Endogenous biological rhythms synchronise human physiology with daily cycles of light-dark, wake-sleep and feeding-fasting. Proper circadian alignment is crucial for physiological function, reflected in the rhythmic expression of molecular clock genes in various tissues, especially in skeletal muscle. Circadian disruption, such as misaligned feeding, dysregulates metabolism and increases the risk of metabolic disorders like type 2 diabetes. Such disturbances are common in critically ill patients, especially those who rely on enteral nutrition. Whilst continuous provision of enteral nutrition is currently the most common practice in critical care, this is largely dictated by convenience rather than evidence. Conversely, some findings indicate that intermittent provision of enteral nutrition aligned with daylight may better support physiological functions and improve clinical/metabolic outcomes. However, there is a critical need for studies of skeletal muscle responses to acutely divergent feeding patterns, in addition to complementary translational research to map tissue-level physiology to whole-body and clinical outcomes.

Musculoskeletal disorders and age-related musculoskeletal decline are major contributors to the burden of ill health seen in older subjects. Despite this increased burden, these chronic disorders of old age receive a relatively small proportion of national research funds. Much has been learned about fundamental processes involved in ageing from basic science research and this is leading to identification of key pathways that mediate ageing which may help the search for interventions to reduce age-related musculoskeletal decline. This short review will focus on the role of reactive oxygen species in age-related skeletal muscle decline and on the implications of this work for potential nutritional interventions in sarcopenia. The key physiological role of reactive oxygen species is now known to be in mediating redox signalling in muscle and other tissues and ageing leads to disruption of such pathways. In muscle, this is reflected in an age-related attenuation of specific adaptations and responses to contractile activity that impacts the ability of skeletal muscle from ageing individuals to respond to exercise. These pathways provides potential targets for identification of logical interventions that may help maintain muscle mass and function during ageing.

Plasma levels of branched-chain amino acids (BCAA) and their metabolites, branched-chain ketoacids (BCKA), are increased in insulin resistance. We previously showed that ketoisocaproic acid (KIC) suppressed insulin-stimulated glucose transport in L6 myotubes, especially in myotubes depleted of branched-chain ketoacid dehydrogenase (BCKD), the enzyme that decarboxylates BCKA. This suggests that upregulating BCKD activity might improve insulin sensitivity. We hypothesised that increasing BCAA catabolism would upregulate insulin-stimulated glucose transport and attenuate insulin resistance induced by BCKA. L6 myotubes were either depleted of BCKD kinase (BDK), the enzyme that inhibits BCKD activity, or treated with BT2, a BDK inhibitor. Myotubes were then treated with KIC (200 μM), leucine (150 μM), BCKA (200 μM), or BCAA (400 μM) and then treated with or without insulin (100 nM). BDK depletion/inhibition rescued the suppression of insulin-stimulated glucose transport by KIC/BCKA. This was consistent with the attenuation of IRS-1 (Ser612) and S6K1 (Thr389) phosphorylation but there was no effect on Akt (Ser473) phosphorylation. The effect of leucine or BCAA on these measures was not as pronounced and BT2 did not influence the effect. Induction of the mTORC1/IRS-1 (Ser612) axis abolished the attenuating effect of BT2 treatment on glucose transport in cells treated with KIC. Surprisingly, rapamycin co-treatment with BT2 and KIC further reduced glucose transport. Our data suggests that the suppression of insulin-stimulated glucose transport by KIC/BCKA in muscle is mediated by mTORC1/S6K1 signalling. This was attenuated by upregulating BCAA catabolic flux. Thus, interventions targeting BCAA metabolism may provide benefits against insulin resistance and its sequelae.

Loss of skeletal muscle strength and mass (sarcopenia) is common in older adults and associated with an increased risk of disability, frailty and premature death. Finding cost-effective prevention and treatment strategies for sarcopenia for the growing ageing population is therefore of great public health interest. Although nutrition is considered an important factor in the aetiology of sarcopenia, its potential for sarcopenia prevention and/or treatment is still being evaluated. Nutrition research for sarcopenia utilises three main approaches to understand muscle-nutrition relationships, evaluating: single nutrients, whole foods and whole diet effects – both alone or combined with exercise. Applying these approaches, we summarise recent evidence from qualitative and quantitative syntheses of findings from observational and intervention studies of healthy older adults, and those with sarcopenia. We consider protein supplements, whole foods (fruits and vegetables) and the Mediterranean diet as exemplars. There is some evidence of beneficial effects of protein supplementation ≥ 0·8 g/kg body weight/d on muscle mass when combined with exercise training in intervention studies of healthy and sarcopenic older adults. In contrast, evidence for effects on muscle function (strength and physical performance) is inconclusive. There is reasonably consistent epidemiological evidence suggesting benefits of higher fruits and vegetables consumption for better physical performance. Similarly, higher adherence to the Mediterranean diet is associated with beneficial effects on muscle function in observational studies. However, intervention studies are lacking. This review discusses how current evidence may inform the development of preventive and intervention strategies for optimal muscle ageing and nutritional public policy aimed at combatting sarcopenia.

