Physiologic and molecular bases of muscle hypertrophy and atrophy: impact of resistance exercise on human skeletal muscle (protein and exercise dose effects)

Growth hormone plus resistance exercise attenuate structural changes in rat myotendinous junctions resulting from chronic unloading

Myotendinous junctions (MTJs) are specialized sites on the muscle surface where forces generated by myofibrils are transmitted across the sarcolemma to the extracellular matrix. At the ultrastructural level, the interface between the sarcolemma and extracellular matrix is highly folded and interdigitated at these junctions. In this study, the effect of exercise and growth hormone (GH) treatments on the changes in MTJ structure that occur during muscle unloading, has been analyzed. Twenty hypophysectomized rats were assigned randomly to one of five groups: ambulatory control, hindlimb unloaded, hindlimb unloaded plus exercise (3 daily bouts of 10 climbs up a ladder with 50% body wt attached to the tail), hindlimb unloaded plus GH (2 daily injections of 1 mg/kg body wt, i.p.), and hindlimb unloaded plus exercise plus GH. MTJs of the plantaris muscle were analyzed by electron microscopy and the contact between muscle and tendon was evaluated using an IL/B ratio, where B is the base and IL is the interface length of MTJ's digit-like processes. After 10 days of unloading, the mean IL/B ratio was significantly lower in unloaded (3.92), unloaded plus exercise (4.18), and unloaded plus GH (5.25) groups than in the ambulatory control (6.39) group. On the opposite, the mean IL/B ratio in the group treated with both exercise and GH (7.3) was similar to control. These findings indicate that the interaction between exercise and GH treatments attenuates the changes in MTJ structure that result from chronic unloading and thus can be used as a countermeasure to these adaptations.

Efficacy and safety of protein supplements for U.S. Armed Forces personnel: consensus statement

To provide evidence-based guidance regarding the efficacy and safety of dietary protein supplement (PS) use by members of the U.S. Armed Forces, a panel of internationally recognized experts in the fields of protein metabolism and dietary supplement research was convened by the Department of Defense Center Alliance for Dietary Supplement Research and the U.S. Army Medical Research and Material Command. To develop a consensus statement, potential benefits, risks, and strategies to optimize military performance through PS use were considered in the context of specific warfighter populations and occupational demands. To maintain muscle mass, strength, and performance during periods of substantial metabolic demand and concomitant negative energy balance the panel recommended that warfighters consume 1.5-2.0 g · kg(-1) · d(-1) of protein. However, if metabolic demand is low, such as in garrison, protein intake should equal the current Military Dietary Reference Intake (0.8-1.5 g · kg(-1) · d(-1)). Although PS use generally appears to be safe for healthy adults, warfighters should be educated on PS quality, given quality-control and contamination concerns with commercial dietary supplements. To achieve recommended protein intakes, the panel strongly urges consumption of high-quality protein-containing whole foods. However, when impractical, the use of PSs (20-25 g per serving or 0.25-0.3 g · kg(-1) per meal), particularly after periods of strenuous physical activity (e.g., military training, combat patrols, and exercise), is acceptable. The committee acknowledges the need for further study of protein requirements for extreme, military-specific environmental conditions and whether unique metabolic stressors associated with military service alter protein requirements for aging warfighters.

MicroRNAs in skeletal muscle and their regulation with exercise, ageing, and disease

Skeletal muscle makes up approximately 40% of the total body mass, providing structural support and enabling the body to maintain posture, to control motor movements and to store energy. It therefore plays a vital role in whole body metabolism. Skeletal muscle displays remarkable plasticity and is able to alter its size, structure and function in response to various stimuli; an essential quality for healthy living across the lifespan. Exercise is an important stimulator of extracellular and intracellular stress signals that promote positive adaptations in skeletal muscle. These adaptations are controlled by changes in gene transcription and protein translation, with many of these molecules identified as potential therapeutic targets to pharmacologically improve muscle quality in patient groups too ill to exercise. MicroRNAs (miRNAs) are recently identified regulators of numerous gene networks and pathways and mainly exert their effect by binding to their target messenger RNAs (mRNAs), resulting in mRNA degradation or preventing protein translation. The role of exercise as a regulatory stimulus of skeletal muscle miRNAs is now starting to be investigated. This review highlights our current understanding of the regulation of skeletal muscle miRNAs with exercise and disease as well as how they may control skeletal muscle health.

