Introduction and Objective: Older adults with pre-diabetes (p-D) or type 2 diabetes (T2D) may be vulnerable to an accelerated loss of muscle mass and function due to poor recovery following physical inactivity. To test this paradigm, we conducted a 10-day bed rest study to characterize the recovery of muscle volume and function in older adults with p-D or T2D. Methods: Thirty-nine older adults with p-D or T2D (p-D/T2D 14M/13F, 65 ± 5.0 y) or who were metabolically healthy (CON, 6M/6F, 68 ± 2.9 y) completed a 10-day bed rest intervention, followed by 4 weeks of ambulatory recovery. Mid-thigh muscle volume was assessed by MRI. Knee extensor power and torque were determined by isokinetic dynamometry. Free living activity was quantified via actigraphy. Mitochondrial oxidative phosphorylation capacity (ATPmax) was assessed by 31P-MRS. All assessments were completed pre- and post-bed rest, and weekly during recovery with analyses performed via repeated measures ANCOVA controlling for baseline. Results: Bed rest decreased muscle volume similarly between groups (~2.3%), with volume returning to baseline levels after only 1 week of ambulatory recovery. However, knee extensor power (CON: 115.8 ± 41.6 vs p-D/T2D: 99.0 ± 43.1 watts, P<0.05) and torque (CON: 87.6 ± 26.6 vs p-D/T2D: 78.0 ± 30.6 N*m, P<0.05) remained significantly lower in the p-D/T2D group during ambulatory recovery. Physical activity levels returned to baseline after 1 week of ambulatory recovery for both groups. ATPmax decreased (~11.5%) and failed to return to baseline in both groups. Conclusion: Older adults with pre-diabetes/T2D had delayed recovery of muscle function after bed rest, despite recovery of muscle volume. Muscle mitochondrial energetics did not recover during ambulatory recovery for both groups. These data suggest that older adults with pre-diabetes or type 2 diabetes are uniquely vulnerable to inactivity induced muscle dysfunction and may require a targeted rehabilitation strategy to facilitate recovery of muscle function. Disclosure C. Elliehausen: None. S.V. Ramos: None. J. Hinkley: None. F. Yi: None. R. Pratley: Consultant; Current; Lilly USA LLC. Research Support; Current; National Institutes of Health, Novo Nordisk. Speaker's Bureau; Current; Novo Nordisk. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Current; Novo Nordisk. Consultant; Current; Pfizer Inc., Recordati Rare Diseases Inc., Regeneron Pharmaceuticals Inc., Response Pharmaceuticals, Rona Therapeutics Ltd. Research Support; Current; Sanofi. Consultant; Current; Scholar Rock Inc. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Current; Sun Pharmaceutical Industries Ltd. Consultant; Current; Third Rock Ventures, Verdiva Bio Dev Limited. Research Support; Current; AstraZeneca AB, Boehringer Ingelheim International GmbH, Abbott Laboratories. Consultant; Current; Abbott Laboratories, AbbVie Inc. Other - Consulting; stock options; Current; Altanine, Inc. Consultant; Current; Amgen Inc., AstraZeneca Pharmaceuticals LP. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Current; Bayer AG, Bayer HealthCare Pharmaceuticals Inc. Research Support; Current; Biomea Fusion. Consultant; Current; Boehringer Ingelheim Pharmaceuticals Inc., Carmot Therapeutics, Inc., Corcept Therapeutics. Research Support; Current; Dompé, Eli Lilly and Company. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Current; Eli Lilly and Company. Research Support; Current; Endogenex Inc. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Current; Endogenex Inc. Consultant; Current; F. Hoffmann-La Roche Ltd. Research Support; Current; Fractyl Health, Inc. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Ended; Gasherbrum Bio Inc., Genprex. Consultant; Current; Hanmi Pharm. Co., Ltd. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Ended; Intas Pharmaceuticals Ltd. Research Support; Current; Lexicon Pharmaceuticals, Inc. Consultant; Current; Lexicon Pharmaceuticals, Inc. Speaker's Bureau; Current; Lilly USA LLC. J.P. DeLany: None. B. Goodpaster: Advisory Panel; Ended; AbbVie Inc., Altimmune, Merck & Co., Inc., Novo Nordisk, Regeneron Pharmaceuticals Inc. P.M. Coen: None. Funding National Institutes of Health (R01AG060153 to PMC)

