Association Between Muscle Mass Determined by D3 -Creatine Dilution and Incident Fractures in a Prospective Cohort Study of Older Men

Association of muscle health impairment and atherosclerosis with major osteoporotic fracture risk in Taiwanese Vegetarians

Objectives

Despite the beneficial effects of "vegetarian style" diet on atherosclerosis, it is also proven potentially detrimental to bone health. The influence of muscle health or atherosclerosis on major osteoporotic fracture (MOF) risk in vegetarians has rarely been explored. This prospective study aimed to investigate an association of MOF risk with muscle health and atherosclerosis in vegetarians.

Materials and Methods

We conducted a questionnaire survey with the Mini-Nutritional Assessment (MNA) on 39 vegetarians. The 10-year probability of MOF was determined using the Taiwanese Fracture Risk Assessment (FRAX®) calculator. Appendicular skeletal muscle (ASM) mass and bone mineral density were measured with dual-energy X-ray absorptiometry. Physical performance was evaluated using the 6-min walk test (6MWT). Common carotid artery intima-media thickness (ccIMT) was determined using sonography. Serum levels of parathyroid hormone (PTH), Vitamin D, adiponectin, and leptin were measured.

Results

Eleven (28.2%) of 39 vegetarians had a moderate-high risk of MOF, defined by FRAX-calculated risk ≥10%. These subjects had lower ASM (P < 0.005) and 6MWT distances (P < 0.01) but greater ccIMT than those with low risk. The MOF risk was negatively correlated with ASM (r = -0.51, P < 0.001) and 6MWT distances (r = -0.62, P < 0.001) but positively correlated with ccIMT (r = 0.56, P < 0.001). Linear regression analysis revealed that MOF risk scores were negatively associated with ASM and 6MWT distance while positively associated with ccIMT. There was no significant association of MOF risk with MNA scores, serum levels of PTH, Vitamin D, adiponectin, or leptin.

Conclusion

Decreased ASM mass, reduced physical performance, and atherosclerosis are significantly associated with MOF risk in vegetarians.

Difference between kidney function by cystatin C versus creatinine and association with muscle mass and frailty

BACKGROUND

A higher difference in estimated glomerular filtration rate by cystatin C versus creatinine (eGFRDiff = eGFRCys - eGFRCreat) is associated with decreased frailty risk. Since eGFRCreat is influenced by muscle more than eGFRCys, muscle mass may explain this association. Previous work could not account for this when considering regional muscle measures by imaging. Deuterated creatine (D3Cr) dilution measures whole body muscle mass (kilograms). We aimed to determine whether eGFRDiff is associated with D3Cr muscle mass and whether muscle mass explains the association between eGFRDiff and frailty.

METHODS

Cross-sectional analysis within the multicenter MrOS Study at Year 14 (visit 4). 490 men of the original cohort of 5994 MrOS participants (aged ≥65 at enrollment) were included. Exposure was eGFRDiff (= eGFRCys - eGFRCreat), calculated using CKD-EPI equations 2012/2021. Primary outcome was D3Cr muscle mass. Secondary outcome was phenotypic pre-frailty (one or two criteria) and frailty (≥three criteria) including the following: weight loss, weakness, slow gait, physical activity, poor energy. The association of eGFRDiff with D3Cr muscle mass was examined by linear regression, that with prefrailty / frailty by multinomial logistic regression.

RESULTS

Mean ± SD age was 84 ± 4 years, eGFRCreat 68 ± 16, eGFRCys 52 ± 16, eGFRDiff -15 ± 12 mL/min/1.73 m2 and D3Cr muscle mass 24 ± 4 kg. For each SD increment in eGFRDiff, D3Cr muscle mass was 1.4 kg higher on average, p < 0.0001 (fully adjusted). Higher eGFRDiff was associated with lower odds of frailty (OR = 0.63 95% CI [0.45;0.89]), but this was partially attenuated and insignificant after additionally adjusting for D3Cr muscle mass (OR = 0.85 95% CI [0.58; 1.24]).

CONCLUSIONS

Higher eGFRDiff is associated with lower odds of frailty among late-life men. D3Cr muscle mass accounts for some of this association. This suggests that non-GFR determinants of creatinine and cystatin C, such as muscle mass, play a role in explaining the association of eGFRDiff with frailty. Future studies are needed to confirm.

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.