Potential mechanisms for a role of metabolic stress in hypertrophic adaptations to resistance training

It is well established that regimented resistance training can promote increases in muscle hypertrophy. The prevailing body of research indicates that mechanical stress is the primary impetus for this adaptive response and studies show that mechanical stress alone can initiate anabolic signalling. Given the dominant role of mechanical stress in muscle growth, the question arises as to whether other factors may enhance the post-exercise hypertrophic response. Several researchers have proposed that exercise-induced metabolic stress may in fact confer such an anabolic effect and some have even suggested that metabolite accumulation may be more important than high force development in optimizing muscle growth. Metabolic stress pursuant to traditional resistance training manifests as a result of exercise that relies on anaerobic glycolysis for adenosine triphosphate production. This, in turn, causes the subsequent accumulation of metabolites, particularly lactate and H(+). Acute muscle hypoxia associated with such training methods may further heighten metabolic buildup. Therefore, the purpose of this paper will be to review the emerging body of research suggesting a role for exercise-induced metabolic stress in maximizing muscle development and present insights as to the potential mechanisms by which these hypertrophic adaptations may occur. These mechanisms include increased fibre recruitment, elevated systemic hormonal production, alterations in local myokines, heightened production of reactive oxygen species and cell swelling. Recommendations are provided for potential areas of future research on the subject.

Blood flow-restricted exercise in space

Prolonged exposure to microgravity results in chronic physiological adaptations including skeletal muscle atrophy, cardiovascular deconditioning, and bone demineralization. To attenuate the negative consequences of weightlessness during spaceflight missions, crewmembers perform moderate- to high-load resistance exercise in conjunction with aerobic (cycle and treadmill) exercise. Recent evidence from ground-based studies suggests that low-load blood flow-restricted (BFR) resistance exercise training can increase skeletal muscle size, strength, and endurance when performed in a variety of ambulatory populations. This training methodology couples a remarkably low exercise training load (approximately 20%-50% one repetition maximum (1RM)) with an inflated external cuff (width, ranging between approximately 30-90 mm; pressure, ranging between approximately 100-250 mmHg) that is placed around the exercising limb. BFR aerobic (walking and cycling) exercise training methods have also recently emerged in an attempt to enhance cardiovascular endurance and functional task performance while incorporating minimal exercise intensity. Although both forms of BFR exercise training have direct implications for individuals with sarcopenia and dynapenia, the application of BFR exercise training during exposure to microgravity to prevent deconditioning remains controversial. The aim of this review is to present an overview of BFR exercise training and discuss the potential usefulness of this method as an adjunct exercise countermeasure during prolonged spaceflight. The work will specifically emphasize ambulatory BFR exercise training adaptations, mechanisms, and safety and will provide directions for future research.

Whey protein supplementation does not affect exercise training-induced changes in body composition and indices of metabolic syndrome in middle-aged overweight and obese adults