BACKGROUND:Age-associated decline in mitochondrial oxidative capacity is associated with increased risk of disease, frailty, and disability. Oral nitrite and nitrate supplementation have been demonstrated to improve mitochondrial energetics and physical function in younger adults, but effects in older adults (age ≥70 years) remain unclear. METHODS:We conducted a randomized, placebo-controlled, double-blind, two-arm trial with a parallel group design to examine the effect of 20 mg sodium nitrite supplements administered three times a day for 12 weeks versus placebo in older (age ≥70 years) sedentary adults. Change in muscle mitochondrial respiration (complex I and II supported maximal oxidative phosphorylation [CI&II MaxOXPHOS]) was the primary outcome. Platelet bioenergetics, cardiorespiratory fitness, and other physical function measures were also assessed. RESULTS:Sixty-four adults (75.7 ± 5.7 years) completed the trial. Nitrite supplementation was not associated with improvements in skeletal muscle mitochondrial respiration, nor improvements in exercise capacity and physical function. However, platelet mitochondrial respiration changed significantly following an acute dose of oral nitrite. Notably, while nitrite levels increased 16- to 30-fold in plasma following an acute dose, levels increased only 1.6-fold in skeletal muscle. CONCLUSIONS:The divergent response of skeletal muscle versus platelet mitochondrial respiration in response to nitrite supplementation suggests tissue-specific pharmacokinetics and pharmacodynamics that likely impact the efficacy of nitrite supplementation. Results also suggest there may be age-related changes in drug delivery, metabolism, and mitochondrial responsiveness compared to the effects of nitrite/nitrate previously demonstrated in younger adults. Clinical Trial Registration Number: ClinicalTrials.gov NCT04405180.

Introduction and Objective: Older adults with pre-diabetes or type 2 diabetes experience hospitalization and bed rest more frequently than healthy older adults. However, the impact of physical inactivity induced by bed rest on insulin sensitivity in older adults with pre- or type 2 diabetes is unknown. To address this, we conducted a 10-day bed rest study to characterize changes in muscle, hepatic, and adipose tissue insulin sensitivity in older adults with or without pre- or type 2 diabetes. Methods: Thirty-nine older adults with pre- or type 2 diabetes (p-D/T2D; 14M/13F, 65 ± 5.0 yrs) or who were metabolically healthy (CON, 6M/6F, 68 ± 2.9 yrs) completed a 10-day bed rest intervention. A 2-step hyperinsulinemic euglycemic clamp (2hrs @ 20 and 2hrs @ 90 mU/m2/min insulin infusion) with U-13C glucose tracer was performed on day 1 and 10 of bed rest. Skeletal muscle insulin sensitivity (Rate of disposal, Rd/Insulin) and hepatic insulin sensitivity index (endogenous glucose production, EGP/Insulin) were determined. Plasma free fatty acid (FFA) suppression during the clamp was measured as an index of adipose tissue insulin sensitivity. Analyses were performed using a mixed model. Results: Bed rest decreased Rd/Insulin by ~31% at step 1(100-120min; time p<0.001, group p=0.033) and ~31.4% at step 2 (220-240min; time p<0.001) for both p-D/T2D and CON. There was no effect of bed rest on EGP/Insulin at step 1 (time p=0.136) or step 2 (time p=0.174) for p-D/T2D and CON. The percent suppression of EGP was also not affected by bed rest. Fasting insulin, plasma FFA, and FFA suppression during the clamp for both p-D/T2D and CON was not affected by bed rest. Conclusion: These data indicate that bed rest-induced physical inactivity reduces skeletal muscle insulin sensitivity in older adults either with or without pre- or type 2 diabetes, while hepatic and adipose insulin sensitivity are unaffected. Disclosure C. Elliehausen: None. J.M. Mucinski: None. S.V. Ramos: None. J. Hinkley: None. F. Yi: None. R. Pratley: Consultant; Current; Lilly USA LLC. Research Support; Current; National Institutes of Health, Novo Nordisk. Speaker's Bureau; Current; Novo Nordisk. