Protein requirements may be lower on a training compared to rest day but are not influenced by moderate training volumes in endurance trained males

The impact of training volume on protein requirements in endurance trained males was investigated with indicator amino acid oxidation (IAAO) methodology on a recovery day (REST) or after a 10 or 20 km run while consuming a single suboptimal protein intake (0.93 g/kg/day). Phenylalanine excretion (F13CO2; inverse proxy for whole body protein synthesis) was greatest and phenylalanine net balance was lowest on REST compared to post-exercise recovery with no difference between training volumes. Single point F13CO2 was indistinguishable from past IAAO studies using multiple protein intakes. Our results suggest that protein requirements may be greatest on recovery days but are not influenced by moderate training volumes in endurance athletes.

Protein Requirements Are Increased in Endurance-Trained Athletes but Similar between Females and Males during Postexercise Recovery

PURPOSE

This study aimed to determine the daily protein requirements of female and male endurance athletes in a home-based setting using noninvasive stable isotope methodology (i.e., indicator amino acid oxidation).

METHODS

Eight males (30 ± 3 yr; 78.6 ± 10.5 kg; 75.6 ± 7.5 mL·kgFFM-1·min-1; mean ± SD) and seven females (30 ± 4 yr; 57.7 ± 5.0 kg; 77.5 ± 7.1 mL·kgFFM-1·min-1) during the midluteal phase were studied. After 2 d of controlled diet (1.4 gprotein·kg-1·d-1) and training (10 and 5 km run·d-1, respectively), participants completed a 20-km run before an at-home indicator amino acid oxidation trial testing a suboptimal, a moderate, and an excess (i.e., 0.2, 1.2, and 2.0 g·kg-1·d-1, respectively) protein intake. Protein was consumed as a crystalline amino acid mixture containing [1-13C]phenylalanine to examine whole-body phenylalanine flux and phenylalanine oxidation (PheOx; the reciprocal of whole-body protein synthesis) through breath and urine sample collection. A modified biphasic linear regression determined the breakpoint in PheOx for each participant to generate an estimated average intake that would maximize whole-body protein synthesis for each sex.

RESULTS

PheOx was different (P < 0.01) between all protein intakes with no effect of sex (P = 0.63). Using a modified three-point curve resulted in a breakpoint that was not different (P = 0.94) between males and females (1.60 and 1.61 g·kg-1·d-1, respectively). The recommended intake (i.e., upper 95% confidence interval) was estimated to be 1.81 and 1.89 g·kg-1·d-1 for males and females, respectively.

CONCLUSIONS

Our findings indicate that endurance athletes consuming a daily protein intake toward the upper end of current consensus recommendations (~1.85 g·kg-1·d-1) will maximize whole-body protein synthesis during postexercise recovery regardless of sex.

Greater plasma essential amino acids and lower 3-methylhistidine with higher protein intake during endurance training: a randomised control trial

Endurance exercise alters amino acid (AA) metabolism that necessitates greater AA intake in the post exercise recovery period to support recovery. Thus, daily AA ingestion during a period of endurance training may affect the metabolically active plasma free AA pool, which is otherwise maintained during periods of inadequate protein intake by the breakdown of skeletal muscle proteins. Nine endurance-trained males completed a 4-day running protocol (20 km, 5 km, 10 km and 20 km on days 1-4, respectively) on three occasions with a controlled diet providing different protein intakes [0.94(LOW), 1.20(MOD) or 1.83gprotein kgbody mass-1 day-1 (HIGH)]. Urine collected over 24 h on day-4 and plasma collected after an overnight fast on day-5 were analyzed for free AA (plasma) and 3-methylhistidine (3MH; plasma and urine), a marker of myofibrillar protein breakdown. There was an effect of protein intake (HIGH > MOD/LOW; P < 0.05) on fasted plasma essential AA, branched chain AA and 3MH but no effect on 24-h urinary 3-MH excretion. Consuming a previously determined optimal daily protein intake of 1.83 g kg-1 day-1 during endurance training maintains fasted plasma free AA and may attenuate myofibrillar protein catabolism, although this latter effect was not detected in 24-h urinary excretion. The maintenance of the metabolically active free plasma AA pool may support greater recovery from exercise and contribute to the previously determined greater whole-body net protein balance in this athletic population. TRN: NCT02801344 (June 15, 2016).

Estimation of skeletal muscle mass in 4-year-old children using the D3-creatine dilution method

BACKGROUND

Given limited experience in applying the creatine-(methyl-D3) (D3Cr) dilution method to measure skeletal muscle mass (SMM) in young children, the feasibility of deployment in a fielding setting and performance of the method was assessed in a cohort of 4-year-old children in Dhaka, Bangladesh.

METHODS

Following D3Cr oral dose (10 mg) administration, single fasting urine samples were collected at 2-4 days (n = 100). Twenty-four-hour post-dose collections and serial spot urine samples on days 2, 3 and 4 were obtained in a subset of participants (n = 10). Urinary creatine, creatinine, D3Cr and D3-creatinine enrichment were analyzed by liquid chromatography-tandem mass spectrometry. Appendicular lean mass (ALM) was measured by dual-energy x-ray absorptiometry and grip strength was measured by a hand-held dynamometer.

RESULTS

SMM was measured successfully in 91% of participants, and there were no adverse events. Mean ± SD SMM was greater than ALM (4.5 ± 0.4 and 3.2 ± 0.6 kg, respectively). Precision of SMM was low (intraclass correlation = 0.20; 95% CI: 0.02, 0.75; n = 10). Grip strength was not associated with SMM in multivariable analysis (0.004 kg per 100 g of SMM; 95% CI: -0.031, 0.038; n = 91).

CONCLUSIONS

The D3Cr dilution method was feasible in a community setting. However, high within-child variability in SMM estimates suggests the need for further optimization of this approach.

IMPACT

The D3-creatine (D3Cr) stable isotope dilution method was considered a feasible method for the estimation of skeletal muscle mass (SMM) in young children in a community setting and was well accepted among participants. SMM was weakly associated with both dual-energy x-ray absorptiometry-derived values of appendicular lean mass and grip strength. High within-child variability in estimated values of SMM suggests that further optimization of the D3Cr stable isotope dilution method is required prior to implementation in community research settings.

Underpinning the Food Matrix Regulation of Postexercise Myofibrillar Protein Synthesis by Comparing Salmon Ingestion With the Sum of Its Isolated Nutrients in Healthy Young Adults

BACKGROUND

Protein is most commonly consumed as whole foods as opposed to single nutrients. However, the food matrix regulation of the postprandial muscle protein synthetic response has received little attention.

OBJECTIVES

The purpose of this study was to assess the effects of eating salmon (SAL) and of ingesting the same nutrients as an isolated mixture of crystalline amino acids and fish oil (ISO) on the stimulation of postexercise myofibrillar protein synthesis (MPS) and whole-body leucine oxidation rates in healthy young adults.

METHODS

Ten recreationally active adults (24 ± 4 y; 5 men, 5 women) performed an acute bout of resistance exercise, followed by the ingestion of SAL or ISO in a crossover fashion. Blood, breath, and muscle biopsies were collected at rest and after exercise during primed continuous infusions of L-[ring-2H5]phenylalanine and L-[1-13C]leucine. All data are presented as means ± SD and/or mean differences (95% CIs).

RESULTS

Postprandial essential amino acid (EAA) concentrations peaked earlier (P = 0.024) in the ISO group than those in the SAL group. Postprandial leucine oxidation rates increased over time (P < 0.001) and peaked earlier in the ISO group (1.239 ± 0.321 nmol/kg/min; 63 ± 25 min) than those in the SAL group (1.230 ± 0.561 nmol/kg/min; 105 ± 20 min; P = 0.003). MPS rates for SAL (0.056 ± 0.022 %/h; P = 0.001) and ISO (0.046 ± 0.025 %/h; P = 0.025) were greater than the basal rates (0.020 ± 0.011 %/h) during the 0- to 5-h recovery period, with no differences between conditions (P = 0.308).

CONCLUSION

We showed that the postexercise ingestion of SAL or ISO stimulate postexercise MPS rates with no differences between the conditions. Thus, our results indicate that ingesting protein from SAL as a whole-food matrix is similarly anabolic to ISO in healthy young adults. This trial was registered at www.

CLINICALTRIALS

gov as NCT03870165.