Little is known about the effects of different quantities of whey protein on exercise training-induced changes in body composition and indices of metabolic syndrome in middle-aged overweight and obese adults. Therefore, we examined the effects of consuming 0.8-MJ supplements with 0 (n = 126), 10 (n = 112), 20 (n = 44), or 30 (n = 45) g whey protein twice daily in conjunction with resistance (2 d/wk) and aerobic (1 d/wk) exercise training in a double-blind, randomized, placebo-controlled, community-based 9-mo study in men (n = 117) and women (n = 210); (age: 48 ± 7.9 y; BMI: 30.0 ± 2.8 kg/m(2)). Whey protein supplementation did not influence any of the following outcomes, some of which were affected by training. Among all participants, strength increased by 15 ± 12% (P < 0.001) and maximal oxygen uptake capacity (VO(2)max) increased by 9 ± 15% (P < 0.001). Body weight was unchanged (0.1 ± 3.7 kg, P = 0.80), lean body mass increased by 1.9 ± 2.8% (0.95 ± 1.3 kg, P < 0.001), and fat mass decreased by 2.6 ± 9.4% (-0.86 ± 3.1 kg, P = 0.001). Oral-glucose-tolerance testing showed that plasma glucose AUC was unchanged (-18.0 ± 170 mmol/L·  3 h, P = 0.16), insulin AUC decreased by 2.6 ± 32% (-7.5 ± 29 nmol/L·  3 h, P = 0.01), and HOMA-IR (0.2 ± 2.0, P = 0.81) and the insulin sensitivity index (0.3 ± 3.0, P = 0.63) were unchanged. Plasma concentrations of TG; total, LDL, and HDL cholesterol; C-reactive protein; plasminogen activator inhibitor-1; blood pressure; and waist circumference were unchanged. Whey protein supplementation did not affect exercise training-induced responses in body composition and indices of metabolic syndrome in middle-aged overweight and obese adults who maintained body weight.

Metabolic slowing with massive weight loss despite preservation of fat-free mass

CONTEXT

An important goal during weight loss is to maximize fat loss while preserving metabolically active fat-free mass (FFM). Massive weight loss typically results in substantial loss of FFM potentially slowing metabolic rate.

OBJECTIVE

Our objective was to determine whether a weight loss program consisting of diet restriction and vigorous exercise helped to preserve FFM and maintain resting metabolic rate (RMR).

PARTICIPANTS AND INTERVENTION

We measured body composition by dual-energy x-ray absorptiometry, RMR by indirect calorimetry, and total energy expenditure by doubly labeled water at baseline (n = 16), wk 6 (n = 11), and wk 30 (n = 16).

RESULTS

At baseline, participants were severely obese (× ± SD; body mass index 49.4 ± 9.4 kg/m(2)) with 49 ± 5% body fat. At wk 30, more than one third of initial body weight was lost (-38 ± 9%) and consisted of 17 ± 8% from FFM and 83 ± 8% from fat. RMR declined out of proportion to the decrease in body mass, demonstrating a substantial metabolic adaptation (-244 ± 231 and -504 ± 171 kcal/d at wk 6 and 30, respectively, P < 0.01). Energy expenditure attributed to physical activity increased by 10.2 ± 5.1 kcal/kg.d at wk 6 and 6.0 ± 4.1 kcal/kg.d at wk 30 (P < 0.001 vs. zero).

CONCLUSIONS

Despite relative preservation of FFM, exercise did not prevent dramatic slowing of resting metabolism out of proportion to weight loss. This metabolic adaptation may persist during weight maintenance and predispose to weight regain unless high levels of physical activity or caloric restriction are maintained.

Protein turnover, amino acid requirements and recommendations for athletes and active populations

Skeletal muscle is the major deposit of protein molecules. As for any cell or tissue, total muscle protein reflects a dynamic turnover between net protein synthesis and degradation. Noninvasive and invasive techniques have been applied to determine amino acid catabolism and muscle protein building at rest, during exercise and during the recovery period after a single experiment or training sessions. Stable isotopic tracers (¹³C-lysine, ¹⁵N-glycine, ²H₅-phenylalanine) and arteriovenous differences have been used in studies of skeletal muscle and collagen tissues under resting and exercise conditions. There are different fractional synthesis rates in skeletal muscle and tendon tissues, but there is no major difference between collagen and myofibrillar protein synthesis. Strenuous exercise provokes increased proteolysis and decreased protein synthesis, the opposite occurring during the recovery period. Individuals who exercise respond differently when resistance and endurance types of contractions are compared. Endurance exercise induces a greater oxidative capacity (enzymes) compared to resistance exercise, which induces fiber hypertrophy (myofibrils). Nitrogen balance (difference between protein intake and protein degradation) for athletes is usually balanced when the intake of protein reaches 1.2 g·kg⁻¹·day⁻¹ compared to 0.8 g·kg⁻¹·day⁻¹ in resting individuals. Muscular activities promote a cascade of signals leading to the stimulation of eukaryotic initiation of myofibrillar protein synthesis. As suggested in several publications, a bolus of 15-20 g protein (from skimmed milk or whey proteins) and carbohydrate (± 30 g maltodextrine) drinks is needed immediately after stopping exercise to stimulate muscle protein and tendon collagen turnover within 1 h.