The impact of computed tomography-defined sarcopenia on outcomes in head and neck cancer has been well described. Sarcopenic obesity (SO) (depleted muscle mass combined with obesity) may pose a more serious risk than either condition alone. We investigated SO and its impact on survival and critical weight loss (≥ 5 %) in patients with head and neck cancer who received curative radiotherapy (± other modalities). Retrospective analysis of computed tomography cross-sectional muscle at cervical (C3), thoracic (T2) and lumbar (L3) regions was conducted. Patients were grouped by BMI and sarcopenia status based on established thresholds. A total of 413 patients were included for analysis, the majority having oropharyngeal carcinoma (52 %), and 56 % received primary concurrent chemoradiotherapy. The majority of the cohort (65 %) was overweight or obese (BMI ≥ 25 kg/m2). Sarcopenia was found in 43 %, with 65 % having SO (n 116), equating to 28 % of the whole cohort. Critical weight loss was experienced by 58 % (n 238). A significantly higher proportion of patients with SO experienced critical weight loss (n 70 v. 19, P < 0·001) and were four times more likely to do so during treatment (OR 4·1; 95 % CI 1·5, 7·1; P = 0·002). SO was not found to impact on overall or cancer-specific survival; however, in patients with sarcopenia, those with SO had better overall survival (median 9·1 v. 7·0 years; 95 % CI 5·2, 16·8; P = 0·021). SO at the time of presentation in patients with head and neck cancer is predictive of critical weight loss during treatment, and muscle evaluation can be useful in identifying patients at nutritional risk regardless of BMI and obvious signs of wasting.

Branched-chain amino acids (BCAA: leucine, isoleucine and valine) are three of the nine indispensable amino acids, and are frequently consumed as a dietary supplement by athletes and recreationally active individuals alike. The popularity of BCAA supplements is largely predicated on the notion that they can stimulate rates of muscle protein synthesis (MPS) and suppress rates of muscle protein breakdown (MPB), the combination of which promotes a net anabolic response in skeletal muscle. To date, several studies have shown that BCAA (particularly leucine) increase the phosphorylation status of key proteins within the mechanistic target of rapamycin (mTOR) signalling pathway involved in the regulation of translation initiation in human muscle. Early research in humans demonstrated that BCAA provision reduced indices of whole-body protein breakdown and MPB; however, there was no stimulatory effect of BCAA on MPS. In contrast, recent work has demonstrated that BCAA intake can stimulate postprandial MPS rates at rest and can further increase MPS rates during recovery after a bout of resistance exercise. The purpose of this evidence-based narrative review is to critically appraise the available research pertaining to studies examining the effects of BCAA on MPS, MPB and associated molecular signalling responses in humans. Overall, BCAA can activate molecular pathways that regulate translation initiation, reduce indices of whole-body and MPB, and transiently stimulate MPS rates. However, the stimulatory effect of BCAA on MPS rates is less than the response observed following ingestion of a complete protein source providing the full complement of indispensable amino acids.

Diminished skeletal muscle strength and size, termed sarcopenia, contributes substantially to physical disability, falls, dependence and reduced quality of life among older people. Physical activity and nutrition are the cornerstones of sarcopenia prevention and treatment. The optimal daily protein intake required to preserve muscle mass and function among older adults is a topic of intense scientific debate. Older adults require protein intakes about 67 % higher than their younger counterparts to maximally stimulate postprandial muscle protein synthesis rates. In addition, evidence suggests a possible benefit of increasing protein intake above the population reference intake (0⋅83 g/kg/d) on lean mass and, when combined with exercise training, muscle strength. In addition to protein quantity, protein quality, the pattern of protein intake over the day and specific amino acids (i.e. leucine) represent key considerations. Long-chain n-3 PUFA (LC n-3 PUFA) supplementation has been shown to enhance muscle protein synthesis rates, increase muscle mass and function and augment adaptations to resistance training in older adults. Yet, these effects are not consistent across all studies. Emerging evidence indicates that an older person's dietary, phenotypic and behavioural characteristics may modulate the efficacy of protein and LC n-3 PUFA interventions for promoting improvements in muscle mass and function, highlighting the potential inadequacy of a ‘one-size-fits-all’ approach. The application of personalised or precision nutrition to sarcopenia represents an exciting and highly novel field of research with the potential to help resolve inconsistencies in the literature and improve the efficacy of dietary interventions for sarcopenia.