Identification of underexplored mesenchymal and vascular-related cell populations in human skeletal muscle

Skeletal muscle repair and maintenance are directly and indirectly supported by stromal populations such as vascular cells and fibro-adipogenic progenitors (FAPs), a subset of which express Twist2 and possess direct myogenic potential in rodents. However, less is understood of the complexity and heterogeneity of human skeletal muscle stromal cells. To this end, we performed single-cell RNA sequencing (scRNAseq) on ~2000 cells isolated from the human semitendinosus muscle of young individuals. This demonstrated the presence of two poorly described cell populations. First, we detected a vascular-related cell type that expressed pericyte and pan-endothelial genes that we localized to large blood vessels within skeletal muscle cross-sections and termed stromal pericytes. RNA velocity analysis demonstrated that stromal pericytes may represent a "transition state" between endothelial cells and pericytes. Analysis of published scRNAseq datasets revealed evidence for stromal pericytes in trunk and heart musculature, which showed transcriptional similarity. Additionally, we identified a subset of FAPs expressing TWIST2 mRNA and protein. Human TWIST2-expressing cells were anatomically and transcriptionally comparable to mouse Twist2 cells as they were restricted to the myofiber interstitium, expressed fibrogenic genes but lacked satellite cell markers and colocalized with the FAPs marker PDGFRα in human muscle cross-sections. Taken together, these results highlight the complexity of stromal cells residing in human skeletal muscle and support the utility of scRNAseq for discovery and characterization of poorly described cell populations.

Walking or body weight squat 'activity snacks' increase dietary amino acid utilization for myofibrillar protein synthesis during prolonged sitting

Interrupting prolonged sitting with intermittent exercise enhances postprandial glycemic control but has unknown effects on sensitizing skeletal muscle to dietary amino acids. We hypothesized that brief walking or body weight squats would enhance the utilization of dietary phenylalanine for myofibrillar protein synthesis (MyoPS) during prolonged sitting. Participants (7 males and 5 females; ~23y; ~25.1kg/m²; ~7300 steps/d) completed three 7.5h trials consisting of prolonged sitting (SIT) or sitting with intermittent (every 30 minutes) walking (WALK) or body weight squatting (SQUAT). Two mixed-macronutrient meals (~55:30:15% carbohydrate:fat:protein), enriched with L-[ring-²H₅]phenylalanine or L-[ring-¹³C₆]phenylalanine, were provided to mimic breakfast and lunch. Tracer incorporation into myofibrillar protein was determined from the vastus lateralis with MyoPS estimated using plasma enrichment as precursor surrogate. Phosphorylation of candidate anabolic signaling proteins were determined by immunoblotting. There was no difference between conditions (p≥0.78) in the time course or area under the curve for plasma phenylalanine enrichment. MyoPS was greater (p<0.05, weighted planned comparison) in SQUAT (0.103±0.030%/h) and WALK (0.118±0.037%/h) compared to SIT (0.080±0.032%/h). Compared to SIT, there were moderate-to-large effect sizes, respectively, for SQUAT (ES=0.75; 95% CI -0.10-1.55) and WALK (ES=1.10; 95% CI 0.20-1.91). Fold change in rpS6^Ser240/244 phosphorylation was greater in SQUAT compared to SIT (7.6±2.7 vs. 1.6±0.45 fold, p<0.05) with no difference (p≥0.21) in any other targets measured (4E-BP1^Thr37/46, eEF2^Thr56, mTOR^Ser2448, ERK1/2^{Thr202/Tyr204}). Interrupting prolonged sitting with short 'activity snacks' improves the utilization of dietary amino acids for MyoPS. The long term impact of this practical lifestyle modification for muscle mass or quality should be investigated.

Trained Integrated Postexercise Myofibrillar Protein Synthesis Rates Correlate with Hypertrophy in Young Males and Females

PURPOSE

Resistance training induces skeletal muscle hypertrophy via the summated effects of postexercise elevations in myofibrillar protein synthesis (MyoPS) that persist for up to 48 h, although research in females is currently lacking. MyoPS is regulated by mTOR translocation and colocalization; however, the effects of resistance training on these intracellular processes are unknown. We hypothesized that MyoPS would correlate with hypertrophy only after training in both sexes and would be associated with intracellular redistribution of mTOR.

METHODS

Recreationally active males and females (n = 10 each) underwent 8 wk of whole-body resistance exercise three times a week. Fasted muscle biopsies were obtained immediately before (REST) and 24 and 48 h after acute resistance exercise in the untrained (UT) and trained (T) states to determine integrated MyoPS over 48 h (D2O ingestion) and intracellular mTOR colocalization (immunofluorescence microscopy).

RESULTS

Training increased (P < 0.01) muscle strength (~20%-126%), muscle thickness (~8%-11%), and average fiber cross-sectional area (~15%-20%). MyoPS increased above REST in UT (P = 0.032) and T (P < 0.01), but to a greater extent in males (~23%; P = 0.023), and was positively (P < 0.01) associated with muscle thickness and fiber cross-sectional area at T only in both males and females. mTOR colocalization with the cell periphery increased (P < 0.01) in T, irrespective of sex or acute exercise. Training increased (P ≤ 0.043) total mTOR, LAMP2 (lysosomal marker), and their colocalization (P < 0.01), although their colocalization was greater in males at 24 and 48 h independent of training status (P < 0.01).

CONCLUSIONS

MyoPS during prolonged recovery from exercise is greater in males but related to muscle hypertrophy regardless of sex only in the trained state, which may be underpinned by altered mTOR localization.

Challenges of rapamycin repurposing as a potential therapeutic candidate for COVID-19: implications for skeletal muscle metabolic health in older persons

The novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged as the causative agent of the ongoing coronavirus disease 2019 (COVID-19) pandemic that has spread worldwide, resulting in over 6 million deaths as of March 2022. Older people have been disproportionately affected by the disease, as they have a greater risk of hospitalization, are more vulnerable to severe infection, and have higher mortality than younger patients. Although effective vaccines have been rapidly developed and administered globally, several clinical trials are ongoing to repurpose existing drugs to combat severe infection. One such drug, rapamycin, is currently under study for this purpose, given its immunosuppressant effects that are mediated by its inhibition of the mechanistic target of rapamycin (mTOR), a master regulator of cell growth. Consistent with this premise, acute rapamycin administration in young healthy humans blocks or attenuates mTOR and its downstream effectors, leading to the inhibition of muscle protein synthesis (MPS). Skeletal muscle mass declines when MPS is chronically lower than muscle protein breakdown. This is consequential for older people who are more susceptible to anabolic resistance (i.e., the blunting of MPS) due to reduced activity, sedentariness, or bed rest such as that associated with COVID-19 hospitalization, and who have also demonstrated a delayed or blunted ability to regain inactivity-induced muscle loss. The lack of studies investigating rapamycin administration on skeletal muscle in older people, and the emergence of effective antiviral medications against severe infection, may indicate the reduced relevance of drug repurposing for present or future pandemics.

A non-invasive 13CO2 breath test detects differences in anabolic sensitivity with feeding and heavy resistance exercise in healthy young males: a randomized control trial

There are limited tools to measure anabolic sensitivity non-invasively in response to acute physiological stimuli, which represents a challenge for research in free-living settings and vulnerable populations. We tested the ability of a stable isotope breath test to detect changes in leucine oxidation (OX) and leucine retention (intake - OX) across a range of anabolic sensitivities. Healthy males ingested a beverage containing 0.25 g·kg-1 protein and 0.75 g·kg-1 carbohydrate with the leucine content enriched to 5% with L-[1-13C]leucine at rest (FED) or after a bout of resistance exercise (EXFED), with a parallel group consuming only the tracer (FAST). Concurrent primed-constant infusions of L-[5,5,5-2H3]leucine revealed high peripheral bioavailability for FED (~81%), EXFED (~80%), and FAST (~117%). After beverage ingestion, whole-body protein synthesis was greater in FED and EXFED than FAST. OX was greater in FED and EXFED than FAST, with EXFED lower than FED. Leucine retention demonstrated expected physiological differences in anabolic sensitivity (EXFED > FED > FAST). We demonstrated that a non-invasive breath test based on an amino acid (leucine) that is preferentially metabolized in peripheral (muscle) tissues can detect differences in anabolic sensitivity. Future studies could examine this test within a variety of populations experiencing muscle growth or atrophy. Novelty Bullets • An oral L-[1-13C]leucine breath test can detect greater anabolic sensitivity after feeding and resistance exercise. • This tool may be applied in growing (e.g., children) or wasting (e.g. aging) populations where invasive procedures are not possible.

Protein quality and the food matrix: defining optimal versus maximal meal-based protein intakes for stimulating muscle protein synthesis

This paper examines the opinion that we should aim to optimize, rather than maximize, protein intakes to avoid over-emphasizing muscle-centric protein requirements. An optimal eating approach strives to minimize amino acid oxidative waste and more efficiently stimulate postprandial muscle protein accretion. To do this, practitioners should acknowledge higher quality protein foods as better in delivering target amounts of amino acids into circulation, and the food matrix (e.g., nutrient-nutrient interactions) can be leveraged to potentiate essential amino acid incorporation into skeletal muscle protein.