Effect of Anamorelin, a Ghrelin Receptor Agonist, on Muscle and Bone in Adults With Osteosarcopenia

CONTEXT

Anamorelin, a ghrelin receptor agonist known to stimulate the pulsatile release of GH from the pituitary, has the potential to improve musculoskeletal health in adults with osteosarcopenia.

OBJECTIVE

To determine the effect of anamorelin treatment for 1 year on muscle mass and strength and on biochemical markers of bone turnover in adults with osteosarcopenia (OS).

DESIGN

Randomized, placebo-controlled, 1-year anamorelin intervention trial.

SETTING

The Bone Metabolism Laboratory at the USDA Nutrition Center at Tufts University.

PARTICIPANTS

26 men and women, age 50 years and older, with OS.

MAIN OUTCOME MEASURES

Muscle mass by D3-creatine dilution and lean body mass (LBM) and bone mineral density (BMD) by dual-energy X-ray absorptiometry, muscle strength, serum IGF-1, and bone turnover markers, serum procollagen 1 intact N-terminal (P1NP), and C-terminal telopeptide (CTX).

RESULTS

Anamorelin did not have a significant effect on muscle mass or LBM; it significantly increased knee flexion torque at 240°/s by 20% (P = .013) and had a similar nonstatistically significant effect on change in knee extension; it increased bone formation (P1NP) by 75% (P = .006) and had no significant effect on bone resorption (CTX) or BMD. Serum IGF-1 increased by 50% in the anamorelin group and did not change in the placebo group (P = .0001 for group difference).

CONCLUSION

In this pilot study, anamorelin did not significantly alter muscle mass; however, it may potentially improve lower extremity strength and bone formation in addition to increasing circulating IGF-1 levels in adults with OS. Further study of anamorelin in this population is warranted.

Sarcopenia: no consensus, no diagnostic criteria, and no approved indication-How did we get here?

In addition to the role of skeletal muscle in movement and locomotion, muscle plays a critical role in a broad array of metabolic processes that can contribute to improved health or risk of disease. The age-associated loss of muscle has been termed sarcopenia. The muscle is the primary site of insulin-stimulated glucose disposal and the largest component of basal metabolic rate, directly and indirectly affects bone density, produces myokines with pleiotropic effect on muscle and other tissues including the brain, and stores essential amino acids essential for the maintenance of protein synthesis during periods of reduced food intake and stress. As such, not surprisingly deterioration of skeletal muscle health, typically operationalized as decline of muscle mass and muscle strength is both a powerful risk factor and main consequence of chronic diseases, disability, and loss of independence, and it is one of the strongest risk factors for mortality. However, skeletal muscle remains one of the most plastic of all tissues, with rapid changes in rates of protein synthesis and degradation in response to physical activity and inactivity, inflammation, and nutritional and hormonal status. This has made the development of pharmacological therapies to increase muscle mass (or prevent loss), an important goal for decades. However, while remarkable advances in the understanding of molecular and cellular regulation of muscle protein metabolism have occurred recently, there are no approved drugs for the treatment of sarcopenia, the loss of skeletal muscle affecting millions of older people. The goal of this paper is to describe the possible reasons for the lack of new and effective pharmacotherapies to treat one of the most important risk factors for age-associated disease and loss of independence.

Associations between D3Cr muscle mass and MR thigh muscle volume with strength, power, physical performance, fitness, and limitations in older adults in the SOMMA study

Background

Different measures to assess muscle size - magnetic resonance (MR) derived thigh muscle volume and d3-creatine dilution derived muscle mass (D3Cr muscle mass) - may have similar associations with strength, power, physical performance, fitness, and functional limitations in older adults.

Methods

Men (N=345) and women (N=482) aged ≥70 years from the Study of Muscle, Mobility and Aging completed exams including leg extension strength (1-repetition max) and cardiopulmonary exercise testing to assess fitness (VO2peak). Correlations and adjusted regression models stratified by sex were used to assess the association between muscle size measures and study outcomes; we tested for sex interactions.

Results

D3Cr muscle mass and MR thigh muscle volume were correlated (men: r=0.62, women: r=0.51, p<.001). Lower D3Cr muscle mass and lower MR thigh muscle volume were associated with lower strength and lower VO2peak in both men and women; D3Cr muscle mass was more strongly associated with strength in men than in women (p-int<0.05). There were correlations, though less consistent, between muscle size or mass with physical performance and function. Associations between the muscle size measures and the study outcomes occasionally varied by sex, and associations of MR thigh muscle volume were, at times, slightly more strongly associated with the study outcomes than was D3Cr muscle mass.