State of the art in anti-aging trends

This chapter is an effort to provide a cursory overview of current paradigms in the realm of anti-aging medicine. This subject will be evaluated according to current philosophic, clinical and scientific perspectives which are predominant in the field at this time.

Blood flow restriction: the metabolite/volume threshold theory

Traditionally it has been thought that muscle hypertrophy occurs primarily from an overload stimulus produced by progressively increasing an external load using at least 70% of one's concentric one repetition maximum (1RM). Blood flow restricted exercise has been demonstrated to result in numerous positive training adaptions, specifically muscle hypertrophy and strength at intensities much lower than this recommendation. The mechanisms behind these adaptions are currently unknown but a commonly cited concept is that acute elevations of systemic hormones, specifically growth hormone (GH), play a large role with resistance training induced muscle hypertrophy, possibly through stimulating muscle protein synthesis (MPS). We hypothesize that the alterations in the intramuscular environment which results in the rapid recruitment of FT fibers, is the large driving force behind the skeletal muscle hypertrophy seen with blood flow restriction, whereas the external load and systemic endogenous hormone elevations may not be as important as once thought. It is further hypothesized that although skeletal muscle hypertrophy can be achieved at low intensities without blood flow restriction when taken to muscular failure, the overall volume of work required is much greater than that needed with blood flow restriction.

Effects of high-intensity exercise and protein supplement on muscle mass in ADL dependent older people with and without malnutrition: a randomized controlled trial

BACKGROUND

Loss of muscle mass is common among old people living in institutions but trials that evaluate interventions aimed at increasing the muscle mass are lacking. Objective, participants and intervention: This randomized controlled trial was performed to evaluate the effect of a high-intensity functional exercise program and a timed protein-enriched drink on muscle mass in 177 people aged 65 to 99 with severe physical or cognitive impairments, and living in residential care facilities.

DESIGN

Three-month high-intensity exercise was compared with a control activity and a protein-enriched drink was compared with a placebo drink. A bioelectrical impedance spectrometer (BIS) was used in the evaluation. The amount of muscle mass and body weight (BW) were followed-up at three and six months and analyzed in a 2 x 2 factorial ANCOVA, using the intention to treat principle, and controlling for baseline values.

RESULTS

At 3-month follow-up there were no differences in muscle mass and BW between the exercise and the control group or between the protein and the placebo group. No interaction effects were seen between the exercise and nutritional intervention. Long-term negative effects on muscle mass and BW was seen in the exercise group at the 6-month follow-up.

CONCLUSION

A three month high-intensity functional exercise program did not increase the amount of muscle mass and an intake of a protein-enriched drink immediately after the exercise did not induce any additional effect on muscle mass. There were negative long-term effects on muscle mass and BW, indicating that it is probably necessary to compensate for an increased energy demand when offering a high-intensity exercise program.

The anabolic response to resistance exercise and a protein-rich meal is not diminished by age

OBJECTIVES

The synergistic effect of resistance exercise and protein ingestion on muscle protein anabolism in young adults has been well described. However, it is unclear if this relationship is maintained in older adults who are at greater risk of sarcopenic muscle loss. To this end, we sought to determine if the synergistic response to a bout of resistance exercise and a protein-rich lean beef meal was altered by age.

