various mechanisms, including anabolic resistance, inflammation, disturbed proteostasis and mitochondrial dysfunction, all of which tip the scales in favour of a negative net protein balance and subsequent muscle loss. Concerningly, recovery from disuse atrophy is more difficult for older adults than their younger counterparts. Resistance training (RT) is a potent anabolic stimulus that can robustly stimulate muscle protein synthesis and mitigate muscle losses in older adults when implemented before, during and following unloading. RT may take the form of traditional weightlifting-focused RT, bodyweight training and lower- and higher-load RT. When combined with sufficient dietary protein, RT can accelerate older adults' recovery from a disuse event, mitigate frailty and improve mobility; however, few older adults regularly participate in RT. A feasible and practical approach to improving the accessibility and acceptability of RT is through the use of resistance bands. Moving forward, RT must be prescribed to older adults to mitigate the negative consequences of disuse atrophy.

Fueling for and recovering from resistance training: The periworkout nutrition practices of competitive powerlifters

PURPOSE

Nutrient timing is a concept that emphasizes the intentional ingestion of whole or fortified foods, and dietary supplements, to adequately fuel for, and recover from, acute and chronic exercise. The nutrition strategies used by powerlifters around training sessions have not, to our knowledge, been previously investigated. This study explored the self-reported periworkout (before, during, and after) nutrition practices of competitive powerlifters, including what, why, and information source that informed practice, with comparison to current sport nutrition guidelines.

METHODS

Actively competing male (n = 240) and female (n = 65) powerlifters completed a cross-sectional online survey of self-reported periworkout nutrition practices in the pre-, intra-, and postexercise periods, fasted training, and supplementation. Data are presented as the number (n) and percentage (%) of all powerlifters practicing a given strategy followed by a % of responses reporting various practices or beliefs within this strategy. Categorical subgroups (sex, age, and weight class; and competitive caliber) were analyzed with a chi-square test or Fisher's exact test and denoted where significant (P ≤ 0.05).

RESULTS

Most powerlifters reported paying specific attention to nutrition practices in the pre-exercise period (n = 261; 85.6%) by ingesting more carbohydrate (CHO) rich foods (n = 234; 89.6%) for the purpose of assisting in training performance (n = 222; 85.1%). Most powerlifters reported intraexercise nutrition strategies (n = 211; 69.2%), of which most included ingesting more CHO rich foods (n = 159; 74.5%) for the purpose of feeling less hungry and/or boosting energy levels during training (n = 129; 61.1%). Most powerlifters reported paying attention to postexercise nutrition (n = 244; 80%), by ingesting more protein rich foods (n = 182; 74.6%) for the purpose of recovering better for the whole day (n = 152; 62.3%) and enhancing the benefits of training (n = 149; 61.1%). Most powerlifters did not complete training sessions in the fasted state (n = 262; 85.9%). Most powerlifters reported paying attention to supplementation before training (n = 237; 77.7%), of which preworkout formulas (n = 137; 57.8%), energy drinks (n = 101; 42.6%), creatine (n = 88; 37.1%), and caffeine pills (n = 70; 29.5%) were most reported. Supplementation was used to assist in training performance (n = 197; 83.1%) and increase wakefulness/alertness (n = 183; 77.2%). Males reported more often than females that they informed multiple elements of their nutrition practices with the information they read or watched somewhere (P = 0.002-0.012).

CONCLUSION

The periworkout nutrition practices used by competitive powerlifters followed current sport nutrition guidelines, by using CHO sources to fuel for training and ensuring the provision of protein postexercise. Competitive powerlifters may wish to exert caution with supplementation, as there is a risk of harm or inadvertent doping.

