MECHANISMS IN ENDOCRINOLOGY: Exogenous insulin does not increase muscle protein synthesis rate when administered systemically: a systematic review

Co-ingestion of cluster dextrin carbohydrate does not increase exogenous protein-derived amino acid release or myofibrillar protein synthesis following a whole-body resistance exercise in moderately trained younger males: a double-blinded randomized controlled crossover trial

Purpose

This study investigates if co-ingestion of cluster dextrin (CDX) augments the appearance of intrinsically labeled meat protein hydrolysate-derived amino acid (D₅-phenylalanine), Akt/mTORC1 signaling, and myofibrillar protein fractional synthetic rate (FSR).

Methods

Ten moderately trained healthy males (age: 21.5 ± 2.1 years, body mass: 75.7 ± 7.6 kg, body mass index (BMI): 22.9 ± 2.1 kg/m²) were included for a double-blinded randomized controlled crossover trial. Either 75 g of CDX or glucose (GLC) was given in conjunction with meat protein hydrolysate (0.6 g protein * FFM⁻¹) following a whole-body resistance exercise. A primed-continuous intravenous infusion of L-[¹⁵N]-phenylalanine with serial muscle biopsies and venous blood sampling was performed.

Results

A time × group interaction effect was found for serum D₅-phenylalanine enrichment (P < 0.01). Serum EAA and BCAA concentrations showed a main effect for group (P < 0.05). T_max serum BCAA was greater in CDX as compared to GLC (P < 0.05). However, iAUC of all serum parameters did not differ between CDX and GLC (P > 0.05). T_max serum EAA showed a trend towards a statistical significance favoring CDX over GLC. The phosphorylation of p70S6K^Thr389, rpS6^Ser240/244, ERK1/2^{Thr202/Tyr204} was greater in CDX compared to GLC (P < 0.05). However, postprandial myofibrillar FSR did not differ between CDX and GLC (P = 0.17).

Conclusion

In moderately trained younger males, co-ingestion of CDX with meat protein hydrolysate does not augment the postprandial amino acid availability or myofibrillar FSR as compared to co-ingestion of GLC during the recovery from a whole-body resistance exercise despite an increased intramuscular signaling.

Trial registration

ClinicalTrials.gov ID: NCT03303729 (registered on October 3, 2017).

Putative Anti-Immobility Action of Acute Insulin Is Attributable to an Increase in Locomotor Activity in Healthy Wistar Rats

BACKGROUND/AIMS

Anti-immobility actions of insulin in diabetic rats that are subjected to the forced swim test (FST) have been reported. In this test, low doses of antidepressants exert actions after long-term treatment, without affecting locomotor activity in healthy rats. Few studies have compared acute and chronic actions of insulin with antidepressants in healthy rats.

METHODS

We hypothesized that if insulin exerts a true anti-immobility action, then its effects must be comparable to fluoxetine in both a 1-day treatment regimen and a 21-day treatment regimen in healthy, gonadally intact female Wistar rats.

RESULTS

The results showed that low levels of glycemia were produced by all treatments, including fluoxetine, and glycemia was lower in proestrus-estrus than in diestrus-metestrus. None of the treatments or regimens produced actions on indicators of anxiety in the elevated plus maze. Insulin in the 1-day regimen increased the number of crossings and rearings in the open field test and caused a low cumulative immobility time in the FST. These actions disappeared in the 21-day regimen. Compared with the other treatments, fluoxetine treatment alone or combined with insulin produced a longer latency to the first period of immobility and a shorter immobility time in the chronic regimen in the FST, without affecting locomotor activity, and more pronounced actions were observed in proestrus-estrus (i.e., a true anti-immobility effect).

CONCLUSION

These results indicate that insulin does not produce a true antidepressant action in healthy rats. The purported antidepressant effects that were observed were instead attributable to an increase in locomotor activity only in the 1-day regimen.