SETTING

The University of Texas Medical Branch, Clinical Research Center, Galveston, Texas.

PARTICIPANTS

Healthy young (n=7, 29±3 y) and older (n=7, 67±2 y) adults.

DESIGN

Mixed muscle fractional synthesis rate (FSR) was calculated during a 3 h post-absorptive/rest period and again during a 5 h period following ingestion of a protein-rich meal (340 g lean beef) and bout of resistance exercise (6 sets of 8 repetitions of isotonic knee extension exercise at 80% one repetition maximum).

MEASUREMENTS

Venous blood samples and vastus lateralis muscle biopsy samples were obtained during a primed (2.0 µmol/kg) constant infusion (0.08 µmol∙kg(-1)min(-1)) of L- [ring-13C6] phenylalanine.

RESULTS

Mixed muscle FSR increased by approximately 108% in both young [pre: 0.073±0.008; post: 0.156±0.021(SE) %/h, p<0.001] and older adults (pre: 0.075±0.004; post: 0.152±0.017 %/h, p=0.003) following the meal and resistance exercise bout.

CONCLUSION

Aging does not diminish the increase in muscle protein synthesis following a high-quality protein rich meal and bout of resistance exercise.

Efficacy and consequences of very-high-protein diets for athletes and exercisers

Athletes and exercisers have utilised high-protein diets for centuries. The objective of this review is to examine the evidence for the efficacy and potential dangers of high-protein diets. One important factor to consider is the definition of a ‘high-protein diet’. There are several ways to consider protein content of a diet. The composition of the diet can be determined as the absolute amount of the protein (or other nutrient of interest), the % of total energy (calories) as protein and the amount of protein ingested per kg of body weight. Many athletes consume very high amounts of protein. High-protein diets most often are associated with muscle hypertrophy and strength, but now also are advocated for weight loss and recovery from intense exercise or injuries. Prolonged intake of a large amount of protein has been associated with potential dangers, such as bone mineral loss and kidney damage. In otherwise healthy individuals, there is little evidence that high protein intake is dangerous. However, kidney damage may be an issue for individuals with already existing kidney dysfunction. Increased protein intake necessarily means that overall energy intake must increase or consumption of either carbohydrate or fat must decrease. In conclusion, high protein intake may be appropriate for some athletes, but there are potential negative consequences that must be carefully considered before adopting such a diet. In particular, care must be taken to ensure that there is sufficient intake of other nutrients to support the training load.

Nutrition for acute exercise-induced injuries

BACKGROUND/AIMS

Injuries are an unavoidable aspect of participation in physical activity. Little information about nutritional support for injuries exists.

REVIEW

Immediately following injury, wound healing begins with an inflammatory response. Excessive anti-inflammatory measures may impair recovery. Many injuries result in limb immobilization. Immobilization results in muscle loss due to increased periods of negative muscle protein balance. Oxidative capacity of muscle is also decreased. Nutrient and energy deficiencies should be avoided. Energy expenditure may be reduced during immobilization, but inflammation, wound healing and the energy cost of ambulation limit the reduction of energy expenditure. There is little rationale for increasing protein intake during immobilization. There is a theoretical rationale for leucine and omega-3 fatty acid supplementation to help reduce muscle atrophy. During rehabilitation and recovery from immobilization, increased activity, in particular resistance exercise will increase muscle protein synthesis and restore sensitivity to anabolic stimuli. Ample, but not excessive, protein and energy must be consumed to support muscle growth. During rehabilitation and recovery, nutritional needs are very much like those for any athlete desiring muscle growth.

CONCLUSION

Nutrition is important for optimal wound healing. The most important consideration is to avoid malnutrition and to apply a risk/benefit approach.