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Current; Novo Nordisk. Consultant; Current; Pfizer Inc., Recordati Rare Diseases Inc., Regeneron Pharmaceuticals Inc., Response Pharmaceuticals, Rona Therapeutics Ltd. Research Support; Current; Sanofi. Consultant; Current; Scholar Rock Inc. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Current; Sun Pharmaceutical Industries Ltd. Consultant; Current; Third Rock Ventures, Verdiva Bio Dev Limited. Research Support; Current; AstraZeneca AB, Boehringer Ingelheim International GmbH, Abbott Laboratories. Consultant; Current; Abbott Laboratories, AbbVie Inc. Other - Consulting; stock options; Current; Altanine, Inc. Consultant; Current; Amgen Inc., AstraZeneca Pharmaceuticals LP. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Current; Bayer AG, Bayer HealthCare Pharmaceuticals Inc. Research Support; Current; Biomea Fusion. Consultant; Current; Boehringer Ingelheim Pharmaceuticals Inc., Carmot Therapeutics, Inc., Corcept Therapeutics. Research Support; Current; Dompé, Eli Lilly and Company. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Current; Eli Lilly and Company. Research Support; Current; Endogenex Inc. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Current; Endogenex Inc. Consultant; Current; F. Hoffmann-La Roche Ltd. Research Support; Current; Fractyl Health, Inc. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Ended; Gasherbrum Bio Inc., Genprex. Consultant; Current; Hanmi Pharm. Co., Ltd. Other - Consulting: Thru 12/31/2023 payment directed to Dr. Pratley's employer; as of 1/1/2024 payment directed to Dr. Pratley personally.; Ended; Intas Pharmaceuticals Ltd. Research Support; Current; Lexicon Pharmaceuticals, Inc. Consultant; Current; Lexicon Pharmaceuticals, Inc. Speaker's Bureau; Current; Lilly USA LLC. B. Goodpaster: Advisory Panel; Ended; AbbVie Inc., Altimmune, Merck & Co., Inc., Novo Nordisk, Regeneron Pharmaceuticals Inc. J.P. DeLany: None. P.M. Coen: None. Funding National Institutes of Health (R01AG060153 to PMC)

In the United States, older adults who self-identify as Black have a disproportionately higher incidence of mobility disability compared to those who are White. Whether older adults who are Black also have lower fitness and mitochondrial energetics has not been adequately investigated. The study of muscle, mobility and aging (SOMMA) examined 879 participants aged ≥ 70 years old, including 116 who self-identified as Black. Mitochondrial respiration (Max OXPHOS) was measured in permeabilized fibers from muscle biopsies. Cardiorespiratory fitness (VO2 peak) was determined by a cardiopulmonary exercise test. Education, income, financial resources, race, sex, and age were determined by self-report. We used propensity score matching to match Blacks with Whites with a 1:1 ratio. Black (n = 90) and White (n = 90) groups were matched for age, sex, SOMMA multimorbidity index, BMI, muscle mass, physical activity, marital status, educational achievement, and whether financial needs were met (all p > 0.05). Despite being well matched for these variables, those who identified as Black had a slower 400-m walking speed (0.97 vs. 1.03 m/s, p = 0.014), lower Max OXPHOS (50.8 vs. 60.9 (pmol/(s*mg)), p = 0.0002), and lower cardiorespiratory fitness (1391 vs. 1566 mL/min, p = 0.007) when compared to those who identified as White. Multivariate regression showed that VO2 peak and Max OXPHOS, but not socioeconomic factors, attenuated the race difference in 400-m walking speed. In conclusion, while the etiology of race differences in mobility is multifactorial, our data indicate that muscle mitochondrial respiration and cardiorespiratory fitness may contribute to the slower walking speed of individuals who identify as Black compared to White.