It is difficult to easily estimate skeletal muscle (SM) volume in children. We aimed to develop regression-based prediction equations to estimate the total body and regional SM volume using calliper measurements of skinfold thickness and limb circumference and to investigate the validity of these equations. In total, 142 healthy, prepubertal, Japanese children, aged 6–12 years, were divided into two groups: the model development group (sixty boys, thirty-eight girls) and the validation group (twenty-six boys, eighteen girls). Contiguous magnetic resonance images were obtained from the first cervical vertebra to the ankle joints as reference data. SM volume was calculated from the summation of the digitised cross-sectional areas. Limb and waist circumferences were measured at mid-upper arm, mid-thigh, maximal calf and at the level of umbilicus. Each girth was corrected for subcutaneous adipose tissue thickness, as estimated by skinfold thickness measurements. Skinfold thickness was measured at the posterior upper arm, anterior thigh, medial calf and lateral to the umbilicus, using callipers. Significant correlations were observed between the site-matched SM volume, measured by MRI, and each corrected girth × standing height value in the model development group. When these SM volume prediction equations were applied to the validation group, the measured total body and regional SM volume were similar to the predicted values. These results suggest that the anthropometric prediction equations developed in this study provide reliable information about the total and regional SM volume in prepubertal Japanese children, with varying degrees of estimation accuracy for each region.

Milk is a source of several nutrients which may be beneficial for skeletal muscle. Evidence that links lower milk intake with declines in muscle strength from midlife to old age is lacking. We used data from the Medical Research Council National Survey of Health and Development to test sex-specific associations between milk consumption from age 36 to 60–64 years, low grip strength (GS) or probable sarcopenia, and GS decline from age 53 to 69 years. We included 1340 men and 1383 women with at least one measure of both milk intake and GS. Milk intake was recorded in 5-d food diaries (aged 36, 43, 53 and 60–64 years), and grand mean of total, reduced-fat and full-fat milk each categorised in thirds (T1 (lowest) to T3 (highest), g/d). GS was assessed at ages 53, 60–64, and 69 years, and probable sarcopenia classified at the age of 69 years. We employed logistic regression to examine the odds of probable sarcopenia and multilevel models to investigate decline in GS in relation to milk intake thirds. Compared with T1, only T2 (58·76–145·25 g/d) of reduced-fat milk was associated with lower odds of sex-specific low GS at the age of 69 years (OR (95 % CI): 0·59 (0·37, 0·94), P = 0·03). In multilevel models, only T3 of total milk (≥ 237·52 g/d) was associated with stronger GS in midlife in men (β (95 % CI) = 1·82 (0·18, 3·45) kg, P = 0·03) compared with T1 (≤ 152·0 g/d), but not with GS decline over time. A higher milk intake across adulthood may promote muscle strength in midlife in men. Its role in muscle health in late life needs further examination.

Severe heat exposure causes mitochondrial fragmentation and dysfunction, which contribute to the pathogenesis of heat-related illness. l-Citrulline is a naturally occurring amino acid and has been suggested to influence heat shock responses. This study aimed to test whether l-citrulline supplementation would preserve mitochondrial integrity and attenuate heat-induced skeletal muscle injury and elucidate the underlying mechanisms. At 37°C, l-citrulline (2 mM) increased mitochondrial elongation in mouse C2C12 myoblasts, a process associated with a reduction in mitochondrial fission protein Drp1 levels. Mechanistic studies revealed that l-citrulline increased cellular nitric oxide (NO) levels, but not S-nitrosylation of Drp1. l-Citrulline caused a decrease in phosphorylation of Drp1 at Ser 616 and an increase in phosphorylation of Drp1 at Ser 637, which resulted in a reduced mitochondrial localisation of Drp1. L-NAME, a non-selective NO synthase inhibitor, abolished the increase in l-citrulline-induced NO levels and inhibited Drp1 phosphorylation changes and mitochondrial elongation, which indicates the involvement of a NO-dependent pathway. Under 43°C heat stress conditions, l-citrulline prevented translocation of Drp1 to mitochondria, mitochondrial fragmentation and decreased membrane potential. Finally, l-citrulline pretreatment inhibited heat-induced reactive oxygen species overproduction, caspase 3/7 activation, apoptotic cell death and improved cell viability. NO inhibitor l-NAME abolished all the above protective effects of l-citrulline under heat stress. Our results suggest that l-citrulline prevents heat-induced mitochondrial dysfunction and cell injury through NO-mediated Drp1 inhibition in C2C12 myoblasts. l-Citrulline may be an effective treatment for heat-related illnesses and other mitochondrial diseases.