Conclusions

Less muscle -measured by either D3Cr muscle mass or MR thigh muscle volume - was associated with lower strength and worse performance. Varied associations by sex and assessment method suggest consideration be given to which measurement to use in future studies.

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).

Safety and Efficacy of Testosterone Therapy on Musculoskeletal Health and Clinical Outcomes in Men: A Systematic Review and Meta-Analysis of Randomized Placebo-Controlled Trials

OBJECTIVE

Age-related declines in muscle and bone, alongside a shift toward greater adiposity, contribute to falls and fracture risk. Testosterone is osteogenic, myogenic, and catabolic to fat. As such, we examined the effects of testosterone therapy on musculoskeletal health and clinical outcomes in men.

METHODS

Electronic databases (Medline, Embase, Web of Science, Central) were systematically searched for randomized controlled trials (RCTs) reporting on the effects of testosterone therapy versus placebo on any primary outcome (bone density, muscle mass, fat mass, muscle strength/physical performance) or secondary outcome (falls, fractures, disability, adverse events) in men (≥18 years). A random effects meta-regression examined the effects of testosterone on prespecified outcomes.

RESULTS

One thousand seven hundred twenty-eight men across 16 RCTs were included (mean age: 77.1 ± 7.6 years). Baseline mean serum testosterone ranged from 7.5 ± 0.3 to 18.9 ± 1.2 nmol/L. Compared to placebo, 6 months of testosterone therapy increased hip bone density and total lean mass, but effects for handgrip and total fat mass did not reach statistical significance. No significant effects of testosterone therapy on musculoskeletal outcomes were evident at 12 months. The limited number of RCTs reporting on adverse events/clinical outcomes, and the low incidence of these events across RCTs, prohibited statistical comparisons.

CONCLUSION

After 6 months, testosterone effectively increases hip bone density and total lean mass in men, but its effects are unclear for lumbar spine bone density and handgrip strength. Further, RCTs are needed to clarify the safety and efficacy of testosterone on musculoskeletal health and clinical outcomes.

The D3 -creatine dilution method non-invasively measures muscle mass in mice

Developing accurate methods to quantify age-related muscle loss (sarcopenia) could greatly accelerate development of therapies to treat muscle loss in the elderly, as current methods are inaccurate or expensive. The current gold standard method for quantifying sarcopenia is dual-energy X-ray absorptiometry (DXA) but does not measure muscle directly-it is a composite measure quantifying "lean mass" (muscle) excluding fat and bone. In humans, DXA overestimates muscle mass, which has led to erroneous conclusions about the importance of skeletal muscle in human health and disease. In animal models, DXA is a popular method for measuring lean mass. However, instrumentation is expensive and is potentially limited by anesthesia concerns. Recently, the D3 -creatine (D3 Cr) dilution method for quantifying muscle mass was developed in humans and rats. This method is faster, cheaper, and more accurate than DXA. Here, we demonstrate that the D3 Cr method is a specific assay for muscle mass in mice, and we test associations with DXA and body weight. We evaluated the D3 Cr method compared to DXA-determined lean body mass (LBM) in aged mice and reported that DXA consistently overestimates muscle mass with age. Overall, we provide evidence that the D3 Cr dilution method directly measures muscle mass in mice. Combined with its ease of use, accessibility, and non-invasive nature, the method may prove to more quickly advance development of preclinical therapies targeting sarcopenia.

Synergistic Strategies to Accelerate the Development of Function-Promoting Therapies: Lessons From Operation Warp Speed and Oncology Drug Development

BACKGROUND

Functional limitations and physical disabilities associated with aging and chronic disease are major concerns for human societies and expeditious development of function-promoting therapies is a public health priority.

METHODS

Expert panel discussion.

RESULTS

The remarkable success of Operation Warp Speed for the rapid development of COVID-19 vaccines, COVID-19 therapeutics, and of oncology drug development programs over the past decade have taught us that complex public health problems such as the development of function-promoting therapies will require collaboration among many stakeholders, including academic investigators, the National Institutes of Health, professional societies, patients and patient advocacy organizations, the pharmaceutical and biotechnology industry, and the U.S. Food and Drug Administration.

CONCLUSIONS

There was agreement that the success of well designed, adequately powered clinical trials will require careful definitions of indication/s, study population, and patient-important endpoints that can be reliably measured using validated instruments, commensurate resource allocation, and versatile organizational structures such as those used in Operation Warp Speed.