Mitochondrial creatine kinase (mtCK) regulates the "fast" export of phosphocreatine to support cytoplasmic phosphorylation of ADP to ATP which is more rapid than direct ATP export. Such "creatine-dependent" phosphate shuttling is attenuated in several muscles, including the heart, of the D2.mdx mouse model of Duchenne muscular dystrophy at only 4 weeks of age. However, the degree to which creatine-dependent and -independent systems of phosphate shuttling progressively worsen or potentially adapt in a hormetic manner throughout disease progression remains unknown. Here, we performed a series of proof-of-principle investigations designed to determine how phosphate shuttling pathways worsen or adapt in later disease stages in D2.mdx (12 months of age). We also determined whether changes in creatine-dependent phosphate shuttling are linked to alterations in mtCK thiol redox state. In permeabilized muscle fibres prepared from cardiac left ventricles, we found that 12-month-old male D2.mdx mice have reduced creatine-dependent pyruvate oxidation and elevated complex I-supported H2O2 emission (mH(2)O(2)). Surprisingly, creatine-independent ADP-stimulated respiration was increased and mH(2)O(2) was lowered suggesting that impairments in the faster mtCK-mediated phosphocreatine export system resulted in compensation of the alternative slower pathway of ATP export. The apparent impairments in mtCK-dependent bioenergetics occurred independent of mtCK protein content but were related to greater thiol oxidation of mtCK and a more oxidized cellular environment (lower GSH:GSSG). Next, we performed a proof-of-principle study to determine whether creatine-dependent bioenergetics could be enhanced through chronic administration of the mitochondrial-targeting, ROS-lowering tetrapeptide, SBT-20. We found that 12 weeks of daily treatment with SBT-20 (from day 4-similar to 12 weeks of age) increased respiration and lowered mH(2)O(2) only in the presence of creatine in D2.mdx mice without affecting calcium-induced mitochondrial permeability transition activity. In summary, creatine-dependent mitochondrial bioenergetics are attenuated in older D2.mdx mice in relation to mtCK thiol oxidation that seem to be countered by increased creatine-independent phosphate shuttling as a unique form of mitohormesis. Separate results demonstrate that creatine-dependent bioenergetics can also be enhanced with a ROS-lowering mitochondrial-targeting peptide. These results demonstrate a specific relationship between redox stress and mitochondrial hormetic reprogramming during dystrophin deficiency with proof-of-principle evidence that creatine-dependent bioenergetics could be modified with mitochondrial-targeting small peptide therapeutics.

Mitochondrial creatine kinase (mtCK) regulates the “fast” export of phosphocreatine to support cytoplasmic phosphorylation of ADP to ATP which is more rapid than direct ATP export. Such “creatine-dependent” phosphate shuttling is attenuated in several muscles, including the heart, of the D2. mdx mouse model of Duchenne muscular dystrophy at only 4 weeks of age. Here, we determined whether such attenuations occur in later stages in D2. mdx (12 months of age) in relation to mtCK thiol redox state, and whether this pathway could be preserved through administration of the mitochondrial-targeting, ROS-lowering tetrapeptide, SBT-20, in the D2. mdx mouse. In permeabilized muscle fibres prepared from cardiac left ventricles, we found that aged male D2. mdx mice have reduced creatine-dependent pyruvate oxidation and elevated complex I-supported H2O2 emission (mH2O2). Surprisingly, creatine-independent ADP-stimulated respiration was increased and mH2O2 was lowered suggesting that impairments in the faster mtCK-mediated phosphocreatine export system resulted in compensation of the alternative slower pathway of ATP export. The apparent impairments in mtCK-dependent bioenergetics occurred independent of mtCK protein content but were related to greater thiol oxidation of mtCK and a more oxidized cellular environment (lower GSH:GSSG). We then found that 12 weeks of daily treatment with SBT-20 (from day 4 to ∼12 weeks of age) increased respiration and lowered mH2O2 only in the presence of creatine in D2. mdx mice without affecting calcium-induced mitochondrial permeability transition pore activity. In summary, creatine-dependent mitochondrial bioenergetics are attenuated in older D2. mdx mice in relation to mtCK thiol oxidation, which can be preserved with a ROS-lowering mitochondrial-targeting peptide. These results demonstrate a specific relationships between redox stress and metabolic reprogramming during dystrophin deficiency that can be targeted with small peptide therapeutics.