The effect of protein or amino acid provision on immobilization-induced muscle atrophy in healthy adults: A systematic review and meta-analysis

Bed rest and limb immobilization are models of muscle disuse associated with skeletal muscle atrophy and reduced strength. The purpose of this systematic review was to examine the impact of protein or amino acid provision before and/or during a period of muscle disuse on muscle atrophy (primary outcome), strength and muscle protein synthesis (secondary outcomes) following a disuse period. We performed a systematic review of Embase, MEDLINE, Web of Science, PubMed and Clinical Trials in December 2022. Eligible studies were randomized controlled trials that combined a dietary protein or amino acid intervention versus control during an experimental model of disuse (bed rest or unilateral limb immobilization) in healthy individuals aged ≥18 years. Nine articles from eight independent trials were identified and rated for risk of bias by two authors. A meta-analysis of muscle mass data revealed no effect (standardized mean difference: 0.2; 95% confidence interval: -0.18 to 0.57, P = 0.31) of protein/amino acid intervention in preventing disuse-induced muscle atrophy. Although the meta-analysis was not conducted on strength or muscle protein synthesis data, there was insufficient evidence in the reviewed articles to support the use of protein/amino acid provision in mitigating the disuse-induced decline in either outcome measurement. Additional high-quality studies, including the reporting of randomization procedures and blinding procedures and the provision of statistical analysis plans, might be required to determine whether protein or amino acid provision serves as an effective strategy to attenuate muscle atrophy during periods of disuse.

Long-chain n -3 polyunsaturated fatty acids for the management of age- and disease-related declines in skeletal muscle mass, strength and physical function

PURPOSE OF REVIEW

This review uses the hierarchy of evidence as a framework to critically evaluate the effect of long chain n -3 polyunsaturated fatty acid (LC n -3 PUFA) ingestion alone, or as an adjunctive intervention to resistance training, on muscle health-related outcomes in healthy and clinical older adult populations.

RECENT FINDINGS

Systematic reviews and meta-analyses of randomized controlled trials consistently report small, but clinically-relevant, effects of LC n -3 PUFA ingestion on strength outcomes, whereas mixed findings have been reported regarding changes in muscle mass and physical function. Cohort studies indicate an association between higher dietary LC n -3 PUFA intake and reduced likelihood of a sarcopenia diagnosis. Acute metabolic studies provide limited evidence for an effect of LC n -3 PUFA ingestion alone, or in combination with resistance training, on free-living integrated rates of MPS, static markers of muscle protein breakdown, or satellite cell activation in healthy older adults.

SUMMARY

Recent data supports the efficacy of LCn-3 PUFA ingestion to facilitate small, but clinically relevant, improvements in muscle strength in healthy and clinical older adult populations. The mechanism(s) that underpin the action of LC n -3 PUFA in promoting strength outcomes remain unknown, but likely relate to neuromuscular function.

The effect of fish oil supplementation on resistance training-induced adaptations

BACKGROUND

Resistance exercise training (RET) is a common and well-established method to induce hypertrophy and improvement in strength. Interestingly, fish oil supplementation (FOS) may augment RET-induced adaptations. However, few studies have been conducted on young, healthy adults.

METHODS

A randomized, placebo-controlled design was used to determine the effect of FOS, a concentrated source of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), compared to placebo (PL) on RET-induced adaptations following a 10-week RET program (3 days·week^-1). Body composition was measured by dual-energy x-ray absorptiometry (LBM, fat mass [FM], percent body fat [%BF]) and strength was measured by 1-repetition maximum barbell back squat (1RM_SQT) and bench press (1RM_BP) at PRE (week 0) and POST (10 weeks). Supplement compliance was assessed via self-report and bottle collection every two weeks and via fatty acid dried blood spot collection at PRE and POST. An a priori α-level of 0.05 was used to determine statistical significance and Cohen's d was used to quantify effect sizes (ES).

RESULTS

Twenty-one of 28 male and female participants (FOS, n = 10 [4 withdrawals]; PL, n = 11 [3 withdrawals]) completed the 10-week progressive RET program and PRE/POST measurements. After 10-weeks, blood EPA+DHA substantially increased in the FOS group (+109.7%, p< .001) and did not change in the PL group (+1.3%, p = .938). Similar between-group changes in LBM (FOS: +3.4%, PL: +2.4%, p = .457), FM (FOS: -5.2%, PL: 0.0%, p = .092), and %BF (FOS: -5.9%, PL: -2.5%, p = .136) were observed, although, the between-group ES was considered large for FM (d = 0.84). Absolute and relative (kg·kg [body mass]^-1) 1RM_BP was significantly higher in the FOS group compared to PL (FOS: +17.7% vs. PL: +9.7%, p = .047; FOS: +17.6% vs. PL: +7.3%, p = .011; respectively), whereas absolute 1RM_SQT was similar between conditions (FOS: +28.8% vs. PL: +20.5%, p = .191). Relative 1RM_SQT was higher in the FOS group (FOS: +29.3% vs. PL: +17.9%, p = .045).