Dietary protein requirements and recommendations for healthy older adults: a critical narrative review of the scientific evidence

Adequate protein intake is essential for the maintenance of whole-body protein mass. Different methodological approaches are used to substantiate the evidence for the current protein recommendations, and it is continuously debated whether older adults require more protein to counteract the age-dependent loss of muscle mass, sarcopenia. Thus, the purpose of this critical narrative review is to outline and discuss differences in the approaches and methodologies assessing the protein requirements and, hence, resulting in controversies in current protein recommendations for healthy older adults. Through a literature search, this narrative review first summarises the historical development of the Food and Agriculture Organization/World Health Organization/United Nations University setting of protein requirements and recommendations for healthy older adults. Hereafter, we describe the various types of studies (epidemiological studies and protein turnover kinetic measurements) and applied methodological approaches founding the basis and the different recommendations with focus on healthy older adults. Finally, we discuss important factors to be considered in future studies to obtain evidence for international agreement on protein requirements and recommendations for healthy older adults. We conclude by proposing future directions to determine 'true' protein requirements and recommendations for healthy older adults.

Ocular surface complications in diabetes: The interrelationship between insulin and enkephalin

Diabetes is a multi-faceted disorder with increasing prevalence and rising healthcare costs. The burden of diabetes is increased because of associated complications affecting nearly all organs including the eye. The underlying pathophysiology for the onset of these ocular surface disorders is not well known. Enkephalins are endogenous opioids that originate in the brain and have numerous actions in the human body. Opioid growth factor (OGF), chemically termed [Met5]-enkephalin, binds to a novel, nuclear-associated receptor and mediates cellular homeostasis. Serum OGF levels are elevated in diabetic individuals and rodent models of diabetes. Sustained blockade of the OGF receptor (OGFr) with opioid receptor antagonists, such as naltrexone (NTX), reverses many complications of diabetes in the animal model, including delayed cutaneous wound healing, dry eye, altered corneal surface sensitivity, and keratopathy. The increased enkephalin levels observed in diabetes suggest a relationship between endogenous opioid peptides and the pathophysiology of diabetes. It is common for diabetic patients to undergo insulin therapy to restore normal blood glucose levels. However, this restoration does not alter OGF serum levels nor ameliorate ocular surface complications in the animal model of diabetes. Moreover, sex differences in the prevalence of diabetes, response to insulin therapy, and abnormalities in the OGF-OGFr axis have been reported. This review highlights current knowledge on the dysregulation of the OGF-OGFr pathway and possible relationships of insulin and enkephalins to the development of ocular surface defects in diabetes. It proposes that this dysregulation is a fundamental mechanism for the pathobiology of diabetic complications.

The Anabolic Response to Plant-Based Protein Ingestion

There is a global trend of an increased interest in plant-based diets. This includes an increase in the consumption of plant-based proteins at the expense of animal-based proteins. Plant-derived proteins are now also frequently applied in sports nutrition. So far, we have learned that the ingestion of plant-derived proteins, such as soy and wheat protein, result in lower post-prandial muscle protein synthesis responses when compared with the ingestion of an equivalent amount of animal-based protein. The lesser anabolic properties of plant-based versus animal-derived proteins may be attributed to differences in their protein digestion and amino acid absorption kinetics, as well as to differences in amino acid composition between these protein sources. Most plant-based proteins have a low essential amino acid content and are often deficient in one or more specific amino acids, such as lysine and methionine. However, there are large differences in amino acid composition between various plant-derived proteins or plant-based protein sources. So far, only a few studies have directly compared the muscle protein synthetic response following the ingestion of a plant-derived protein versus a high(er) quality animal-derived protein. The proposed lower anabolic properties of plant- versus animal-derived proteins may be compensated for by (i) consuming a greater amount of the plant-derived protein or plant-based protein source to compensate for the lesser quality; (ii) using specific blends of plant-based proteins to create a more balanced amino acid profile; (iii) fortifying the plant-based protein (source) with the specific free amino acid(s) that is (are) deficient. Clinical studies are warranted to assess the anabolic properties of the various plant-derived proteins and their protein sources in vivo in humans and to identify the factors that may or may not compromise the capacity to stimulate post-prandial muscle protein synthesis rates. Such work is needed to determine whether the transition towards a more plant-based diet is accompanied by a transition towards greater dietary protein intake requirements.

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.