Optimizing the Design of Clinical Trials to Evaluate the Efficacy of Function-Promoting Therapies

BACKGROUND

Several candidate molecules that may have application in treating physical limitations associated with aging and chronic diseases are in development. Challenges in the framing of indications, eligibility criteria, and endpoints and the lack of regulatory guidance have hindered the development of function-promoting therapies.

METHODS

Experts from academia, pharmaceutical industry, National Institutes of Health (NIH), and Food and Drug Administration (FDA) discussed optimization of trial design including the framing of indications, eligibility criteria, and endpoints.

RESULTS

Mobility disability associated with aging and chronic diseases is an attractive indication because it is recognized by geriatricians as a common condition associated with adverse outcomes, and it can be ascertained reliably. Other conditions associated with functional limitation in older adults include hospitalization for acute illnesses, cancer cachexia, and fall injuries. Efforts are underway to harmonize definitions of sarcopenia and frailty. Eligibility criteria should reconcile the goals of selecting participants with the condition and ensuring generalizability and ease of recruitment. An accurate measure of muscle mass (eg, D3 creatine dilution) could be a good biomarker in early-phase trials. Performance-based and patient-reported measures of physical function are needed to demonstrate whether treatment improves how a person lives, functions, or feels. Multicomponent functional training that integrates training in balance, stability, strength, and functional tasks with cognitive and behavioral strategies may be needed to translate drug-induced muscle mass gains into functional improvements.

CONCLUSIONS

Collaborations among academic investigators, NIH, FDA, pharmaceutical industry, patients, and professional societies are needed to conduct well-designed trials of function-promoting pharmacological agents with and without multicomponent functional training.

Comparing the Fracture Profile of Osteosarcopenic Older Adults with Osteopenia/Osteoporosis Alone

PURPOSE

To determine whether osteosarcopenia is associated with a greater likelihood of recurrent fractures, as well as type of fracture, than osteopenia/osteoporosis or sarcopenia alone.

METHODS

Anthropometry (height/weight; scales and stadiometer), body composition (bone mineral density [BMD] and appendicular lean mass; dual-energy x-ray absorptiometry), grip strength (hydraulic dynamometer), and gait speed (4 m) were measured in an outpatient clinic. WHO definition for osteopenia/osteoporosis (BMD T-score below -1 SDs) while sarcopenia was defined by SDOC or EWGSOP2. Number and location of fractures within the past 5 years were self-reported and verified by medical records (unverified fractures excluded). Univariable and multivariable regressions were used to examine the association between the exposure and outcome while adjusting for confounders.

RESULTS

481 community-dwelling older adults (median age: 78, IQR: 72, 83; 75.9% women) were included. Prevalence of osteosarcopenia depended on the definition (SDOC: 179 (37.2%); EWGSOP2: 123 (25.6%)). In multivariable analysis adjusting for age, sex, alcohol, smoking, BMI, lowest BMD T-score, physical activity, and comorbidities, the likelihood of recurrent fractures (≥ 2 vs 0-1) was significantly higher in those with osteosarcopenia versus osteopenia/osteoporosis irrespective of the definition (SDOC: odds ratio [OR]: 1.63, 95% CI: 1.03, 2.59, p = 0.037; EWGSOP2: OR: 1.83, 95% CI: 1.12, 3.01, p = 0.016]. Associations with sarcopenia alone (SDOC: 10; EWGSOP2: 7) were not possible due to the extremely low prevalence of this condition in those with normal BMD.

CONCLUSION

Our data suggest osteosarcopenia is associated with a greater likelihood of recurrent fractures versus osteopenia/osteoporosis alone. Further studies are needed to evaluate the relationship with sarcopenia alone.

Biochemical Markers of Musculoskeletal Health and Aging to be Assessed in Clinical Trials of Drugs Aiming at the Treatment of Sarcopenia: Consensus Paper from an Expert Group Meeting Organized by the European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases (ESCEO) and the Centre Académique de Recherche et d'Expérimentation en Santé (CARES SPRL), Under the Auspices of the World Health Organization Collaborating Center for the Epidemiology of Musculoskeletal Conditions and Aging