CONCLUSIONS

When combined with RET, FOS improves absolute and relative 1RM upper-body and relative 1RM lower-body strength to a greater extent than that observed in the PL group of young, recreationally trained adults.

Pre-sleep protein supplementation does not improve performance, body composition, and recovery in British Army recruits (part 1)

Dietary protein is crucial for optimising physical training adaptations such as muscular strength and mass, which are key aims for athletic populations, including British Army recruits. New recruits fail to meet the recommended protein intake during basic training (BT), with negligible amounts consumed in the evening. This study assessed the influence of a daily bolus of protein prior to sleep on performance adaptations, body composition and recovery in British Army recruits. 99 men and 23 women [mean ± standard deviation (SD): age: 21.3 ± 3.5 years, height: 174.8 ± 8.4 cm, body mass 75.4 ± 12.2 kg] were randomised into a dietary control (CON), carbohydrate placebo (PLA), moderate (20 g) protein (MOD) or high (60 g) protein (HIGH) supplementation group. Supplements were isocaloric and were consumed on weekday evenings between 2000 and 2100 for 12 weeks during BT. Performance tests (mid-thigh pull, medicine ball throw, 2 km run time, maximal push-up, and maximal vertical jump) and body composition were assessed at the start and end of BT. Dietary intake, energy expenditure, salivary hormones, urinary nitrogen balance, perceived muscle soreness, rating of perceived exertion, mood, and fatigue were assessed at the start, middle and end of BT. Protein supplementation increased protein intake in HIGH (2.16 ± 0.50 g⸱kg-1⸱day-1) and MOD (1.71 ± 0.48 g⸱kg-1⸱day-1) compared to CON (1.17 ± 0.24 g⸱kg-1⸱day-1) and PLA (1.31 ± 0.29 g⸱kg-1⸱day-1; p < 0.001). Despite this, there was no impact of supplementation on mid-thigh pull performance (CON = 7 ± 19%, PLA = 7 ± 19%, MOD = 0 ± 16%, and HIGH = 4 ± 14%; p = 0.554) or any other performance measures (p > 0.05). Fat-free mass changes were also similar between groups (CON = 4 ± 3%, PLA = 4 ± 4%, MOD = 3 ± 3%, HIGH = 5 ± 4%, p = 0.959). There was no impact of protein supplementation on any other body composition or recovery measure. We conclude no benefits of pre-bed protein supplementation to improve performance, body composition and recovery during BT. It is possible the training stimulus was great enough, limiting the impact of protein supplementation. However, the high degree of inter-participant variability suggests an individualised use of protein supplementation should be explored, particularly in those who consume sub-optimal (<1.6 g⸱kg-1⸱day-1) habitual amounts of protein. Clinical trial registration: The study was registered with ClinicalTrials.gov, U.S. national institutes (identifier: NCT05998590).

Mechanisms of mechanical overload-induced skeletal muscle hypertrophy: current understanding and future directions

Mechanisms underlying mechanical overload-induced skeletal muscle hypertrophy have been extensively researched since the landmark report by Morpurgo (1897) of "work-induced hypertrophy" in dogs that were treadmill trained. Much of the preclinical rodent and human resistance training research to date supports that involved mechanisms include enhanced mammalian/mechanistic target of rapamycin complex 1 (mTORC1) signaling, an expansion in translational capacity through ribosome biogenesis, increased satellite cell abundance and myonuclear accretion, and postexercise elevations in muscle protein synthesis rates. However, several lines of past and emerging evidence suggest that additional mechanisms that feed into or are independent of these processes are also involved. This review first provides a historical account of how mechanistic research into skeletal muscle hypertrophy has progressed. A comprehensive list of mechanisms associated with skeletal muscle hypertrophy is then outlined, and areas of disagreement involving these mechanisms are presented. Finally, future research directions involving many of the discussed mechanisms are proposed.