Casein Protein Processing Strongly Modulates Post-Prandial Plasma Amino Acid Responses In Vivo in Humans

Micellar casein is characterized as a slowly digestible protein source, and its structure can be modulated by various food processing techniques to modify its functional properties. However, little is known about the impact of such modifications on casein protein digestion and amino acid absorption kinetics and the subsequent post-prandial plasma amino acid responses. In the present study, we determined post-prandial aminoacidemia following ingestion of isonitrogenous amounts of casein protein (40 g) provided as micellar casein (Mi-CAS), calcium caseinate (Ca-CAS), or cross-linked sodium caseinate (XL-CAS). Fifteen healthy, young men (age: 26 ± 4 years, BMI: 23 ± 1 kg·m-2) participated in this randomized cross-over study and ingested 40 g Mi-Cas, Ca-CAS, and XL-CAS protein, with a ~1 week washout between treatments. On each trial day, arterialized blood samples were collected at regular intervals during a 6 h post-prandial period to assess plasma amino acid concentrations using ultra-performance liquid chromatography. Plasma amino acid concentrations were higher following the ingestion of XL-CAS when compared to Mi-CAS and Ca-CAS from t = 15 to 90 min (all p < 0.05). Plasma amino acid concentrations were higher following ingestion of Mi-CAS compared to Ca-CAS from t = 30 to 45 min (both p < 0.05). Plasma total amino acids iAUC were higher following the ingestion of XL-CAS when compared to Ca-CAS (294 ± 63 vs. 260 ± 75 mmol·L-1, p = 0.006), with intermediate values following Mi-CAS ingestion (270 ± 63 mmol·L-1, p > 0.05). In conclusion, cross-linked sodium caseinate is more rapidly digested when compared to micellar casein and calcium caseinate. Protein processing can strongly modulate the post-prandial rise in plasma amino acid bioavailability in vivo in humans.

Carbohydrate restriction: Friend or foe of resistance-based exercise performance?

It is commonly accepted that adequate carbohydrate availability is necessary for optimal endurance performance. However, for strength- and physique-based athletes, sports nutrition research and recommendations have focused on protein ingestion, with far less attention given to carbohydrates. Varying resistance exercise protocols, such as differences in intensity, volume, and intraset rest prescriptions between strength-training and physique-training goals elicit different metabolic responses, which may necessitate different carbohydrate needs. The results of several acute and chronic training studies suggest that although severe carbohydrate restriction may not impair strength adaptations during a resistance training program, consuming an adequate amount of carbohydrate in the days leading up to testing may enhance maximal strength and strength-endurance performance. Although several molecular studies demonstrate no additive increases in postexercise mammalian target of rapamycin 1 phosphorylation with carbohydrate and protein compared with protein ingestion alone, the effects of chronic resistance training with carbohydrate restriction on muscle hypertrophy are conflicting and require further research to determine a minimal carbohydrate threshold necessary to optimize muscle hypertrophy. This review summarizes the current knowledge regarding carbohydrate availability and resistance training outcomes and poses new research questions that will better help guide carbohydrate recommendations for strength and physique athletes. In addition, given that success in physique sports is based on subjective appearance, and not objective physical performance, we also review the effects of subchronic carbohydrate ingestion during contest preparation on aesthetic appearance.

Protein Intake at Breakfast Promotes a Positive Whole-Body Protein Balance in a Dose-Response Manner in Healthy Children: A Randomized Trial

Background

Protein ingestion promotes whole-body net protein balance (NB) in children, which is a prerequisite for growth. Determining how much protein is required at breakfast to promote a positive NB, which may be negative after the traditional overnight fast in children, has yet to be determined.

Objective

We determined the impact of incremental doses of milk protein at breakfast as well as the impact of daily dietary protein distribution on NB in children.

Methods

A total of 28 children [14 boys, 14 girls; age range: 7-11 y; body mass index (mean ± SD, in kg/m2): 16.0 ± 1.9] completed 2 intervention trials. During the breakfast meal, participants consumed an isoenergetic beverage with different amounts of protein (0, 7, 14, or 21 g for Groups A-D, respectively) and [15N]-glycine to measure whole body protein metabolism. Whole-body nitrogen turnover, protein synthesis (PS), protein breakdown, and NB were measured over 9 and 24 h.