In clinical trials, biochemical markers provide useful information on the drug's mode of action, therapeutic response and side effect monitoring and can act as surrogate endpoints. In pharmacological intervention development for sarcopenia management, there is an urgent need to identify biomarkers to measure in clinical trials and that could be used in the future in clinical practice. The objective of the current consensus paper is to provide a clear list of biochemical markers of musculoskeletal health and aging that can be recommended to be measured in Phase II and Phase III clinical trials evaluating new chemical entities for sarcopenia treatment. A working group of the European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases (ESCEO) proposed classifying biochemical markers into 2 series: biochemical markers evaluating musculoskeletal status and biochemical markers evaluating causal factors. For series 1, the group agreed on 4 biochemical markers that should be assessed in Phase II or Phase III trials (i.e., Myostatin-Follistatin, Brain Derived Neurotrophic Factor, N-terminal Type III Procollagen and Serum Creatinine to Serum Cystatin C Ratio - or the Sarcopenia Index). For series 2, the group agreed on 6 biochemical markers that should be assessed in Phase II trials (i.e., the hormones insulin-like growth factor-1 (IGF-I), dehydroepiandrosterone sulphate, and cortisol, and the inflammatory markers C-reactive protein (CRP), interleukin-6 and tumor necrosis factor-α), and 2 in Phase III trials (i.e., IGF-I and CRP). The group also proposed optional biochemical markers that may provide insights into the mode of action of pharmacological therapies. Further research and development of new methods for biochemical marker assays may lead to the evolution of these recommendations.

Comparing Prevalence of Sarcopenia Using Twelve Sarcopenia Definitions in a Large Multinational European Population of Community-Dwelling Older Adults

OBJECTIVES

Multinational prevalence data on sarcopenia among generally healthy older adults is limited. The aim of the study was to assess prevalence of sarcopenia in the DO-HEALTH European trial based on twelve current sarcopenia definitions.

SETTING AND PARTICIPANTS

This is an analysis of the DO-HEALTH study including 1495 of 2157 community-dwelling participants age 70+ years from Germany, France, Portugal, and Switzerland with complete measurements of the sarcopenia toolbox including muscle mass by DXA, grip strength, and gait speed.

MEASUREMENTS

The twelve sarcopenia definitions applied were Asian Working Group on Sarcopenia (AWGS1), AWGS2, Baumgartner, Delmonico, European Working Group on Sarcopenia in Older People (EWGSOP1), EWGSOP2, EWGSOP2-lower extremities, Foundation for the National Institutes of Health (FNIH1), FNIH2, International Working Group on Sarcopenia in Older People (IWGS), Morley, and Sarcopenia Definitions and Outcomes Consortium (SDOC).

RESULTS

Mean age was 74.9 years (SD 4.4); 63.3% were women. Sarcopenia prevalence ranged between 0.7% using the EWGSOP2 or AWGS2 definition, up to 16.8% using the Delmonico definition. Overall, most sarcopenia definitions, including Delmonico (16.8%), Baumgartner (12.8%), FNIH1(10.5%), IWGS (3.6%), EWGSOP1 (3.4%), SDOC (2.0%), Morley (1.3%), and AWGS1 (1.1%) tended to be higher than the prevalence based on EWGSOP2 (0.7%). In contrast, the definitions AWGS2 (0.7%), EWGSOP2-LE (1.1%), FNIH2 (1.0%) - all based on muscle mass and muscle strength - showed similar lower prevalence as EWGSOP2 (0.7%). Moreover, most sarcopenia definitions did not overlap on identifying sarcopenia on an individual participant-level.

CONCLUSION

In this multinational European trial of community-dwelling older adults we found major discordances of sarcopenia prevalence both on a population- and on a participant- level between various sarcopenia definitions. Our findings suggest that the concept of sarcopenia may need to be rethought to reliably and validly identify people with impaired muscle health.

D3 -creatine dilution for skeletal muscle mass measurement: historical development and current status