RPS6 phosphorylation occurs to a greater extent in the periphery of human skeletal muscle fibers, near focal adhesions, after anabolic stimuli

Following anabolic stimuli (mechanical loading and/or amino acid provision) the mechanistic target of rapamycin complex 1 (mTORC1), a master regulator of protein synthesis, translocates toward the cell periphery. However, it is unknown if mTORC1-mediated phosphorylation events occur in these peripheral regions or prior to translocation (i.e. in central regions). We therefore aimed to determine the cellular location of a mTORC1-mediated phosphorylation event, RPS6^Ser240/244, in human skeletal muscle following anabolic stimuli. Fourteen young, healthy males either ingested a protein-carbohydrate beverage (0.25g/kg protein, 0.75g/kg carbohydrate) alone (n=7;23±5yrs;76.8±3.6kg;13.6±3.8%BF, FED) or following a whole-body resistance exercise bout (n=7;22±2yrs;78.1±3.6kg;12.2±4.9%BF, EXFED). Vastus lateralis muscle biopsies were obtained at rest (PRE) and 120 and 300min following anabolic stimuli. RPS6^Ser240/244 phosphorylation measured by immunofluorescent staining or immunoblot was positively correlated (r=0.76, p<0.001). Peripheral staining intensity of p-RPS6^Ser240/244 increased above PRE in both FED and EXFED at 120min (~54% and ~138% respectively, p<0.05) but was greater in EXFED at both post-stimuli time points (p<0.05). The peripheral-central ratio of p-RPS6^240/244 staining displayed a similar pattern, even when corrected for total RPS6 distribution, suggesting RPS6 phosphorylation occurs to a greater extent in the periphery of fibers. Moreover, p-RPS6^Ser240/244 intensity within paxillin-positive regions, a marker of focal adhesion complexes, was elevated at 120min irrespective of stimulus (p=0.006) before returning to PRE at 300min. These data confirm that RPS6^Ser240/244 phosphorylation occurs in the region of human muscle fibers to which mTOR translocates following anabolic stimuli and identifies focal adhesion complexes as a potential site of mTORC1 regulation in vivo.

Effect of maternal prenatal and postpartum vitamin D supplementation on offspring bone mass and muscle strength in early childhood: follow-up of a randomized controlled trial

BACKGROUND

Maternal vitamin D status during pregnancy and lactation is a modifiable factor that may influence offspring musculoskeletal outcomes. However, few randomized trials have tested the effects of prenatal or postpartum vitamin D supplementation on offspring bone and muscle development.

OBJECTIVES

The aim was to examine hypothesized effects of improvements in early-life vitamin D status on childhood musculoskeletal health in Dhaka, Bangladesh.

METHODS

In a previously completed, double-blind, dose-ranging trial, healthy pregnant women (n = 1300) were recruited at 17-24 weeks' gestation and randomly assigned to a prenatal/postpartum regimen of 0/0, 4200/0, 16,800/0, 28,000/0, or 28,000/28,000 IU cholecalciferol (vitamin D3)/wk until 26 wk postpartum. In this new report, we describe additional follow-up at 4 y of age (n = 642) for longer-term outcomes. Bone mineral content (BMC) and areal bone mineral density (aBMD) were measured by DXA. Grip strength was tested using a hand-held dynamometer. The primary comparison was children of women assigned to 28,000 IU/wk prenatally compared with placebo. Differences are expressed as means and 95% CIs.

RESULTS

Total-body-less-head (TBLH) BMC, TBLH aBMD, and grip strength were similar in the combined high-dose prenatal (28,000/0 and 28,000/28,000 IU/wk) compared with placebo groups (mean difference [95% CI] = 0.61 g [-10.90, 12.13], 0.0004 g/cm2 [-0.0089, 0.0097], and 0.02 kg [-0.26, 0.31], respectively). In dose-ranging analyses, TBLH BMC and aBMD, whole-body BMC and aBMD, and grip strength in each of the prenatal vitamin D groups were not significantly different from placebo (P > 0.05 for all comparisons). Only head aBMD was greater in children of women assigned to the 28,000/28,000-IU regimen compared with placebo (mean difference [95% CI] = 0.024 g/cm2 [0.0009, 0.047], P = 0.042); the effect was attenuated upon adjustment for child height, weight, and sex (P = 0.11).

CONCLUSIONS

Maternal prenatal, with or without postpartum, vitamin D supplementation does not improve child BMC, aBMD, or grip strength at 4 y of age. The MDIG trial and present follow-up study were registered prospectively at www.clinicaltrials.gov as NCT01924013 and NCT03537443, respectively.

Incorporation of Dietary Amino Acids Into Myofibrillar and Sarcoplasmic Proteins in Free-Living Adults Is Influenced by Sex, Resistance Exercise, and Training Status

BACKGROUND

Acute exercise increases the incorporation of dietary amino acids into de novo myofibrillar proteins after a single meal in controlled laboratory studies in males. It is unclear whether this extends to free-living settings or is influenced by training or sex.

OBJECTIVES

We determined the effects of exercise, training status, and sex on 24-hour free-living dietary phenylalanine incorporation into skeletal muscle proteins.

METHODS

In a parallel group design, recreationally active males (mean ± SD age, 23 ± 3 years;  BMI. 23.4 ± 2.9 kg/m2; n = 10) and females (age 24 ± 5 years;  BMI, 23.1 ± 3.9 kg/m2; n = 9) underwent 8 weeks of whole-body resistance exercise 3 times a week. Controlled diets containing 1.6 g/kg-1/d-1 (amino acids modelled after egg), enriched to 10% with [13C6] or [2H5]phenylalanine, were consumed before and after an acute bout of resistance exercise. Fasted muscle biopsies were obtained before [untrained, pre-exercise condition (REST ] and 24 hours after an acute bout of resistance exercise in untrained (UT) and trained (T) states to determine dietary phenylalanine incorporation into myofibrillar (ΔMyo) and sarcoplasmic (ΔSarc) proteins, intracellular mechanistic target of rapamycin (mTOR) colocalization with ulex europaeus agglutinin-1 (UEA-1; capillary marker; immunofluorescence), and amino acid transporter expression (Western blotting).

RESULTS

The ΔMyo values were ∼62% greater (P < 0.01) in females than males at REST. The ΔMyo values increased above REST by ∼51% during UT and ∼30% in T (both P < 0.01) in males, remained unchanged in females during UT, and were ∼33% lower at T when compared to UT (P = 0.013). Irrespective of sex, ΔMyo and ΔSarc were decreased at T compared to UT (P ≤ 0.026). Resistance training increased mTOR colocalization with UEA-1 (P = 0.004), while L amino acid transporter 1, which was greater in males (P < 0.01), and sodium-coupled neutral amino acid transporter 2 protein expression were not affected by acute exercise (P ≥ 0.33) or training (P ≥ 0.45).

CONCLUSIONS

The exercise-induced incorporation of dietary phenylalanine into myofibrillar and sarcoplasmic proteins is attenuated after training regardless of sex, suggesting a reduced reliance on dietary amino acids for postexercise skeletal muscle remodeling in the T state.

A muscle-centric view of time-restricted feeding for older adults

PURPOSE OF REVIEW

The practice of time-restricted feeding (TRF) has received fervent interest in recent years as a strategy to mitigate obesity and metabolic disease. We sought to review the implications of TRF for skeletal muscle health and function in aging.

RECENT FINDINGS

TRF has high adherence and can promote body weight loss in older populations. Body weight reductions favor fat mass in the young, however, there is also the potential for undesirable losses in lean mass. There is currently no evidence to support TRF for skeletal muscle function and metabolism in older persons, and only tentative findings in the young. With a narrow eating window of 6-8 h and a prolonged fasting period to minimize daily insulin exposure, TRF may contradict recommended dietary practices for optimizing skeletal muscle anabolism in older people.

SUMMARY

TRF might represent a promising intervention to address obesity and its associated metabolic diseases, however, at present there is insufficient evidence for optimizing skeletal muscle mass or health in older individuals. Further research is needed to: (1) ascertain the impact of TRF on body composition, skeletal muscle anabolism, and autophagy in aging, and; (2) delineate the potentially myoprotective roles of dietary protein and exercise within the framework of TRF in older persons.

LAT1 and SNAT2 Protein Expression and Membrane Localization of LAT1 Are Not Acutely Altered by Dietary Amino Acids or Resistance Exercise Nor Positively Associated with Leucine or Phenylalanine Incorporation in Human Skeletal Muscle

The influx of essential amino acids into skeletal muscle is primarily mediated by the large neutral amino acid transporter 1 (LAT1), which is dependent on the glutamine gradient generated by the sodium-dependent neutral amino acid transporter 2 (SNAT2). The protein expression and membrane localization of LAT1 may be influenced by amino acid ingestion and/or resistance exercise, although its acute influence on dietary amino acid incorporation into skeletal muscle protein has not been investigated. In a group design, healthy males consumed a mixed carbohydrate (0.75 g·kg^-1) crystalline amino acid (0.25 g·kg^-1) beverage enriched to 25% and 30% with LAT1 substrates L-[1-¹³C]leucine (LEU) and L-[ring-²H₅]phenylalanine (PHE), respectively, at rest (FED: n = 7, 23 ± 5 y, 77 ± 4 kg) or after a bout of resistance exercise (EXFED: n = 7, 22 ± 2 y, 78 ± 11 kg). Postprandial muscle biopsies were collected at 0, 120, and 300 min to measure transporter protein expression (immunoblot), LAT1 membrane localization (immunofluorescence), and dietary amino acid incorporation into myofibrillar protein (ΔLEU and ΔPHE). Basal LAT1 and SNAT2 protein contents were correlated with each other (r = 0.55, p = 0.04) but their expression did not change across time in FED or EXFED (all, p > 0.05). Membrane localization of LAT1 did not change across time in FED or EXFED whether measured as outer 1.5 µm intensity or membrane-to-fiber ratio (all, p > 0.05). Basal SNAT2 protein expression was not correlated with ΔLEU or ΔPHE (all, p ≥ 0.05) whereas basal LAT1 expression was negatively correlated with ΔPHE in FED (r = -0.76, p = 0.04) and EXFED (r = -0.81, p = 0.03) but not ΔLEU (p > 0.05). Basal LAT1 membrane localization was not correlated with ΔLEU or ΔPHE (all, p > 0.05). Our results suggest that LAT1/SNAT2 protein expression and LAT1 membrane localization are not influenced by acute anabolic stimuli and do not positively influence the incorporation of dietary amino acids for de novo myofibrillar protein synthesis in healthy young males.