Results

Following an overnight fast, children were in negative NB (-64.5 mg · kg-1 · h-1). Protein ingestion at breakfast induced a stepwise increase in NB over 9 h [Groups A (6.2 mg · kg-1 · h-1) < B (27.9 mg · kg-1 · h-1) < C (46.9 mg · kg-1 · h-1) < D (66.0 mg · kg-1 · h-1)] with all conditions different from each other (all P < 0.01). PS was 42% greater in Group D than in Group A over 9 h (P < 0.05).

Conclusions

Consuming ≥7 g of the total daily protein intake at breakfast attenuates the observed overnight protein losses in children during the subsequent 9 h following breakfast consumption. The dose-dependent increase in NB over a daytime fed period, inclusive of breakfast and lunch, highlights the importance of breakfast protein intake on acute anabolism in healthy active children. This trial was registered at clinicaltrials.gov as NCT02465151.

International Society of Sports Nutrition Position Stand: protein and exercise

The International Society of Sports Nutrition (ISSN) provides an objective and critical review related to the intake of protein for healthy, exercising individuals. Based on the current available literature, the position of the Society is as follows:An acute exercise stimulus, particularly resistance exercise, and protein ingestion both stimulate muscle protein synthesis (MPS) and are synergistic when protein consumption occurs before or after resistance exercise.For building muscle mass and for maintaining muscle mass through a positive muscle protein balance, an overall daily protein intake in the range of 1.4-2.0 g protein/kg body weight/day (g/kg/d) is sufficient for most exercising individuals, a value that falls in line within the Acceptable Macronutrient Distribution Range published by the Institute of Medicine for protein.Higher protein intakes (2.3-3.1 g/kg/d) may be needed to maximize the retention of lean body mass in resistance-trained subjects during hypocaloric periods.There is novel evidence that suggests higher protein intakes (>3.0 g/kg/d) may have positive effects on body composition in resistance-trained individuals (i.e., promote loss of fat mass).Recommendations regarding the optimal protein intake per serving for athletes to maximize MPS are mixed and are dependent upon age and recent resistance exercise stimuli. General recommendations are 0.25 g of a high-quality protein per kg of body weight, or an absolute dose of 20-40 g.Acute protein doses should strive to contain 700-3000 mg of leucine and/or a higher relative leucine content, in addition to a balanced array of the essential amino acids (EAAs).These protein doses should ideally be evenly distributed, every 3-4 h, across the day.The optimal time period during which to ingest protein is likely a matter of individual tolerance, since benefits are derived from pre- or post-workout ingestion; however, the anabolic effect of exercise is long-lasting (at least 24 h), but likely diminishes with increasing time post-exercise.While it is possible for physically active individuals to obtain their daily protein requirements through the consumption of whole foods, supplementation is a practical way of ensuring intake of adequate protein quality and quantity, while minimizing caloric intake, particularly for athletes who typically complete high volumes of training. Rapidly digested proteins that contain high proportions of essential amino acids (EAAs) and adequate leucine, are most effective in stimulating MPS. Different types and quality of protein can affect amino acid bioavailability following protein supplementation. Athletes should consider focusing on whole food sources of protein that contain all of the EAAs (i.e., it is the EAAs that are required to stimulate MPS). Endurance athletes should focus on achieving adequate carbohydrate intake to promote optimal performance; the addition of protein may help to offset muscle damage and promote recovery. Pre-sleep casein protein intake (30-40 g) provides increases in overnight MPS and metabolic rate without influencing lipolysis.

Carbohydrate intake and resistance-based exercise: are current recommendations reflective of actual need?