We aimed to investigate the anthropometric and body composition parameters associated with the metabolically obese normal-weight (MONW) phenotype. This cross-sectional study was conducted with 506 adolescents in Brazil (aged 10–19 y). The MONW phenotype was defined as normal-weight, according to BMI/age, and at least one metabolic alteration. Anthropometric measurements were obtained and the DEXA was used for body composition analysis. Crude and adjusted Poisson regression models with robust variance were used to estimate the associations. The phenotype was positively associated with waist circumference (male: prevalence ratio (PR) = 1·05; 95% CI 1·01, 1·09; female: PR = 1·06; 95% CI 1·02, 1·09), waist:height ratio (male: PR = 1·26; 95% CI 1·07, 1·49; female: PR = 1·29; 95% CI 1·07, 1·56) and android:gynoid fat ratio (male: PR = 1·25; 95% CI 1·03, 1·51; female: PR = 1·39; 95% CI 1·20, 1·62), in both sexes. Furthermore, there was a positive association of phenotype with waist:hip ratio (PR = 1·32; 95% CI 1·06, 1·65) and trunk:arm fat ratio (PR = 1·13; 95% CI 1·02, 1·24) only in males and with trunk:leg fat ratio (PR = 2·84; 95% CI 1·46, 5·53), BAIp (PR = 1·06; 95% CI 1·01, 1·12), fat mass index (PR = 1·24; 95% CI 1·10, 1·41) and regional indices of metabolic load and capacity (PR = 1·29; 95% CI 1·09, 1·53), in females. Anthropometric and body composition parameters indicative of central and total fat are associated with the MONW phenotype.

Maternal exercise has shown beneficial effects on mother and child. Literature confirm progeny’s cognition improvement, and upregulation in neurotrophins, antioxidant network, and DNA repair system. Considering that there is a lack of information demonstrating the impact of maternal exercise on offspring’s skeletal muscle, we aimed to investigate the mitochondrial and redox effects elicited by maternal swimming. Adult female Wistar rats were divided into three groups: control sedentary, free swimming, and swimming with overload (2% of the body weight). Exercised groups were submitted weekly to five swimming sessions (30 min/day), starting 1 week prior to the mating and lasting to the delivery. Gastrocnemius and soleus muscle from 60-day-old offspring were analyzed. Our results clearly showed a sex-dependent effect. Male soleus showed increased mitochondrial functionality in the overload group. Female muscle from the overload group adapted deeply. Considering the redox status, the female offspring delivered to overload exercised dams presented reduced oxidants levels and protein damage, allied to downregulated antioxidant defenses. We also observed an increase in the mitochondrial function in the gastrocnemius muscle of the female offspring born from overload exercised dams. Soleus from female delivered to the overload exercise group presented reduced mitochondrial activity, as well as reduced reactive species, protein carbonyls, and antioxidant network, when compared to the male. In conclusion, maternal exercise altered the redox status and mitochondrial function in the offspring’s skeletal muscle in a sex-dependent way. The clinical implication was not investigated; however, the sexual dimorphism in response to maternal exercise might impact exercise resilience in adulthood.

Modern electron microscopy permits scientists to study the fine detail of cells and tissues, using both two-dimensional and three-dimensional imaging modalities. However, achieving optimal preservation of ultrastructure for a variety of biological samples remains a challenge. Here, we describe practical methods to preserve the fine structure of mouse skeletal muscle and sciatic nerve and to obtain high-resolution images of mitochondria in cultured cells, flow-sorted T-cells, and mouse urothelium. We also propose an effective and economical workflow for three-dimensional electron microscopy in the context of a microscopy core facility.