Protein Requirements for Master Athletes: Just Older Versions of Their Younger Selves

It is established that protein requirements are elevated in athletes to support their training and post-exercise recovery and adaptation, especially within skeletal muscle. However, research on the requirements for this macronutrient has been performed almost exclusively in younger athletes, which may complicate their translation to the growing population of Master athletes (i.e. > 35 years old). In contrast to older (> 65 years) untrained adults who typically demonstrate anabolic resistance to dietary protein as a primary mediator of the ‘normal’ age-related loss of muscle mass and strength, Master athletes are generally considered successful models of aging as evidenced by possessing similar body composition, muscle mass, and aerobic fitness as untrained adults more than half their age. The primary physiology changes considered to underpin the anabolic resistance of aging are precipitated or exacerbated by physical inactivity, which has led to higher protein recommendations to stimulate muscle protein synthesis in older untrained compared to younger untrained adults. This review puts forth the argument that Master athletes have similar muscle characteristics, physiological responses to exercise, and protein metabolism as young athletes and, therefore, are unlikely to have protein requirements that are different from their young contemporaries. Recommendations for protein amount, type, and pattern will be discussed for Master athletes to enhance their recovery from and adaptation to resistance and endurance training.

High-intensity interval training or resistance training versus usual care in men with prostate cancer on active surveillance: A three-arm feasibility randomized controlled trial

BACKGROUND

This study assessed the feasibility of a phase II randomized controlled trial of high-intensity interval training (HIIT), resistance training (RT), and usual care (UC) in men with prostate cancer (PCa) on active surveillance (AS) and evaluated changes in clinically relevant outcomes.

METHODS

Eighteen men undergoing AS for PCa were randomized to HIIT (n=5), RT (n=7), or UC (n=6). Exercise participants attended two supervised sessions weekly and were instructed to complete one home-based session weekly for 8 weeks. UC participants were provided with physical activity guidelines.

RESULTS

Feasibility was met for attendance, compliance, and retention, but not recruitment. HIIT increased leg press (mean: +8.2kg, 95%CI 1.1, 15.3) from baseline to 8 weeks. RT increased seated row (mean: +11.7kg, 95%CI 6.1, 17.3) and chest press (mean: +10.4kg, 95%CI 5.3, 15.5), leg press (mean: +13.1kg, 95%CI 5.9, 20.3), serum insulin-like binding protein-3 (IGFBP-3) (mean: +400.0ng/ml, 95%CI 94.5, 705.5), and decreased interferon-γ (mean: -3.1pg/ml, 95%CI -5.7, -0.4). No changes were observed in the UC group.

CONCLUSION

HIIT and RT may be effective strategies for improving muscle strength; however, only RT may increase serum IGFBP-3. Strategies that can enhance recruitment in men on AS are important prior to conducting a phase II trial.

TRIAL REGISTRATION NUMBER

ClinicalTrials.gov number NCT04266262 Novelty bullets • High-intensity interval training or resistance training are feasible during active surveillance for prostate cancer. • Resistance training may suppress the tumor-promoting effects of insulin-like growth factor-I (IGF-I) via increased expression of IGFBP-3.

Early resistance training-mediated stimulation of daily muscle protein synthetic responses to higher habitual protein intake in middle-aged adults

KEY POINTS

The ingestion of protein potentiates the stimulation of myofibrillar protein synthesis rates after an acute bout of resistance exercise. Protein supplementation (eating above the protein Recommended Dietary Allowance) during resistance training has been shown to maximize lean mass and strength gains in healthy young and older adults. Here, contractile, oxidative, and structural protein synthesis were assessed in skeletal muscle in response to a moderate or higher protein diet during the early adaptive phase of resistance training in middle-aged adults. The stimulation of myofibrillar, mitochondrial or collagen protein synthesis rates during 0-3 weeks of resistance training is not further enhanced by a higher protein diet. These results show that moderate protein diets are sufficient to support the skeletal muscle adaptive response during the early phase of a resistance training programme.

ABSTRACT

Protein ingestion augments muscle protein synthesis (MPS) rates acutely after resistance exercise and can offset age-related loss in muscle mass. Skeletal muscle contains a variety of protein pools, such as myofibrillar (contractile), mitochondrial (substrate oxidation), and collagen (structural support) proteins, and the sensitivity to nutrition and exercise seems to be dependent on the major protein fraction studied. However, it is unknown how free-living conditions with high dietary protein density and habitual resistance exercise mediates muscle protein subfraction synthesis. Therefore, we investigated the effect of moderate (MOD: 1.06 ± 0.22 g kg^-1  day^-1 ) or high (HIGH: 1.55 ± 0.25 g kg^-1  day^-1 ) protein intake on daily MPS rates within the myofibrillar (MyoPS), mitochondrial (MitoPS) and collagen (CPS) protein fractions in middle-aged men and women (n = 20, 47 ± 1 years, BMI 28 ± 1 kg m^-2 ) during the early phase (0-3 weeks) of a dietary counselling-controlled resistance training programme. Participants were loaded with deuterated water, followed by daily maintenance doses throughout the intervention. Muscle biopsies were collected at baseline and after weeks 1, 2 and 3. MyoPS in the HIGH condition remained constant (P = 1.000), but MOD decreased over time (P = 0.023). MitoPS decreased after 0-3 weeks when compared to 0-1 week (P = 0.010) with no effects of protein intake (P = 0.827). A similar decline with no difference between groups (P = 0.323) was also observed for CPS (P = 0.007). Our results demonstrated that additional protein intake above moderate amounts does not potentiate the stimulation of longer-term MPS responses during the early stage of resistance training adaptations in middle-aged adults.

Satellite cell and myonuclear accretion is related to training-induced skeletal muscle fiber hypertrophy in young males and females

Satellite cells (SC) play an integral role in the recovery from skeletal muscle damage and supporting muscle hypertrophy. Acute resistance exercise typically elevates type I and type II SC content 24-96 h post exercise in healthy young males, although comparable research in females is lacking. We aimed to elucidate whether sex-based differences exist in fiber type-specific SC content after resistance exercise in the untrained (UT) and trained (T) states. Ten young males (23.0 ± 4.0 yr) and females (23.0 ± 4.8 yr) completed an acute bout of resistance exercise before and after 8 wk of whole body resistance training. Muscle biopsies were taken from the vastus lateralis immediately before and 24 and 48 h after each bout to determine SC and myonuclear content by immunohistochemistry. Males had greater SC associated with type II fibers (P ≤ 0.03). There was no effect of acute resistance exercise on SC content in either fiber type (P ≥ 0.58) for either sex; however, training increased SC in type II fibers (P < 0.01) irrespective of sex. The change in mean 0-48 h type II SC was positively correlated with muscle fiber hypertrophy in type II fibers (r = 0.47; P = 0.035). Furthermore, the change in myonuclei per fiber was positively correlated with type I and type II fiber hypertrophy (both r = 0.68; P < 0.01). Our results suggest that SC responses to acute and chronic resistance exercise are similar in males and females and that SC and myonuclear accretion is related to training-induced muscle fiber hypertrophy.NEW & NOTEWORTHY We demonstrate that training-induced increase in SC content in type II fibers and myonuclear content in type I and II fibers is similar between males and females. Furthermore, these changes are related to the extent of muscle fiber hypertrophy. Thus, SC and myonuclear accretion appear to contribute to muscle hypertrophy irrespective of sex, highlighting the importance of these muscle stem cells in human skeletal muscle growth.

A Muscle-Centric Perspective on Intermittent Fasting: A Suboptimal Dietary Strategy for Supporting Muscle Protein Remodeling and Muscle Mass?