Substantial research has been completed examining the impact of carbohydrate (CHO) intake on endurance exercise, whereas its role in resistance-based exercise performance, adaptation and cell signalling has yet to be fully characterised. This empirical shortcoming has precluded the ability to establish specific CHO recommendations for resistance exercise. This results in recommendations largely stemming from findings based on endurance exercise and/or anecdotal evidence despite the distinct energetic demands and molecular responses mediating adaptation from endurance- and resistance-based exercise. Moreover, the topic of CHO and exercise has become one of polarising nature with divergent views - some substantiated, others lacking evidence. Current literature suggests a moderately high daily CHO intake (3-7 g/kg per d) for resistance training, which is thought to prevent glycogen depletion and facilitate performance and adaptation. However, contemporary investigation, along with an emerging understanding of the molecular underpinnings of resistance exercise adaptation, may suggest that such an intake may not be necessary. In addition to the low likelihood of true glycogen depletion occurring in response to resistance exercise, a diet restrictive in CHO may not be detrimental to acute resistance exercise performance or the cellular signalling activity responsible for adaptation, even when muscle glycogen stores are reduced. Current evidence suggests that signalling of the mammalian target of rapamycin complex 1, the key regulatory kinase for gene translation (protein synthesis), is unaffected by CHO restriction or low muscular glycogen concentrations. Such findings may call into question the current view and subsequent recommendations of CHO intake with regard to resistance-based exercise.

Does the muscle protein synthetic response to exercise and amino acid-based nutrition diminish with advancing age? A systematic review

The precise role of age-related muscle anabolic resistance in the progression of sarcopenia and functional decline in older individuals is unclear. The present aim was to assess whether the muscle protein synthesis (MPS) response to acute exercise (endurance or resistance) and/or amino acid-based nutrition is attenuated in older compared with young individuals. A systematic review was conducted on studies that directly examined the influence of age on the MPS response to exercise and/or amino acid-based nutrition. Each study arm was synthesized and reported as providing sufficient or insufficient "evidence of age-related muscle anabolic resistance". Subsequently, three models were established to compare age-related differences in the MPS response to 1) exercise alone, 2) amino acid-based nutrition alone, or 3) the combination of exercise and amino acid-based nutrition. Following exercise alone, 8 of the 17 study arms provided sufficient evidence of age-related muscle anabolic resistance, while in response to amino acid-based nutrition alone, 8 of the 21 study arms provided sufficient evidence of age-related muscle anabolic resistance. When exercise and amino acid-based nutrition were combined, only 2 of the 10 study arms provided sufficient evidence of age-related muscle anabolic resistance. Our results highlight that optimization of exercise and amino acid-based nutrition is sufficient to induce a comparable MPS response between young and older individuals. However, the exercise volume completed and/or the amino acid/protein dose and leucine content must exceed a certain threshold to stimulate equivalent MPS rates in young and older adults, below which age-related muscle anabolic resistance may become apparent.

Innovative biomarkers for predicting type 2 diabetes mellitus: relevance to dietary management of frailty in older adults

Type 2 diabetes mellitus (T2DM) increases in prevalence in the elderly. There is evidence for significant muscle loss and accelerated cognitive impairment in older adults with T2DM; these comorbidities are critical features of frailty. In the early stages of T2DM, insulin sensitivity can be improved by a "healthy" diet. Management of insulin resistance by diet in people over 65 years of age should be carefully re-evaluated because of the risk for falling due to hypoglycaemia. To date, an optimal dietary programme for older adults with insulin resistance and T2DM has not been described. The use of biomarkers to identify those at risk for T2DM will enable clinicians to offer early dietary advice that will delay onset of disease and of frailty. Here we have used an in silico literature search for putative novel biomarkers of T2DM risk and frailty. We suggest that plasma bilirubin, plasma, urinary DPP4-positive microparticles and plasma pigment epithelium-derived factor merit further investigation as predictive biomarkers for T2DM and frailty risk in older adults. Bilirubin is screened routinely in clinical practice. Measurement of specific microparticle frequency in urine is less invasive than a blood sample so is a good choice for biomonitoring. Future studies should investigate whether early dietary changes, such as increased intake of whey protein and micronutrients that improve muscle function and insulin sensitivity, affect biomarkers and can reduce the longer term complication of frailty in people at risk for T2DM.