### Competing Interest StatementStealth Biotherapeutics provided SBT-20 through a material transfer agreement but did not provide funding for this study.* ADP : adenosine diphosphate ATP : adenosine triphosphate ANT : adenine nucleotide translocase cCK : cytosolic creatine kinase mH2O2 : mitochondrial H2O2 emission mtCK : mitochondrial creatine kinase PCr : phosphocreatine PDC : pyruvate dehydrogenase complex VDAC : voltage dependent anion carrier

Background The geroscience hypothesis posits that aging biological processes contribute to many age-related deficits, including the accumulation of multiple chronic diseases. Though only one facet of mitochondrial function, declines in muscle mitochondrial bioenergetic capacities may contribute to this increased susceptibility to multimorbidity. Methods The Study of Muscle, Mobility and Aging (SOMMA) assessed ex vivo muscle mitochondrial energetics in 764 older adults (mean age =76.4, 56.5% women, 85.9% non-Hispanic white) by high-resolution respirometry of permeabilized muscle fibers. We estimated the proportional odds ratio (POR [95%CI]) for the likelihood of greater multimorbidity (four levels: 0 conditions, N=332; 1 condition, N=299; 2 conditions, N=98; or 3+ conditions, N=35) from an index of 11 conditions, per SD decrement in muscle mitochondrial energetic parameters. Distribution of conditions allowed for testing the associations of maximal muscle energetics with some individual conditions. Results Lower oxidative phosphorylation supported by fatty acids and/or complex-I and -II linked carbohydrates (e.g., Max OXPHOSCI+CII) was associated with a greater multimorbidity index score (POR=1.32[1.13,1.54]) and separately with diabetes mellitus (OR=1.62[1.26,2.09]), depressive symptoms (OR=1.45[1.04,2.00]) and possibly chronic kidney disease (OR=1.57[0.98,2.52]) but not significantly with other conditions (e.g., cardiac arrhythmia, chronic obstructive pulmonary disease). Conclusions Lower muscle mitochondrial bioenergetic capacities was associated with a worse composite multimorbidity index score. Our results suggest that decrements in muscle mitochondrial energetics may contribute to a greater global burden of disease and is more strongly related to some conditions than others.

Objective:Examine the association of ectopic adipose tissue (AT) with skeletal muscle (SM) mitochondrial bioenergetics in older adults.Methods:Cross-sectional data from 829 older adults ≥70 years was used. Total abdominal, subcutaneous, and visceral AT; and thigh muscle fat infiltration (MFI) was quantified by MRI. SM mitochondrial energetics were characterized using in vivo 31 P-MRS (ATP max ) and ex vivo high-resolution respirometry (maximal oxidative phosphorylation (OXPHOS)). ActivPal was used to measure PA (step count). Linear regression models adjusted for covariates were applied, with sequential adjustment for BMI and PA.Results:Independent of BMI, total abdominal (standardized (Std.) β=-0.21; R 2 =0.09) and visceral AT (Std. β=-0.16; R 2 =0.09) were associated with ATP max ( p <0.01), but not after further adjustment for PA (p≥0.05). Visceral AT (Std. β=-0.16; R 2 =0.25) and thigh MFI (Std. β=-0.11; R 2 =0.24) were negatively associated with carbohydrate-supported maximal OXPHOS independent of BMI and PA ( p <0.05). Total abdominal AT (Std. β=-0.19; R 2 =0.24) and visceral AT (Std. β=-0.17; R 2 =0.24) were associated with fatty acid-supported maximal OXPHOS independent of BMI and PA (p<0.05).Conclusions:Skeletal MFI and abdominal visceral, but not subcutaneous AT, are inversely associated with SM mitochondrial bioenergetics in older adults independent of BMI. Associations between ectopic AT and in vivo mitochondrial bioenergetics are attenuated by PA.