Muscle protein is constantly “turning over” through the breakdown of old/damaged proteins and the resynthesis of new functional proteins, the algebraic difference determining net muscle gain, maintenance, or loss. This turnover, which is sensitive to the nutritional environment, ultimately determines the mass, quality, and health of skeletal muscle over time. Intermittent fasting has become a topic of interest in the health community as an avenue to improve health and body composition primarily via caloric deficiency as well as enhanced lipolysis and fat oxidation secondary to attenuated daily insulin response. However, this approach belies the established anti-catabolic effect of insulin on skeletal muscle. More importantly, muscle protein synthesis, which is the primary regulated turnover variable in healthy humans, is stimulated by the consumption of dietary amino acids, a process that is saturated at a moderate protein intake. While limited research has explored the effect of intermittent fasting on muscle-related outcomes, we propose that infrequent meal feeding and periods of prolonged fasting characteristic of models of intermittent fasting may be counter-productive to optimizing muscle protein turnover and net muscle protein balance. The present commentary will discuss the regulation of muscle protein turnover across fasted and fed cycles and contrast it with studies exploring how dietary manipulation alters the partitioning of fat and lean body mass. It is our position that intermittent fasting likely represents a suboptimal dietary approach to remodel skeletal muscle, which could impact the ability to maintain or enhance muscle mass and quality, especially during periods of reduced energy availability.

Fuelling the female athlete: Carbohydrate and protein recommendations

Optimal carbohydrate and protein intakes are vital for modulating training adaptation, recovery, and exercise performance. However, the research base underpinning contemporary sport nutrition guidelines has largely been conducted in male populations with a lack of consensus on whether the menstrual phase and associated changes in sex hormones allow broad application of these principles to female athletes. The present review will summarise our current understanding of carbohydrate and protein requirements in female athletes across the menstrual cycle and provide a critical analysis on how they compare to male athletes. On the basis of current evidence, we consider it premature to conclude that female athletes require sex specific guidelines in relation to CHO or protein requirements provided energy needs are met. However, there is a need for further research using sport-specific competition and training related exercise protocols that rigorously control for prior exercise, CHO/energy intake, contraceptive use and phase of menstrual cycle. Our overarching recommendation is to use current recommendations as a basis for adopting an individualised approach that takes into account athlete specific training and competition goals whilst also considering personal symptoms associated with the menstrual cycle.

Trolley dilemma in the sky: Context matters when civilians and cadets make remotely piloted aircraft decisions

Crews operating remotely piloted aircrafts (RPAs) in military operations may be among the few that truly experience tragic dilemmas similar to the famous Trolley Problem. In order to analyze decision-making and emotional conflict of RPA operators within Trolley-Problem-like dilemma situations, we created an RPA simulation that varied mission contexts (firefighter, military and surveillance as a control condition) and the social “value” of a potential victim. We found that participants (Air Force cadets and civilian students) were less likely to make the common utilitarian choice (sacrificing one to save five), when the value of the one increased, especially in the military context. However, in the firefighter context, this decision pattern was much less pronounced. The results demonstrate behavioral and justification differences when people are more invested in a particular context despite ostensibly similar dilemmas.

Interrupting prolonged sitting with repeated chair stands or short walks reduces postprandial insulinemia in healthy adults

We determined if interrupting prolonged sitting with practical "activity snacks" could reduce postprandial glycemia and insulinemia in healthy adults. Fourteen participants (7 males, 7 females; 24 ± 5 yr; 25 ± 5 kg/m²; 40 ± 8 mL/kg/min; 7,033 ± 2,288 steps/day) completed three 7.5-h trials in a randomized order consisting of uninterrupted sitting (SIT), sitting with intermittent (every 30 min) walking (WALK; 2 min at 3.1 mph), or sitting with intermittent squats (SQUAT; 15 chair stands with calf raise). Mixed-macronutrient liquid meals provided 20% ("breakfast") and 30% ("lunch") of daily energy needs to mimic Western meal patterns. Blood samples were obtained for analysis of postprandial plasma glucose and insulin concentrations, and skeletal muscle biopsy samples were collected to measure markers of contraction- and insulin-mediated glucose uptake signaling. Postprandial glucose and insulin did not differ across conditions following breakfast. After lunch, peak insulin concentration was lower in SQUAT (52 ± 27, P < 0.01) and WALK (62 ± 35, P < 0.05) compared with SIT (79 ± 43 μIU/mL). The insulin incremental area under the curve (iAUC) 1 h following lunch was 37 and 29% lower in SQUAT (P < 0.01) and WALK (P < 0.05) compared with SIT, respectively; however, 3-h insulin iAUC was reduced in SQUAT only (24% vs. SIT, P < 0.05). The 3-h insulin:glucose iAUC was reduced following lunch in both SQUAT (30%) and WALK (23%) compared with SIT (P < 0.05). Phosphorylation of AKT^Thr308, AKT^Ser473, and AS160^Ser318 was not different between conditions (P > 0.05). Interrupting prolonged sitting with short walks or repeated chair stands reduces postprandial insulinemia in healthy adults. Our results may have implications for mitigating cardiometabolic disease risk in adults who engage in periods of prolonged sitting.NEW & NOTEWORTHY Breaking up prolonged sitting with intermittent walking breaks can improve glycemic control. Here, we demonstrated that interrupting prolonged sitting every 30 min with 1 min of repeated chair stands was as effective as 2-min treadmill walks for lowering postprandial insulinemia in healthy adults. Markers of contraction- and insulin-mediated muscle glucose uptake were unchanged. Repeated chair stands as a form of body-weight resistance activity may represent a cost- and space-efficient activity break for mitigating cardiometabolic-disease risk.

Protein Timing Does Not Affect Next-Day Recovery of Strength or Power but May Enhance Aerobic Adaptations to Short-Term Variable Intensity Exercise Training in Recreationally Active Males: A Pilot Study

Background: Variable intensity training (VIT) characteristic of stop-and-go team sport exercise may reduce performance capacity when performed on successive days but also represent a strategy to induce rapid training-induced increases in exercise capacity. Although post-exercise protein enhances muscle protein synthesis, the timing of protein ingestion following variable intensity training (VIT) on next-day recovery and short-term performance adaptation is unknown. Purpose: To determine if immediate (IMM) as compared to delayed (DEL) protein ingestion supports greater acute recovery of exercise performance during successive days of VIT and/or supports chronic training adaptations. Methods: Sixteen habitually active men performed 5 consecutive days of variable intensity training (VIT) in the evening prior to consuming a beverage providing carbohydrate and whey protein (IMM; 0.7 g and 0.3 g/kg, respectively) or carbohydrates alone (DEL; 1 g/kg) with the reciprocal beverage consumed the following morning. Performance was assessed before each VIT (recovery) and 2 days after the final VIT (adaptation). Results: Five consecutive days of VIT progressively decreased anaerobic peak power (~7%) and muscle strength (MVC; ~8%) with no impact of protein timing. Following 2 days of recovery, VIT increased maximal voluntary contraction and predicted VO_2peak by ~10 and ~5%, respectively, with a moderate beneficial effect of IMM on predicted VO_2peak (ES = 0.78). Conclusion: Successive days of simulated team sport exercise decreases markers of next-day performance capacity with no effect of protein timing on acute recovery. However, practical VIT increases muscle strength and aerobic capacity in as little as 5 days with the latter potentially enhanced by immediate post-exercise protein consumption.

More than just a garbage can: emerging roles of the lysosome as an anabolic organelle in skeletal muscle

Skeletal muscle is a highly plastic tissue capable of remodeling in response to a range of physiological stimuli, including nutrients and exercise. Historically, the lysosome has been considered an essentially catabolic organelle contributing to autophagy, phagocytosis, and exo-/endocytosis in skeletal muscle. However, recent evidence has emerged of several anabolic roles for the lysosome, including the requirement for autophagy in skeletal muscle mass maintenance, the discovery of the lysosome as an intracellular signaling hub for mechanistic target of rapamycin complex 1 (mTORC1) activation, and the importance of transcription factor EB/lysosomal biogenesis-related signaling in the regulation of mTORC1-mediated protein synthesis. We, therefore, propose that the lysosome is an understudied organelle with the potential to underpin the skeletal muscle adaptive response to anabolic stimuli. Within this review, we describe the molecular regulation of lysosome biogenesis and detail the emerging anabolic roles of the lysosome in skeletal muscle with particular emphasis on how these roles may mediate adaptations to chronic resistance exercise. Furthermore, given the well-established role of amino acids to support muscle protein remodeling, we describe how dietary proteins "labeled" with stable isotopes could provide a complementary research tool to better understand how lysosomal biogenesis, autophagy regulation, and/or mTORC1-lysosomal repositioning can mediate the intracellular usage of dietary amino acids in response to anabolic stimuli. Finally, we provide avenues for future research with the aim of elucidating how the regulation of this important organelle could mediate skeletal muscle anabolism.