Efficacy of α-lactalbumin and milk protein on weight loss and body composition during energy restriction

Our objective was to examine whether elevated α-lactalbumin (αlac) protein intake compared to elevated supra sustained milk protein (SSP) and sustained milk protein (SP) intake results into a difference in body weight and body composition over a 6-month energy-restriction intervention. Body weight, body composition, resting energy expenditure (REE), satiety and blood- and urine-parameters of 87 subjects (BMI 31 ± 5 kg/m(2) and fat percentage 40 ± 8%) were assessed before and after daily energy intakes of 100, 33, and 67% for 1, 1, and 2 months respectively (periods 1, 2, and 3), with protein intake from meal replacements and 2 months of 67% with ad libitum protein intake additional to the meal replacements (period 4). The diets resulted in 0.8 ± 0.3 g/kg body mass (BM) for SP and significant higher protein intake (24-h nitrogen) of 1.2 ± 0.3 and 1.0 ± 0.3 g/kgBM for SSP and αlac (P < 0.05). Body weight and fat percentage was decreased in all groups after 6 months (SP -7 ± 5 kg and -5 ± 3%; SSP -6 ± 3 kg and -5 ± 3%; αlac -6 ± 4 kg and -4 ± 4%, P < 0.001; there was no significant group by time difference). Furthermore, sparing of fat-free mass (FFM) and preservation of REE in function of FFM during weight loss was not significantly different between the αlac-group and the SSP- and SP-groups. In conclusion, the efficacy of αlac in reduction of body weight and fat mass (FM), and preservation of FFM does not differ from the efficacy of similar daily intakes of milk protein during 6 months of energy restriction.

Resistance exercise and appropriate nutrition to counteract muscle wasting and promote muscle hypertrophy

PURPOSE OF REVIEW

Loss of skeletal muscle mass is a common feature of a number of clinical scenarios including limb casting, bed rest, and various disorders such as HIV-AIDS, sepsis, cancer cachexia, heart failure, and uremia. Commonly, muscle disuse (hypodynamia) is the sole reason, or a large part, of why muscle mass is lost. The reduction in strength, or dynapenia, that accompanies these conditions is also a function of the degree of hypodynamia and is related to muscle loss.

RECENT FINDINGS

The major and consistent finding in a number of human-based models of muscle wasting is a decline in the synthesis of new muscle proteins both in the postabsorptive and fed states. Thus, countermeasures are best suited to those that augment muscle protein synthesis and not those that attempt to counteract proteolysis. Our main thesis is that retention of muscle mass in wasting conditions will be achieved to the greatest extent by focussing on increased muscle use with moderate-to-high resistance loads as the primary countermeasure with a secondary countermeasure being to provide adequate nutritional support. Either intervention alone will alleviate some part of hypodynamia-induced muscle mass loss and dynapenia; however, together nutrition and muscular contraction will result in greater mitigation of muscle loss.

SUMMARY

Advances in our understanding of hypodynamia-induced muscle loss, a condition common to almost all syndromes of muscle wasting, has led to a focus on reduced basal and feeding-induced elevations in protein synthesis. Countermeasures for wasting should focus on stimulating anabolism rather than alleviating catabolism.

Protein intake induced an increase in exercise stimulated fat oxidation during stable body weight

BACKGROUND

Protein-rich weight-loss diets spare fat-free mass at the cost of fat mass. The objective was to examine if there is a change in stimulated fat oxidation related to protein intake during stable body weight.

METHODS

Subjects' (BMI 22±2kg/m(2), age 25±8 years) maximal fat oxidation (Fat(max)) was assessed during a graded bicycle test, before and after a 3-month dietary-intervention of 2MJ/day supplements exchanged with 2MJ/d of habitual energy intake. The parallel design consisted of protein-rich supplements in the protein group and an isocaloric combination of carbohydrate and fat supplements in the control group. Daily protein intake was determined according to 24-h urine nitrogen. Body composition was measured according to a 4-compartment model by a combination of underwater-weighing technique, deuterium-dilution technique and whole-body dual-energy X-ray absorptiometry (DXA).