BACKGROUND:Phenotypic frailty syndrome identifies older adults at greater risk for adverse health outcomes. Despite the critical role of mitochondria in maintaining cellular function, including energy production, the associations between muscle mitochondrial energetics and frailty have not been widely explored in a large, well-phenotyped, older population. METHODS:The Study of Muscle, Mobility and Aging (SOMMA) assessed muscle energetics in older adults (N = 879, mean age = 76.3 years, 59.2% women). 31Phosporous magnetic resonance spectroscopy measured maximal production of adenosine triphosphate (ATPmax) in vivo, while ex vivo high-resolution respirometry of permeabilized muscle fibers from the vastus lateralis measured maximal oxygen consumption supported by fatty acids and complex I- and II-linked carbohydrates (e.g., Max OXPHOSCI+CII). Five frailty criteria, shrinking, weakness, exhaustion, slowness, and low activity, were used to classify participants as robust (0, N = 397), intermediate (1-2, N = 410), or frail (≥ 3, N = 66). We estimated the proportional odds ratio (POR) for greater frailty, adjusted for multiple potential confounders. RESULTS:One-SD decrements of most respirometry measures (e.g., Max OXPHOSCI+CII, adjusted POR = 1.5, 95%CI [1.2,1.8], p = 0.0001) were significantly associated with greater frailty classification. The associations of ATPmax with frailty were weaker than those between Max OXPHOSCI+CII and frailty. Muscle energetics was most strongly associated with slowness and low physical activity components. CONCLUSIONS:Our data suggest that deficits in muscle mitochondrial energetics may be a biological driver of frailty in older adults. On the other hand, we did observe differential relationships between measures of muscle mitochondrial energetics and the individual components of frailty.

Greater perceived physical fatigability and lower skeletal muscle energetics are predictors of mobility decline. Characterizing associations between muscle energetics and perceived fatigability may provide insight into potential targets to prevent mobility decline. We examined associations of in vivo (maximal ATP production, ATPmax) and ex vivo (maximal carbohydrate supported oxidative phosphorylation [max OXPHOS] and maximal fatty acid supported OXPHOS [max FAO OXPHOS]) measures of mitochondrial energetics with two measures of perceived physical fatigability, Pittsburgh Fatigability Scale (PFS, 0-50, higher=greater) and Rating of Perceived Exertion (RPE Fatigability, 6-20, higher=greater) after a slow treadmill walk. Participants from the Study of Muscle, Mobility and Aging (N=873) were 76.3±5.0 years old, 59.2% women, and 85.3% White. Higher muscle energetics (both in vivo and ex vivo ) were associated with lower perceived physical fatigability, all p<0.03. When stratified by sex, higher ATPmax was associated with lower PFS Physical for men only; higher max OXPHOS and max FAO OXPHOS were associated with lower RPE fatigability for both sexes. Higher skeletal muscle energetics were associated with 40-55% lower odds of being in the most (PFS≥25, RPE Fatigability≥12) vs least (PFS 0-4, RPE Fatigability 6-7) severe fatigability strata, all p<0.03. Being a woman was associated with 2-3 times higher odds of being in the most severe fatigability strata when controlling for ATPmax but not the in vivo measures (p<0.05). Better mitochondrial energetics were linked to lower fatigability and less severe fatigability in older adults. Findings imply that improving skeletal muscle energetics may mitigate perceived physical fatigability and prolong healthy aging.