Leucine-enriched amino acids maintain peripheral mTOR-Rheb localization independent of myofibrillar protein synthesis and mTORC1 signaling postexercise

Postexercise protein ingestion can elevate rates of myofibrillar protein synthesis (MyoPS), mTORC1 activity, and mTOR translocation/protein-protein interactions. However, it is unclear if leucine-enriched essential amino acids (LEAA) can similarly facilitate intracellular mTOR trafficking in humans after exercise. The purpose of this study was to determine the effect of postexercise LEAA (4 g total EAAs, 1.6 g leucine) on acute MyoPS and mTORC1 translocation and signaling. Recreationally active men performed lower-body resistance exercise (5 × 8-10 leg press and leg extension) to volitional failure. Following exercise participants consumed LEAA (n = 8) or an isocaloric carbohydrate drink (PLA; n = 10). MyoPS was measured over 1.5-4 h of recovery by oral pulse of l-[ring-²H₅]-phenylalanine. Phosphorylation of proteins in the mTORC1 pathway were analyzed via immunoblotting and mTORC1-LAMP2/WGA/Rheb colocalization via immunofluorescence microscopy. There was no difference in MyoPS between groups (LEAA = 0.098 ± 0.01%/h; PL = 0.090 ± 0.01%/h; P > 0.05). Exercise increased (P < 0.05) rpS6^Ser240/244(LEAA = 35.3-fold; PLA = 20.6-fold), mTOR^Ser2448(LEAA = 1.8-fold; PLA = 1.2-fold) and 4EBP1^Thr37/46(LEAA = 1.5-fold; PLA = 1.4-fold) phosphorylation irrespective of nutrition (P > 0.05). LAT1 and SNAT2 protein expression were not affected by exercise or nutrient ingestion. mTOR-LAMP2 colocalization was greater in LEAA preexercise and decreased following exercise and supplement ingestion (P < 0.05), yet was unchanged in PLA. mTOR-WGA (cell periphery marker) and mTOR-Rheb colocalization was greater in LEAA compared with PLA irrespective of time-point (P < 0.05). In conclusion, the postexercise consumption of 4 g of LEAA maintains mTOR in peripheral regions of muscle fibers, in closer proximity to its direct activator Rheb, during prolonged recovery independent of differences in MyoPS or mTORC1 signaling compared with PLA ingestion. This intracellular localization of mTOR may serve to "prime" the kinase for future anabolic stimuli.NEW & NOTEWORTHY This is the first study to investigate whether postexercise leucine-enriched amino acid (LEAA) ingestion elevates mTORC1 translocation and protein-protein interactions in human skeletal muscle. Here, we observed that although LEAA ingestion did not further elevate postexercise MyoPS or mTORC1 signaling compared with placebo, mTORC1 peripheral location and interaction with Rheb were maintained. This may serve to "prime" mTORC1 for subsequent anabolic stimuli.

An Acute Reduction in Habitual Protein Intake Attenuates Post Exercise Anabolism and May Bias Oxidation-Derived Protein Requirements in Resistance Trained Men

Protein recommendations for resistance-trained athletes are generally lower than their habitual intakes. Excess protein consumption increases the capacity to oxidize amino acids, which can attenuate post-exercise anabolism and may impact protein requirements determined by stable isotope techniques predicated on amino acid tracer oxidation. We aimed to determine the impact of an acute (5d) reduction in dietary protein intake on post-exercise anabolism in high habitual consumers using the indicator amino acid oxidation (IAAO) technique. Resistance trained men [n = 5; 25 ± 7 y; 73.0 ± 5.7 kg; 9.9 ± 2.9% body fat; 2.69 ± 0.38 g·kg⁻¹·d⁻¹ habitual protein intake) consumed a high (H; 2.2 g·kg⁻¹·d⁻¹) and moderate (M; 1.2 g·kg⁻¹·d⁻¹) protein diet while training every other day. During the High protein phase, participants consumed a 2d controlled diet prior to determining whole body phenylalanine turnover, net balance (NB), and ¹³CO₂ excretion (F¹³CO₂) after exercise via oral [¹³C]phenylalanine. During the Moderate phase, participants consumed 2.2 g protein·kg⁻¹·d⁻¹ for 2d prior to consuming 1.2 g protein·kg⁻¹·d⁻¹ for 5d. Phenylalanine metabolism was measured on days 1, 3, and 5 (M1, M3, and M5, respectively) of the moderate intake. F¹³CO₂, the primary outcome for IAAO, was ~72 and ~55% greater on the 1st day (M1, P < 0.05) and the third day of the moderate protein diet (M3, P = 0.07), respectively, compared to the High protein trial. Compared to the High protein trial, NB was ~25% lower on the 1st day (M1, P < 0.01) and 15% lower on the third day of the moderate protein diet (M3, P = 0.09). High habitual protein consumption may bias protein requirements determined by traditional IAAO methods that use only a 2d pre-trial controlled diet. Post-exercise whole body anabolism is attenuated following a reduction in protein intake in resistance trained men and may require ~3–5d to adapt. This trial is registered at clinicaltrials.gov as NCT03845569.

Protein Intake to Maximize Whole-Body Anabolism during Postexercise Recovery in Resistance-Trained Men with High Habitual Intakes is Severalfold Greater than the Current Recommended Dietary Allowance

BACKGROUND

Dietary protein supports resistance exercise-induced anabolism primarily via the stimulation of protein synthesis rates. The indicator amino acid oxidation (IAAO) technique provides a noninvasive estimate of the protein intake that maximizes whole-body protein synthesis rates and net protein balance.

OBJECTIVE

We utilized IAAO to determine the maximal anabolic response to postexercise protein ingestion in resistance-trained men.

METHODS

Seven resistance-trained men (mean ± SD age 24 ± 3 y; weight 80 ± 9 kg; 11 ± 5% body fat; habitual protein intake 2.3 ± 0.6 g·kg-1·d-1) performed a bout of whole-body resistance exercise prior to ingesting hourly mixed meals, which provided a variable amount of protein (0.20-3.00 g·kg-1·d-1) as crystalline amino acids modeled after egg protein. Steady-state protein kinetics were modeled with oral l-[1-13C]-phenylalanine. Breath and urine samples were taken at isotopic steady state to determine phenylalanine flux (PheRa), phenylalanine excretion (F13CO2; reciprocal of protein synthesis), and net balance (protein synthesis - PheRa). Total amino acid oxidation was estimated from the ratio of urinary urea and creatinine.

RESULTS

Mixed model biphasic linear regression revealed a plateau in F13CO2 (mean: 2.00; 95% CI: 1.62, 2.38 g protein·kg-1·d-1) (r2 = 0.64; P ˂ 0.01) and in net balance (mean: 2.01; 95% CI: 1.44, 2.57 g protein·kg-1·d-1) (r2 = 0.63; P ˂ 0.01). Ratios of urinary urea and creatinine concentrations increased linearly (r = 0.84; P ˂ 0.01) across the range of protein intakes.

CONCLUSIONS

A breakpoint protein intake of ∼2.0 g·kg-1·d-1, which maximized whole-body anabolism in resistance-trained men after exercise, is greater than previous IAAO-derived estimates for nonexercising men and is at the upper range of current general protein recommendations for athletes. The capacity to enhance whole-body net balance may be greater than previously suggested to maximize muscle protein synthesis in resistance-trained athletes accustomed to a high habitual protein intake. This trial was registered at clinicaltrials.gov as NCT03696264.

Protein to Maximize Whole-Body Anabolism in Resistance-trained Females after Exercise

INTRODUCTION

Current athlete-specific protein recommendations are based almost exclusively on research in males.

PURPOSE

Using the minimally invasive indicator amino acid oxidation technique, we determined the daily protein intake that maximizes whole-body protein synthesis (PS) and net protein balance (NB) after exercise in strength-trained females.

METHODS

Eight resistance-trained females (23 ± 3.5 yr, 67.0 ± 7.7 kg, 163.3 ± 3.7 cm, 24.4% ± 6.9% body fat; mean ± SD) completed a 2-d controlled diet during the luteal phase before performing an acute bout of whole-body resistance exercise. During recovery, participants consumed eight hourly meals providing a randomized test protein intake (0.2-2.9 g·kg·d) as crystalline amino acids modeled after egg protein, with constant phenylalanine (30.5 mg·kg·d) and excess tyrosine (40.0 mg·kg·d) intakes. Steady-state whole-body phenylalanine rate of appearance (Ra), oxidation (Ox; the reciprocal of PS), and NB (PS - Ra) were determined from oral [C] phenylalanine ingestion. Total protein oxidation was estimated from the urinary urea-creatinine ratio (U/Cr).

RESULTS

A mixed model biphase linear regression revealed a break point (i.e., estimated average requirement) of 1.49 ± 0.44 g·kg·d (mean ± 95% confidence interval) in Ox (r = 0.64) and 1.53 ± 0.32 g·kg·d in NB (r = 0.65), indicating a saturation in whole-body anabolism. U/Cr increased linearly with protein intake (r = 0.56, P < 0.01).

CONCLUSIONS

Findings from this investigation indicate that the safe protein intake (upper 95% confidence interval) to maximize anabolism and minimize protein oxidation for strength-trained females during the early ~8-h postexercise recovery period is at the upper end of the recommendations of the American College of Sports Medicine for athletes (i.e., 1.2-2.0 g·kg·d).