The FNDC5 gene encodes the fibronectin type III domain-containing protein 5 that is a membrane protein mainly expressed in skeletal muscle, and the FNDC5 rs3480 polymorphism may be associated with liver disease severity in non-alcoholic fatty liver disease (NAFLD). We investigated the influence of the FNDC5 rs3480 polymorphism on the relationship between sarcopenia and the histological severity of NAFLD. A total of 370 adult individuals with biopsy-proven NAFLD were studied. The association between the key exposure sarcopenia and the outcome liver histological severity was investigated by binary logistic regression. Stratified analyses were undertaken to examine the impact of FNDC5 rs3480 polymorphism on the association between sarcopenia and the severity of NAFLD histology. Patients with sarcopenia had more severe histological grades of steatosis and a higher prevalence of significant fibrosis and definite non-alcoholic steatohepatitis than those without sarcopenia. There was a significant association between sarcopenia and significant fibrosis (adjusted OR 2·79, 95 % CI 1·31, 5·95, P = 0·008), independent of established risk factors and potential confounders. Among patients with sarcopenia, significant fibrosis occurred more frequently in the rs3480 AA genotype carriers than in those carrying the FNDC5 rs3480 G genotype (43·8 v. 17·2 %, P = 0·031). In the association between sarcopenia and liver fibrosis, there was a significant interaction between the FNDC5 genotype and sarcopenia status (P value for interaction = 0·006). Sarcopenia is independently associated with significant liver fibrosis, and the FNDC5 rs3480 G variant influences the association between sarcopenia and liver fibrosis in patients with biopsy-proven NAFLD.

Animal-derived dietary protein ingestion and physical activity stimulate myofibrillar protein synthesis rates in older adults. We determined whether a non-animal-derived diet can support daily myofibrillar protein synthesis rates to the same extent as an omnivorous diet. Nineteen healthy older adults (aged 66 (sem 1) years; BMI 24 (sem 1) kg/m2; twelve males, seven females) participated in a randomised, parallel-group, controlled trial during which they consumed a 3-d isoenergetic high-protein (1·8 g/kg body mass per d) diet, where the protein was provided from predominantly (71 %) animal (OMNI; n 9; six males, three females) or exclusively vegan (VEG; n 10; six males, four females; mycoprotein providing 57 % of daily protein intake) sources. During the dietary control period, participants conducted a daily bout of unilateral resistance-type leg extension exercise. Before the dietary control period, participants ingested 400 ml of deuterated water, with 50-ml doses consumed daily thereafter. Saliva samples were collected throughout to determine body water 2H enrichments, and muscle samples were collected from rested and exercised muscle to determine daily myofibrillar protein synthesis rates. Deuterated water dosing resulted in body water 2H enrichments of approximately 0·78 (sem 0·03) %. Daily myofibrillar protein synthesis rates were 13 (sem 8) (P = 0·169) and 12 (sem 4) % (P = 0·016) greater in the exercised compared with rested leg (1·59 (sem 0·12) v. 1·77 (sem 0·12) and 1·76 (sem 0·14) v. 1·93 (sem 0·12) %/d) in OMNI and VEG groups, respectively. Daily myofibrillar protein synthesis rates did not differ between OMNI and VEG in either rested or exercised muscle (P > 0·05). Over the course of a 3-d intervention, omnivorous- or vegan-derived dietary protein sources can support equivalent rested and exercised daily myofibrillar protein synthesis rates in healthy older adults consuming a high-protein diet.

Muscle mass may play an important role in the metabolic profile of individuals with or without excess weight. Metabolic phenotypes classify individuals as healthy or unhealthy based on certain metabolic conditions. We investigated the association between skeletal mass indices (SMI) and the metabolically unhealthy phenotype in normal-weight and overweight/obese adults. A total of 660 adults aged 20 to 59 years were assessed by a population-based cross-sectional study. Muscle mass of the limbs or appendicular lean mass (ALM) adjusted for weight (SMIweight) and BMI (SMIBMI) was used to evaluate SMI. Logistic regression was employed to estimate the association between SMIweight, SMIBMI and metabolic phenotypes of normal-weight and overweight/obese individuals. Metabolically unhealthy individuals were older in both sexes. Metabolically unhealthy men had lower SMI values and higher fat percentage than metabolically healthy men. SMIweight was inversely associated with the metabolically unhealthy phenotype, both in normal-weight men (OR 0·49, 95 % CI 0·24, 0·99, P = 0·04) and in overweight/obese men (OR 0·32, 95 % CI 0·16, 0·64, P = 0·001). SMIBMI was inversely associated with the metabolically unhealthy phenotype in overweight/obese men (OR 0·36, 95 % CI 0·18, 0·72, P = 0·004), but not in normal-weight men (OR 0·70, 95 % CI 0·34, 1·43, P = 0·33). Among women, SMI showed no significant association with the phenotypes. In conclusion, the SMI are inversely associated with the metabolically unhealthy phenotype in men, especially among overweight/obese men.