Eggs: Healthy or Risky? A Review of Evidence from High Quality Studies on Hen's Eggs

Hen's eggs (from Gallus gallus domesticus) provide choline, folate, vitamin D, iodine, B vitamins and high-quality protein and are no longer viewed by national bodies as a risk factor for hypercholesterolaemia and cardiovascular disease (CVD). Yet, questions remain about the benefits and risks of eating eggs regularly. This review evaluates recent high-quality evidence from randomised controlled trials (RCT) and meta-analyses of observational studies and considers new areas of interest, such as weight management, protein metabolism, allergy risk and sustainability. In several RCT, eggs increased muscle protein synthesis and lowered fat mass, which could support optimal body composition. Eggs within a meal improved satiety, which could translate into lower energy intakes, although more RCT are needed. In observational studies, higher egg consumption was associated with a null effect or a modest reduced risk of CVD. For type 2 diabetes (T2D) incidence and risk of CVD in people with T2D, there were inconsistencies between observational and RCT data, with the former noting positive associations and the latter seeing no effect of higher egg intake on markers of T2D and CVD. Sustainability metrics suggest that eggs have the lowest planetary impact amongst animal proteins. To lower allergy risk, earlier introduction of eggs into weaning diets is warranted. In conclusion, the balance of evidence points to eggs being a nutritious food suggesting there are broad health benefits from including eggs in the diet at intakes higher than that currently consumed by European populations.

Acute Quark Ingestion Increases Muscle Protein Synthesis Rates at Rest with a Further Increase after Exercise in Young and Older Adult Males in a Parallel-Group Intervention Trial

BACKGROUND

Ingestion of protein concentrates or isolates increases muscle protein synthesis rates in young and older adults. There is far less information available on the anabolic response following the ingestion of dairy wholefoods, which are commonly consumed in a normal diet.

OBJECTIVES

This study investigates whether ingestion of 30 g protein provided as quark increases muscle protein synthesis rates at rest and whether muscle protein synthesis rates are further increased after resistance exercise in young and older adult males.

METHODS

In this parallel-group intervention trial, 14 young (18-35 y) and 15 older (65-85 y) adult males ingested 30 g protein provided as quark after a single-legged bout of resistance exercise on leg press and leg extension machines. Primed, continuous intravenous L-[ring-¹³C₆]-phenylalanine infusions were combined with the collection of blood and muscle tissue samples to assess postabsorptive and 4-h postprandial muscle protein synthesis rates at rest and during recovery from exercise. Data represent means ± SDs; η² was used to measure the effect size.

RESULTS

Plasma total amino acid and leucine concentrations increased after quark ingestion in both groups (both time: P < 0.001; η² > 0.8), with no differences between groups (time × group: P = 0.127 and P = 0.172, respectively; η²<0.1). Muscle protein synthesis rates increased following quark ingestion at rest in both young (from 0.030 ± 0.011 to 0.051 ± 0.011 %·h^-1) and older adult males (from 0.036 ± 0.011 to 0.062 ± 0.013 %·h^-1), with a further increase in the exercised leg (to 0.071 ± 0.023 %·h^-1 and to 0.078 ± 0.019 %·h^-1, respectively; condition: P < 0.001; η² = 0.716), with no differences between groups (condition × group: P = 0.747; η² = 0.011).

CONCLUSIONS

Quark ingestion increases muscle protein synthesis rates at rest with a further increase following exercise in both young and older adult males. The postprandial muscle protein synthetic response following quark ingestion does not differ between healthy young and older adult males when an ample amount of protein is ingested. This trial was registered at the Dutch Trial register, which is accessible via trialsearch.who.int www.trialregister.nl as NL8403.

Longing for a Longitudinal Proxy: Acutely Measured Surface EMG Amplitude is not a Validated Predictor of Muscle Hypertrophy

Surface electromyography amplitudes are commonly measured in acute sports and exercise science studies to make inferences about muscular strength, performance, and hypertrophic adaptations that may result from different exercises or exercise-related variables. Here, we discuss the presumptive logic and assumptions underlying these inferences, focusing on hypertrophic adaptations for simplicity's sake. We present counter-evidence for each of its premises and discuss evidence both for and against the logical conclusion. Given the limited evidence validating the amplitude of surface electromyograms as a predictor of longitudinal hypertrophic adaptations, coupled with its weak mechanistic foundation, we suggest that acute comparative studies that wish to assess stimulus potency be met with scrutiny.