The Combined Oral Stable Isotope Assessment of Muscle (COSIAM) reveals D-3 creatine derived muscle mass as a standout cross-sectional biomarker of muscle physiology vitality in older age

Skeletal muscle dysfunction with advancing age

As a result of advances in medical treatments and associated policy over the last century, life expectancy has risen substantially and continues to increase globally. However, the disconnect between lifespan and 'health span' (the length of time spent in a healthy, disease-free state) has also increased, with skeletal muscle being a substantial contributor to this. Biological ageing is accompanied by declines in both skeletal muscle mass and function, termed sarcopenia. The mechanisms underpinning sarcopenia are multifactorial and are known to include marked alterations in muscle protein turnover and adaptations to the neural input to muscle. However, to date, the relative contribution of each factor remains largely unexplored. Specifically, muscle protein synthetic responses to key anabolic stimuli are blunted with advancing age, whilst alterations to neural components, spanning from the motor cortex and motoneuron excitability to the neuromuscular junction, may explain the greater magnitude of function losses when compared with mass. The consequences of these losses can be devastating for individuals, their support networks, and healthcare services; with clear detrimental impacts on both clinical (e.g., mortality, frailty, and post-treatment complications) and societal (e.g., independence maintenance) outcomes. Whether declines in muscle quantity and quality are an inevitable component of ageing remains to be completely understood. Nevertheless, strategies to mitigate these declines are of vital importance to improve the health span of older adults. This review aims to provide an overview of the declines in skeletal muscle mass and function with advancing age, describes the wide-ranging implications of these declines, and finally suggests strategies to mitigate them, including the merits of emerging pharmaceutical agents.

Estimating Muscle Mass Using D3-Creatine Dilution: A Narrative Review of Clinical Implications and Comparison With Other Methods

BACKGROUND

The D3-creatine (D3-Cr) dilution method is of emerging interest for estimating total-body skeletal muscle mass. This review explores the association of muscle mass estimated via D3-Cr with various clinical outcomes and provides a summary of the literature comparing D3-Cr with other body composition techniques.

METHODS

A literature search was conducted on PubMed/MEDLINE and Web of Science for studies using D3-Cr to measure muscle in adult populations (ie, ≥18 years old) from inception until September 2023.

RESULTS

Out of the 23 included studies, 15 investigated the correlation between D3-Cr and clinical outcomes. More consistent associations were reported for mortality (100%, n = 2), mobility disability (100%; n = 5), falls and fractures (100%; n = 3), physical performance (63.3%; n = 11), muscle strength (44.4%; n = 9), and muscle composition (33.3%; n = 3). However, conflicting findings were also reported for such correlations. Among the 23 studies, 14 compared D3-Cr-estimated muscle with other body composition techniques, including magnetic resonance imaging (MRI) as a reference method. Strong and positive correlations were found between D3-Cr and MRI. Nonetheless, variations in muscle measurements were noted, with differences in D3-Cr values ranging from 0.62 kg lower to 13.47 kg higher compared to MRI.

CONCLUSIONS

D3-Cr-estimated muscle mass may be a valuable predictor of clinical outcomes showing consistent associations with falls and fractures, mobility disability, and mortality. However, less consistent associations were found with muscle strength and composition, and physical performance. Although a strong correlation exists between D3-Cr-estimated muscle mass and MRI measurements, under- or overestimation may occur.

Interactions Between HR-pQCT Bone Density and D3 Cr Muscle Mass (or HR-pQCT Bone Structure and HR-pQCT Muscle Density) in Predicting Fractures: The Osteoporotic Fractures in Men Study

We examined if an interaction exists between bone and muscle in predicting fractures in older men. Prospective data from the Osteoporotic Fractures in Men study was used to build Cox proportional hazards models. Predictors included HR-pQCT total volumetric BMD (Tt.BMD), trabecular BMD (Tb.BMD), cortical BMD (Ct.BMD) and cortical area (Ct.Ar) at distal radius/tibia, HR-pQCT muscle volume and density (diaphyseal tibia), D3 -creatine dilution (D3 Cr) muscle mass, and grip strength and leg force, analyzed as continuous variables and as quartiles. Incident fractures were self-reported every 4 months via questionnaires and centrally adjudicated by physician review of radiology reports. Potential confounders (demographics, comorbidities, lifestyle factors, etc.) were considered. A total of 1353 men (mean age 84.2 ± 4.0 years, 92.7% white) were followed for 6.03 ± 2.11 years. In the unadjusted (continuous) model, there were no interactions (p > 0.05) between any muscle variable (D3 Cr muscle mass, muscle volume, muscle density, grip strength or leg force) and Tt.BMD at distal radius/tibia for fractures (all: n = 182-302; nonvertebral: n = 149-254; vertebral: n = 27-45). No consistent interactions were observed when interchanging Tt.BMD for Tb.BMD/Ct.BMD or for Ct.Ar (bone structure) at the distal radius/tibia in the unadjusted (continuous) models. Compared with men in quartiles (Q) 2-4 of D3 Cr muscle mass and Q2-4 of distal tibia Tt.BMD, men in Q1 of both had increased risk for all fractures (hazard ratio (HR) = 2.00; 95% confidence interval [CI] 1.24-3.23, p = 0.005) and nonvertebral fractures (HR = 2.10; 95% CI 1.25-3.52, p < 0.001) in the multivariable-adjusted model. Confidence intervals overlapped (p > 0.05) when visually inspecting other quartile groups in the multivariable-adjusted model. In this prospective cohort study of older men, there was no consistent interactions between bone and muscle variables on fracture risk. Larger sample sizes and longer follow-up may be needed to clarify if there is an interaction between bone and muscle on fracture risk in men. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