RESULTS

Subjects were weight stable and did not change their physical activity. The protein group (n=12) increased protein intake (11±14g, P<0.05) and had significantly higher daily protein intake vs. control (n=4) (80±21 vs.59±11g, P<0.05). Fat(max) increased significantly in the protein group (0.08±0.08g/min, P<0.01). Fat-free mass increased independent of change in body weight (P<0.01), and fat mass and fat percentage decreased (P<0.05). Change in Fat(max) was a function of change in protein intake (r=0.623, P<0.05), and not of changes in body composition or VO(2)max.

CONCLUSION

Increased stimulated fat oxidation was related to increased protein intake.

Ectopic expression of eIF2Bepsilon in rat skeletal muscle rescues the sepsis-induced reduction in guanine nucleotide exchange activity and protein synthesis

Eukaryotic initiation factor 2B (eIF2B) is a guanine nucleotide exchange factor (GEF) whose activity is both tightly regulated and rate-controlling with regard to global rates of protein synthesis. Skeletal muscle eIF2B activity and expression of its catalytic epsilon-subunit (eIF2Bepsilon) have been implicated as potential contributors to the altered rates of protein synthesis in a number of physiological conditions and experimental models. The objective of this study was to directly examine the effects of exogenously expressed eIF2Bepsilon in vivo on GEF activity and protein synthetic rates in rat skeletal muscle. A plasmid encoding FLAG-eIF2Bepsilon was transfected into the tibialis anterior (TA) of one leg, while the contralateral TA received a control plasmid. Ectopic expression of eIF2Bepsilon resulted in increased GEF activity in TA homogenates of healthy rats, demonstrating that the expressed protein was catalytically active. In an effort to restore a deficit in eIF2B activity, we utilized an established model of chronic sepsis in which skeletal muscle eIF2B activity is known to be impaired. Ectopic expression of eIF2Bepsilon in the TA rescued the sepsis-induced deficit in GEF activity and muscle protein synthesis. The results demonstrate that modulation of eIF2Bepsilon expression may be sufficient to correct deficits in skeletal muscle protein synthesis associated with sepsis and other muscle-wasting conditions.

Skeletal dysplasias associated with mild myopathy-a clinical and molecular review

Musculoskeletal system is a complex assembly of tissues which acts as scaffold for the body and enables locomotion. It is often overlooked that different components of this system may biomechanically interact and affect each other. Skeletal dysplasias are diseases predominantly affecting the development of the osseous skeleton. However, in some cases skeletal dysplasia patients are referred to neuromuscular clinics prior to the correct skeletal diagnosis. The muscular complications seen in these cases are usually mild and may stem directly from the muscle defect and/or from the altered interactions between the individual components of the musculoskeletal system. A correct early diagnosis may enable better management of the patients and a better quality of life. This paper attempts to summarise the different components of the musculoskeletal system which are affected in skeletal dysplasias and lists several interesting examples of such diseases in order to enable better understanding of the complexity of human musculoskeletal system.

Leucine supplementation favors liver protein status but does not reduce body fat in rats during 1 week of food restriction

An important role in protein–energy metabolism has been attributed to leucine because of its long-term effects on body fat reduction and on the improvement of some indicators of protein status in rodents. The present study investigated the influence of leucine supplementation on the body composition and protein status of rats during the early phase of weight loss, which is characterized by a rapid loss of body weight. Thirty adult male Wistar rats were divided into 2 groups, a control and a leucine group (diet supplemented with 0.59% L-leucine), and were submitted to 1 week of 50% food restriction. The following parameters were evaluated: chemical carcass composition, protein and RNA content in liver and gastrocnemius muscle, and serum concentrations of insulin-like growth factor-1 and corticosterone. A higher liver weight and liver protein content were observed in the supplemented group (p < 0.05). However, no difference in body fat was found between groups (p > 0.05). The results indicate that low-dose leucine supplementation favors liver protein status but does not reduce body fat in rats during the early phase of rapid weight loss.