Maximizing Post-exercise Anabolism: The Case for Relative Protein Intakes

Maximizing the post-exercise increase in muscle protein synthesis, especially of the contractile myofibrillar protein fraction, is essential to facilitate effective muscle remodeling, and enhance hypertrophic gains with resistance training. MPS is the primary regulated variable influencing muscle net balance with dietary amino acid ingestion representing the single most important nutritional variable enhancing post-exercise rates of muscle protein synthesis. Dose-response studies in average (i.e., ~80 kg) males have reported an absolute 20 g dose of high quality, rapidly digested protein maximizes mixed, and myofibrillar protein synthetic rates. However, it is unclear if these absolute protein intakes can be viewed in a “one size fits all” solution. Re-analysis of published literature in young adults suggests a relative single meal intake of ~0.31 g/kg of rapidly digested, high quality protein (i.e., whey) should be considered as a nutritional guideline for individuals of average body composition aiming to maximize post-exercise myofibrillar protein synthesis while minimizing irreversible amino acid oxidative catabolism that occurs with excessive intakes of this macronutrient. This muscle-specific bolus intake is lower than that reported to maximize whole body anabolism (i.e., ≥0.5 g/kg). Review of the available literature suggests that potential confounders such as the co-ingestion of carbohydrate, sex, and amount of active muscle mass do not represent significant barriers to the translation of this objectively determined relative protein intake. Additional research is warranted to elucidate the effective dose for proteins with suboptimal amino acid compositions (e.g., plant-based), and/or slower digestion rates as well as whether recommendations are appreciably affected by other physiological conditions such endurance exercise, high habitual daily protein ingestion, aging, obesity, and/or periods of chronic negative energy balance.

Molecular regulation of human skeletal muscle protein synthesis in response to exercise and nutrients: a compass for overcoming age-related anabolic resistance

Skeletal muscle mass, a strong predictor of longevity and health in humans, is determined by the balance of two cellular processes, muscle protein synthesis (MPS) and muscle protein breakdown. MPS seems to be particularly sensitive to changes in mechanical load and/or nutritional status; therefore, much research has focused on understanding the molecular mechanisms that underpin this cellular process. Furthermore, older individuals display an attenuated MPS response to anabolic stimuli, termed anabolic resistance, which has a negative impact on muscle mass and function, as well as quality of life. Therefore, an understanding of which, if any, molecular mechanisms contribute to anabolic resistance of MPS is of vital importance in formulation of therapeutic interventions for such populations. This review summarizes the current knowledge of the mechanisms that underpin MPS, which are broadly divided into mechanistic target of rapamycin complex 1 (mTORC1)-dependent, mTORC1-independent, and ribosomal biogenesis-related, and describes the evidence that shows how they are regulated by anabolic stimuli (exercise and/or nutrition) in healthy human skeletal muscle. This review also summarizes evidence regarding which of these mechanisms may be implicated in age-related skeletal muscle anabolic resistance and provides recommendations for future avenues of research that can expand our knowledge of this area.

Obesity Alters the Muscle Protein Synthetic Response to Nutrition and Exercise

Improving the health of skeletal muscle is an important component of obesity treatment. Apart from allowing for physical activity, skeletal muscle tissue is fundamental for the regulation of postprandial macronutrient metabolism, a time period that represents when metabolic derangements are most often observed in adults with obesity. In order for skeletal muscle to retain its capacity for physical activity and macronutrient metabolism, its protein quantity and composition must be maintained through the efficient degradation and resynthesis for proper tissue homeostasis. Life-style behaviors such as increasing physical activity and higher protein diets are front-line treatment strategies to enhance muscle protein remodeling by primarily stimulating protein synthesis rates. However, the muscle of individuals with obesity appears to be resistant to the anabolic action of targeted exercise regimes and protein ingestion when compared to normal-weight adults. This indicates impaired muscle protein remodeling in response to the main anabolic stimuli to human skeletal muscle tissue is contributing to poor muscle health with obesity. Deranged anabolic signaling related to insulin resistance, lipid accumulation, and/or systemic/muscle inflammation are likely at the root of the anabolic resistance of muscle protein synthesis rates with obesity. The purpose of this review is to discuss the impact of protein ingestion and exercise on muscle protein remodeling in people with obesity, and the potential mechanisms underlining anabolic resistance of their muscle.

Dietary Protein Quantity, Quality, and Exercise Are Key to Healthy Living: A Muscle-Centric Perspective Across the Lifespan

A healthy eating pattern, regardless of age, should consist of ingesting high quality protein preferably in adequate amounts across all meals throughout the day. Of particular relevance to overall health is the growth, development, and maintenance of skeletal muscle tissue. Skeletal muscle not only contributes to physical strength and performance, but also contributes to efficient macronutrient utilization and storage. Achieving an optimal amount of muscle mass begins early in life with transitions to "steady-state" maintenance as an adult, and then safeguarding against ultimate decline of muscle mass with age, all of which are influenced by physical activity and dietary (e.g., protein) factors. Current protein recommendations, as defined by recommended dietary allowances (RDA) for the US population or the population reference intakes (PRI) in Europe, are set to cover basic needs; however, it is thought that a higher protein intake might be necessary for optimizing muscle mass, especially for adults and individuals with an active lifestyle. It is necessary to balance the accurate assessment of protein quality (e.g., digestible indispensable amino acid score; DIAAS) with methods that provide a physiological correlate (e.g., established measures of protein synthesis, substrate oxidation, lean mass retention, or accrual, etc.) in order to accurately define protein requirements for these physiological outcomes. Moreover, current recommendations need to shift from single nutrient guidelines to whole food based guidelines in order to practically acknowledge food matrix interactions and other required nutrients for potentially optimizing the health effects of food. The aim of this paper is to discuss protein quality and amount that should be consumed with consideration to the presence of non-protein constituents within a food matrix and potential interactions with physical activity to maximize muscle mass throughout life.

The Effect of Dietary Protein on Protein Metabolism and Performance in Endurance-trained Males

Recommendations for dietary protein are primarily based on intakes that maintain nitrogen (i.e., protein) balance rather than optimize metabolism and/or performance.

PURPOSE

This study aimed to determine how varying protein intakes, including a new tracer-derived safe intake, alter whole body protein metabolism and exercise performance during training.

METHODS

Using a double-blind randomized crossover design, 10 male endurance-trained runners (age, 32 ± 8 yr; V˙O2peak, 65.9 ± 7.9 mL O2·kg·min) performed three trials consisting of 4 d of controlled training (20, 5, 10, and 20 km·d, respectively) while consuming diets providing 0.94 (LOW), 1.20 (MOD), and 1.83 (HIGH) g protein·kg·d. Whole body protein synthesis, breakdown, and net balance were determined by oral [N]glycine on the first and last day of the 4-d controlled training period, whereas exercise performance was determined from maximum voluntary isometric contraction, 5-km time trial, and countermovement jump impulse (IMP) and peak force before and immediately after the 4-d intervention.

RESULTS

Synthesis and breakdown were not affected by protein intake, whereas net balance showed a dose-response (HIGH > MOD > LOW, P < 0.05) with only HIGH being in positive balance (P < 0.05). There was a trend (P = 0.06) toward an interaction in 5-km Time Trial with HIGH having a moderate effect over LOW (effect size = 0.57) and small effect over MOD (effect size = 0.26). IMP decreased with time (P < 0.01) with no effect of protein (P = 0.56). There was no effect of protein intake (P ≥ 0.06) on maximum voluntary isometric contraction, IMP, or peak force performance.

CONCLUSION

Our data suggest that athletes who consume dietary protein toward the upper end of the current recommendations by the American College of Sports Medicine (1.2-2 g·kg) would better maintain protein metabolism and potentially exercise performance during training.

Translocation and protein complex co-localization of mTOR is associated with postprandial myofibrillar protein synthesis at rest and after endurance exercise

Translocation and colocalization of mechanistic target of rapamycin complex 1 (mTORC1) with regulatory proteins represents a critical step in translation initiation of protein synthesis in vitro. However, mechanistic insight into the control of postprandial skeletal muscle protein synthesis rates at rest and after an acute bout of endurance exercise in humans is lacking. In crossover trials, eight endurance‐trained men received primed‐continuous infusions of L‐[ring‐²H₅]phenylalanine and consumed a mixed‐macronutrient meal (18 g protein, 60 g carbohydrates, 17 g fat) at rest (REST) and after 60 min of treadmill running at 70% VO_2peak (EX). Skeletal muscle biopsies were collected to measure changes in phosphorylation and colocalization in the mTORC1‐pathway, in addition to rates of myofibrillar (MyoPS) and mitochondrial (MitoPS) protein synthesis. MyoPS increased (P < 0.05) above fasted in REST (~2.1‐fold) and EX (~twofold) during the 300 min postprandial period, with no corresponding changes in MitoPS (P > 0.05). TSC2/Rheb colocalization decreased below fasted at 60 and 300 min after feeding in REST and EX (P < 0.01). mTOR colocalization with Rheb increased above fasted at 60 and 300 min after feeding in REST and EX (P < 0.01), which was consistent with an increased phosphorylation 4E‐BP1^Thr37/46 and rpS6^ser240/244 at 60 min. Our data suggest that MyoPS, but not MitoPS, is primarily nutrient responsive in trained young men at rest and after endurance exercise. The postprandial increase in MyoPS is associated with an increase in mTOR/Rheb colocalization and a reciprocal decrease in TSC2/Rheb colocalization and thus likely represent important regulatory events for in vivo skeletal muscle myofibrillar mRNA translation in humans.