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.

Multisystem physiological perspective of human frailty and its modulation by physical activity

"Frailty" is a term used to refer to a state characterized by enhanced vulnerability to, and impaired recovery from, stressors compared with a nonfrail state, which is increasingly viewed as a loss of resilience. With increasing life expectancy and the associated rise in years spent with physical frailty, there is a need to understand the clinical and physiological features of frailty and the factors driving it. We describe the clinical definitions of age-related frailty and their limitations in allowing us to understand the pathogenesis of this prevalent condition. Given that age-related frailty manifests in the form of functional declines such as poor balance, falls, and immobility, as an alternative we view frailty from a physiological viewpoint and describe what is known of the organ-based components of frailty, including adiposity, the brain, and neuromuscular, skeletal muscle, immune, and cardiovascular systems, as individual systems and as components in multisystem dysregulation. By doing so we aim to highlight current understanding of the physiological phenotype of frailty and reveal key knowledge gaps and potential mechanistic drivers of the trajectory to frailty. We also review the studies in humans that have intervened with exercise to reduce frailty. We conclude that more longitudinal and interventional clinical studies are required in older adults. Such observational studies should interrogate the progression from a nonfrail to a frail state, assessing individual elements of frailty to produce a deep physiological phenotype of the syndrome. The findings will identify mechanistic drivers of frailty and allow targeted interventions to diminish frailty progression.

Associations between leukocyte telomere length and osteosarcopenia in 20,400 adults aged 60 years and over: Data from the UK Biobank

PURPOSE

Two mechanisms implicated in telomere shortening are oxidative stress and inflammation, both of which are linked to bone and muscle loss suggesting a pathological link between telomere attrition and osteosarcopenia. Using older adults aged 60 years and over in the UK Biobank, we examined the association between leukocyte telomere length and osteosarcopenia.

METHODS

Baseline leukocyte telomere length was measured using a multiplex qPCR technique and expressed as the amount of the telomere amplification product (T) to that of a single-copy gene (S) (T/S ratio). Osteosarcopenia data was from the first imaging visit and defined by WHO criteria (femoral neck bone density T score ≤ -1) for osteopenia/osteoporosis plus either the EWGSOP2 (low appendicular lean mass/height² and low grip strength) or SDOC (low grip strength and slow walking pace) criteria for sarcopenia. Binary or multinomial logistic regression models were used to associate telomere length and osteosarcopenia or its components, adjusting for the covariates: age, sex, race, education, Townsend deprivation index, alcohol, smoking, BMI/weight, physical activity levels.

RESULTS

Among 20,400 older adults (mean age: 67.79 ± 4.9 years, 53% men), the prevalence of osteosarcopenia by EWGSOP2 (n = 96, 0.47%) or SDOC (n = 205, 1%) criteria was low at the first imaging visit (mean 8.82 years after baseline). Baseline telomere length was not associated with osteosarcopenia by EWGSOP2 (Relative Risk (RR): 1.00, 95% CI: 0.82-1.23 comparing osteosarcopenia to normal (non-osteopenic, non-osteoporotic, and non-sarcopenic) per Standard Deviation (SD) increase in telomere length) or SDOC (RR: 0.95, 95% CI: 0.83-1.09) criteria. Longer telomere length was associated with a lower risk of slow walking pace (Odds Ratio: 0.92, 95% CI: 0.87-0.99 per SD increase in telomere length, p = 0.021). Telomere length, however, was not associated with low grip strength, low bone density or low appendicular lean mass/height² (p > 0.05).

CONCLUSIONS

In this population-based study, telomere length was not associated with osteosarcopenia; however, slow walking pace was. Further studies are needed to reexamine this relationship, including a greater number of the oldest-old (≥75 years) where osteosarcopenia is more prevalent.