The French chemist Michel Eugène Chevreul discovered creatine in meat two centuries ago. Extensive biochemical and physiological studies of this organic molecule followed with confirmation that creatine is found within the cytoplasm and mitochondria of human skeletal muscles. Two groups of investigators exploited these relationships five decades ago by first estimating the creatine pool size in vivo with ¹⁴ C and ¹⁵ N labelled isotopes. Skeletal muscle mass (kg) was then calculated by dividing the creatine pool size (g) by muscle creatine concentration (g/kg) measured on a single muscle biopsy or estimated from the literature. This approach for quantifying skeletal muscle mass is generating renewed interest with the recent introduction of a practical stable isotope (creatine-(methyl-d₃ )) dilution method for estimating the creatine pool size across the full human lifespan. The need for a muscle biopsy has been eliminated by assuming a constant value for whole-body skeletal muscle creatine concentration of 4.3 g/kg wet weight. The current single compartment model of estimating creatine pool size and skeletal muscle mass rests on four main assumptions: tracer absorption is complete; tracer is all retained; tracer is distributed solely in skeletal muscle; and skeletal muscle creatine concentration is known and constant. Three of these assumptions are false to varying degrees. Not all tracer is retained with urinary isotope losses ranging from 0% to 9%; an empirical equation requiring further validation is used to correct for spillage. Not all tracer is distributed in skeletal muscle with non-muscle creatine sources ranging from 2% to 10% with a definitive value lacking. Lastly, skeletal muscle creatine concentration is not constant and varies between muscles (e.g. 3.89-4.62 g/kg), with diets (e.g. vegetarian and omnivore), across age groups (e.g. middle-age, ~4.5 g/kg; old-age, 4.0 g/kg), activity levels (e.g. athletes, ~5 g/kg) and in disease states (e.g. muscular dystrophies, <3 g/kg). Some of the variability in skeletal muscle creatine concentrations can be attributed to heterogeneity in the proportions of wet skeletal muscle as myofibres, connective tissues, and fat. These observations raise serious concerns regarding the accuracy of the deuterated-creatine dilution method for estimating total body skeletal muscle mass as now defined by cadaver analyses of whole wet tissues and in vivo approaches such as magnetic resonance imaging. A new framework is needed in thinking about how this potentially valuable method for measuring the creatine pool size in vivo can be used in the future to study skeletal muscle biology in health and disease.

Greater pQCT Calf Muscle Density Is Associated with Lower Fracture Risk, Independent of FRAX, Falls and BMD: A Meta-Analysis in the Osteoporotic Fractures in Men (MrOS) Study

We investigated the predictive performance of peripheral quantitative computed tomography (pQCT) measures of both calf muscle density (an established surrogate for muscle adiposity, with higher values indicating lower muscle adiposity and higher muscle quality) and size (cross-sectional area [CSA]) for incident fracture. pQCT (Stratec XCT2000/3000) measurements at the tibia were undertaken in Osteoporotic Fractures in Men (MrOS) United States (US), Hong Kong (HK), and Swedish (SW) cohorts. Analyses were by cohort and synthesized by meta-analysis. The predictive value for incident fracture outcomes, illustrated here for hip fracture (HF), using an extension of Poisson regression adjusted for age and follow-up time, was expressed as hazard ratio (HR) per standard deviation (SD) increase in exposure (HR/SD). Further analyses adjusted for femoral neck (fn) bone mineral density (BMD) T-score, Fracture Risk Assessment Tool (FRAX) 10-year fracture probability (major osteoporotic fracture) and prior falls. We studied 991 (US), 1662 (HK), and 1521 (SW) men, mean ± SD age 77.0 ± 5.1, 73.9 ± 4.9, 80 ± 3.4 years, followed for a mean ± SD 7.8 ± 2.2, 8.1 ± 2.3, 5.3 ± 2.0 years, with 31, 47, and 78 incident HFs, respectively. Both greater muscle CSA and greater muscle density were associated with a lower risk of incident HF [HR/SD: 0.84; 95% confidence interval [CI], 0.72-1.0 and 0.78; 95% CI, 0.66-0.91, respectively]. The pattern of associations was not materially changed by adjustment for prior falls or FRAX probability. In contrast, after inclusion of fn BMD T-score, the association for muscle CSA was no longer apparent (1.04; 95% CI, 0.88-1.24), whereas that for muscle density was not materially changed (0.69; 95% CI, 0.59-0.82). Findings were similar for osteoporotic fractures. pQCT measures of greater calf muscle density and CSA were both associated with lower incidence of fractures in older men, but only muscle density remained an independent risk factor for fracture after accounting for fn BMD. These findings demonstrate a complex interplay between measures of bone, muscle size, and quality, in determining fracture risk. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Commentary on Guidance for assessment of the muscle mass phenotypic criterion for the Global Leadership Initiative on Malnutrition (GLIM) diagnosis of malnutrition

The assessment of body composition and its relation to health outcomes has been an integral component of medical research. While body mass index is used as an index of overweight and obesity, the effects of the major individual components of body mass, including fat and skeletal muscle, on outcomes depends upon the accuracy of the method being used. Body composition may be measured using several different methods, each of which rely on assumptions that may not be true in all populations, ages, or clinical conditions. This article is protected by copyright. All rights reserved.