Protein Metabolism in Active Youth: Not Just Little Adults

Understanding how exercise and dietary protein alter the turnover and synthesis of body proteins in youth can provide guidelines for the optimal development of lean mass. This review hypothesizes that active youth obtain similar anabolic benefits from exercise and dietary protein as adults, but the requirement for amino acids to support growth renders them more sensitive to these nutrients.

Reduced physical activity in young and older adults: metabolic and musculoskeletal implications

BACKGROUND

Although the health benefits of regular physical activity and exercise are well established and have been incorporated into national public health recommendations, there is a relative lack of understanding pertaining to the harmful effects of physical inactivity. Experimental paradigms including complete immobilization and bed rest are not physiologically representative of sedentary living. A useful 'real-world' approach to contextualize the physiology of societal downward shifts in physical activity patterns is that of short-term daily step reduction.

RESULTS

Step-reduction studies have largely focused on musculoskeletal and metabolic health parameters, providing relevant disease models for metabolic syndrome, type 2 diabetes (T2D), nonalcoholic fatty liver disease (NAFLD), sarcopenia and osteopenia/osteoporosis. In untrained individuals, even a short-term reduction in physical activity has a significant impact on skeletal muscle protein and carbohydrate metabolism, causing anabolic resistance and peripheral insulin resistance, respectively. From a metabolic perspective, short-term inactivity-induced peripheral insulin resistance in skeletal muscle and adipose tissue, with consequent liver triglyceride accumulation, leads to hepatic insulin resistance and a characteristic dyslipidaemia. Concomitantly, various inactivity-related factors contribute to a decline in function; a reduction in cardiorespiratory fitness, muscle mass and muscle strength.

CONCLUSIONS

Physical inactivity maybe particularly deleterious in certain patient populations, such as those at high risk of T2D or in the elderly, considering concomitant sarcopenia or osteoporosis. The effects of short-term physical inactivity (with step reduction) are reversible on resumption of habitual physical activity in younger people, but less so in older adults. Nutritional interventions and resistance training offer potential strategies to prevent these deleterious metabolic and musculoskeletal effects.

IMPACT

Individuals at high risk of/with cardiometabolic disease and older adults may be more prone to these acute periods of inactivity due to acute illness or hospitalization. Understanding the risks is paramount to implementing countermeasures.

Whole-body net protein balance plateaus in response to increasing protein intakes during post-exercise recovery in adults and adolescents

Background

Muscle protein synthesis and muscle net balance plateau after moderate protein ingestion in adults. However, it has been suggested that there is no practical limit to the anabolic response of whole-body net balance to dietary protein. Moreover, limited research has addressed the anabolic response to dietary protein in adolescents. The present study determined whether whole-body net balance plateaued in response to increasing protein intakes during post-exercise recovery and whether there were age- and/or sex-related dimorphisms in the anabolic response.

Methods

Thirteen adults [7 males (M), 6 females (F)] and 14 adolescents [7 males (AM), 7 females (AF) within ~ 0.4 y from peak height velocity] performed ~ 1 h variable intensity exercise (i.e., Loughborough Intermittent Shuttle Test) prior to ingesting hourly mixed meals that provided a variable amount of protein (0.02-0.25 g·kg^- 1·h^- 1) as crystalline amino acids modeled after egg protein. Steady-state protein kinetics were modeled noninvasively with oral L-[1-¹³C]phenylalanine. Breath and urine samples were taken at plateau to determine phenylalanine oxidation and flux (estimate of protein breakdown), respectively. Whole-body net balance was determined by the difference between protein synthesis (flux - oxidation) and protein breakdown. Total amino acid oxidation was estimated from the ratio of urinary urea/creatinine.

Results

Mixed model biphasic linear regression explained a greater proportion of net balance variance than linear regression (all, r ² ≥ 0.56; P < 0.01), indicating an anabolic plateau. Net balance was maximized at ~ 0.15, 0.12, 0.12, and 0.11 g protein·kg^- 1·h^- 1 in M, F, AM, and AF, respectively. When collapsed across age, the y-intercept (net balance at very low protein intake) was greater (overlapping CI did not contain zero) in adolescents vs. adults. Urea/creatinine excretion increased linearly (all, r ≥ 0.76; P < 0.01) across the range of protein intakes. At plateau, net balance was greater (P < 0.05) in AM vs. M.

Conclusions

Our data suggest there is a practical limit to the anabolic response to protein ingestion within a mixed meal and that higher intakes lead to deamination and oxidation of excess amino acids. Consistent with a need to support lean mass growth, adolescents appear to have greater anabolic sensitivity and a greater capacity to assimilate dietary amino acids than adults.

Whole egg, but not egg white, ingestion induces mTOR colocalization with the lysosome after resistance exercise

We have recently demonstrated that whole egg ingestion induces a greater muscle protein synthetic (MPS) response when compared with isonitrogenous egg white ingestion after resistance exercise in young men. Our aim was to determine whether whole egg or egg white ingestion differentially influenced colocalization of key regulators of mechanistic target of rapamycin complex 1 (mTORC1) as means to explain our previously observed divergent postexercise MPS response. In crossover trials, 10 healthy resistance-trained men (21 ± 1 yr; 88 ± 3 kg; body fat: 16 ± 1%; means ± SE) completed lower body resistance exercise before ingesting whole eggs (18 g protein, 17 g fat) or egg whites (18 g protein, 0 g fat). Muscle biopsies were obtained before exercise and at 120 and 300 min after egg ingestion to assess, by immunofluorescence, protein colocalization of key anabolic signaling molecules. After resistance exercise, tuberous sclerosis 2-Ras homolog enriched in brain (Rheb) colocalization decreased ( P < 0.01) at 120 and 300 min after whole egg and egg white ingestion with concomitant increases ( P < 0.01) in mTOR-Rheb colocalization. After resistance exercise, mTOR-lysosome-associated membrane protein 2 (LAMP2) colocalization significantly increased at 120 and 300 min only after whole egg ingestion ( P < 0.01), and mTOR-LAMP2 colocalization correlated with rates of MPS at rest and after exercise ( r = 0.40, P < 0.05). We demonstrated that the greater postexercise MPS response with whole egg ingestion is related in part to an enhanced recruitment of mTORC1-Rheb complexes to the lysosome during recovery. These data suggest nonprotein dietary factors influence the postexercise regulation of mRNA translation in human skeletal muscle.

Dysregulated Handling of Dietary Protein and Muscle Protein Synthesis After Mixed-Meal Ingestion in Maintenance Hemodialysis Patients

Introduction

Skeletal muscle loss is common in patients with renal failure who receive maintenance hemodialysis (MHD) therapy. Regular ingestion of protein-rich meals are recommended to help offset muscle protein loss in MHD patients, but little is known about the anabolic potential of this strategy.

Methods

Eight MHD patients (age: 56 ± 5 years; body mass index [BMI]: 32 ± 2 kg/m²) and 8 nonuremic control subjects (age: 50 ± 2 years: BMI: 31 ± 1 kg/m²) received primed continuous L-[ring-²H₅]phenylalanine and L-[1-¹³C]leucine infusions with blood and muscle biopsy sampling on a nondialysis day. Participants consumed a mixed meal (546 kcal; 20-g protein, 59-g carbohydrates, 26-g fat) with protein provided as L-[5,5,5-²H₃]leucine-labeled eggs.

Results

Circulating dietary amino acid availability was reduced in MHD patients (41 ± 5%) versus control subjects (61 ± 4%; P = 0.03). Basal muscle caspase-3 protein content was elevated (P = 0.03) and large neutral amino acid transporter 1 (LAT1) protein content was reduced (P = 0.02) in MHD patients versus control subjects. Basal muscle protein synthesis (MPS) was ∼2-fold higher in MHD patients (0.030 ± 0.005%/h) versus control subjects (0.014 ± 0.003%/h) (P = 0.01). Meal ingestion failed to increase MPS in MHD patients (absolute change from basal: 0.0003 ± 0.007%/h), but stimulated MPS in control subjects (0.009 ± 0.002%/h; P = 0.004).

Conclusions

MHD patients demonstrated muscle anabolic resistance to meal ingestion. This blunted postprandial MPS response in MHD patients might be related to high basal MPS, which results in a stimulatory ceiling effect and/or reduced plasma dietary amino acid availability after mixed-meal ingestion.