Annual banned-substance review: analytical approaches in human sports drug testing

The aim of improving anti-doping efforts is predicated on several different pillars, including, amongst others, optimized analytical methods. These commonly result from exploiting most recent developments in analytical instrumentation as well as research data on elite athletes' physiology in general, and pharmacology, metabolism, elimination, and downstream effects of prohibited substances and methods of doping, in particular. The need for frequent and adequate adaptations of sports drug testing procedures has been incessant, largely due to the uninterrupted emergence of new chemical entities but also due to the apparent use of established or even obsolete drugs for reasons other than therapeutic means, such as assumed beneficial effects on endurance, strength, and regeneration capacities. Continuing the series of annual banned-substance reviews, literature concerning human sports drug testing published between October 2014 and September 2015 is summarized and reviewed in reference to the content of the 2015 Prohibited List as issued by the World Anti-Doping Agency (WADA), with particular emphasis on analytical approaches and their contribution to enhanced doping controls.

Protein supplementation does not alter intramuscular anabolic signaling or endocrine response after resistance exercise in trained men

The mammalian/mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway appears to be the primary regulator of muscle protein synthesis. A variety of stimuli including resistance exercise, amino acids, and hormonal signals activate mTORC1 signaling. The purpose of this study was to investigate the effect of a protein supplement on mTORC1 signaling following a resistance exercise protocol designed to promote elevations in circulating hormone concentrations. We hypothesized that the protein supplement would augment the intramuscular anabolic signaling response. Ten resistance-trained men (age, 24.7 ± 3.4 years; weight, 90.1 ± 11.3 kg; height, 176.0 ± 4.9 cm) received either a placebo or a supplement containing 20 g protein, 6 g carbohydrates, and 1 g fat after high-volume, short-rest lower-body resistance exercise. Blood samples were obtained at baseline, immediately, 30 minutes, 1 hour, 2 hours, and 5 hours after exercise. Fine-needle muscle biopsies were completed at baseline, 1 hour, and 5 hours after exercise. Myoglobin, lactate dehydrogenase, and lactate concentrations were significantly elevated after resistance exercise (P < .0001); however, no differences were observed between trials. Resistance exercise also elicited a significant insulin, growth hormone, and cortisol response (P < .01); however, no differences were observed between trials for insulin-like growth factor-1, insulin, testosterone, growth hormone, or cortisol. Intramuscular anabolic signaling analysis revealed significant elevations in RPS6 phosphorylation after resistance exercise (P = .001); however, no differences were observed between trials for signaling proteins including Akt, mTOR, p70S6k, and RPS6. The endocrine response and phosphorylation status of signaling proteins within the mTORC1 pathway did not appear to be altered by ingestion of supplement after resistance exercise in resistance-trained men.

Nutritional interventions to augment resistance training-induced skeletal muscle hypertrophy

Skeletal muscle mass is regulated by a balance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). In healthy humans, MPS is more sensitive (varying 4–5 times more than MPB) to changes in protein feeding and loading rendering it the primary locus determining gains in muscle mass. Performing resistance exercise (RE) followed by the consumption of protein results in an augmentation of MPS and, over time, can lead to muscle hypertrophy. The magnitude of the RE-induced increase in MPS is dictated by a variety of factors including: the dose of protein, source of protein, and possibly the distribution and timing of post-exercise protein ingestion. In addition, RE variables such as frequency of sessions, time under tension, volume, and training status play roles in regulating MPS. This review provides a brief overview of our current understanding of how RE and protein ingestion can influence gains in skeletal muscle mass in young, healthy individuals. It is the goal of this review to provide nutritional recommendations for optimal skeletal muscle adaptation. Specifically, we will focus on how the manipulation of protein intake during the recovery period following RE augments the adaptive response.

Exercise and the Regulation of Skeletal Muscle Hypertrophy

Skeletal muscle is a critical organ serving as the primary site for postprandial glucose disposal and the generation of contractile force. The size of human skeletal muscle mass is dependent upon the temporal relationship between changes in muscle protein synthesis (MPS) and muscle protein breakdown. The aim of this chapter is to review our current understanding of how resistance exercise influences protein turnover with a specific emphasis on the molecular factors regulating MPS. We also will discuss recent data relating to the prescription of resistance exercise to maximize skeletal muscle hypertrophy. Finally, we evaluate the impact of age and periods of disuse on the loss of muscle mass and the controversy surround the etiology of muscle disuse atrophy.