Abstract Low muscle mass and fat infiltration are core features of sarcopenic obesity. We examined skeletal muscle mitochondrial respiration and walking speed in older adults with normal muscle composition (NMC), only low muscle volume (LMV), only high fat infiltration (HFI) and the combination of LMV and HFI, (adverse muscle composition (AMC)). Muscle composition was determined by whole body MRI and high resolution respirometry was performed on muscle biopsies from N=828 participants (76.3 ± 5.0 years old, 59.4% female) in the Study of Muscle, Mobility and Aging (SOMMA). We found that mitochondrial respiration was significantly lower in individuals with LMV (-10%), HFI (-9%) and AMC (-8%), compared to those with NMC. In multivariate regression models, 4m and 400m walking speeds were significantly slower in LMV (400m: β=-0.10 4m: β=-0.07, both p< 0.05), HFI (400m: β=-0.14, 4m: β=-0.13, both p< 0.01) and AMC (400m: β=-0.19, 4m: β=-0.16, both, p< 0.01) groups, compared to NMC (400m: 1.11m/s, 4m:1.10m/s). Compared to NMC, mitochondrial respiration explained ~20% of the variance in 400m walking speed in LMV, ~64% in HFI, and ~47% in AMC, while all three groups remained significantly slower. Compared to those with NMC, mitochondrial respiration explained ~29%, and ~77% of the variance in 4m walking speed in LMV and HFI respectively, abolishing group differences in 4m walking speed, while mitochondrial respiration explained ~50% of the variance in ACM. In conclusion, low muscle volume, high fat infiltration and AMC phenotypes are linked to lower mitochondrial respiration and slower walking speed in older adults.

The age-related decline in muscle mitochondrial energetics contributes to the loss of mobility in older adults. Women experience a higher prevalence of mobility impairment compared to men, but it is unknown whether sex-specific differences in muscle energetics underlie this disparity. In the Study of Muscle, Mobility and Aging (SOMMA), muscle energetics were characterized using in vivo phosphorus-31 magnetic resonance spectroscopy and high-resolution respirometry of vastus lateralis biopsies in 773 participants (56.4% women, age 70-94 years). A Short Physical Performance Battery score ≤ 8 was used to define lower-extremity mobility impairment. Muscle mitochondrial energetics were lower in women compared to men (e.g. Maximal Complex I&II OXPHOS: Women=55.06 +/- 15.95; Men=65.80 +/- 19.74; p<0.001) and in individuals with mobility impairment compared to those without (e.g., Maximal Complex I&II OXPHOS in women: SPPB≥9=56.59 +/- 16.22; SPPB≤8=47.37 +/- 11.85; p<0.001). Muscle energetics were negatively associated with age only in men (e.g., Maximal ETS capacity: R=-0.15, p=0.02; age/sex interaction, p=0.04), resulting in muscle energetics measures that were significantly lower in women than men in the 70-79 age group but not the 80+ age group. Similarly, the odds of mobility impairment were greater in women than men only in the 70-79 age group (70-79 age group, OR age-adjusted =1.78, 95% CI=1.03, 3.08, p=0.038; 80+ age group, OR age-adjusted =1.05, 95% CI=0.52, 2.15, p=0.89). Accounting for muscle energetics attenuated up to 75% of the greater odds of mobility impairment in women. Women had lower muscle mitochondrial energetics compared to men, which largely explain their greater odds of lower-extremity mobility impairment.

OBJECTIVE:Our objective was to investigate the overall and sex-specific relationships between the presence and severity of knee osteoarthritis (KOA) and muscle composition, power, and energetics in older adults. METHODS:Male and female patients (n = 655, mean ± SD age 76.1 ± 4.9 years; 57% female) enrolled in the Study of Muscle, Mobility, and Aging completed standing knee radiographs and knee pain assessments. Participants were divided into three groups using Kellgren-Lawrence grade (KLG) of KOA severity (0-1, 2, or 3-4). Outcome measures included whole-body muscle mass, thigh fat-free muscle (FFM) volume and muscle fat infiltration (MFI), leg power, specific power (power normalized to muscle volume), and muscle mitochondrial energetics. RESULTS:Overall, the presence and severity of KOA is associated with greater MFI, lower leg power and specific power, and reduced oxidative phosphorylation (P trend < 0.036). Sex-specific analysis revealed reduced energetics only in female patients with KOA (P trend < 0.007) compared to female patients without KOA. In models adjusted for age, sex, race, nonsteroidal anti-inflammatory drug administration, site or technician, physical activity, height, and participants with abdominal adiposity with KLG 3 to 4 had greater MFI (mean 0.008%, 95% confidence interval [CI] 0.004%-0.011%) and lower leg power (mean -51.56 W, 95% CI -74.03 to -29.10 W) and specific power (mean -5.38 W/L, 95% CI -7.31 to -3.45 W/L) than those with KLG 0 to 1. No interactions were found between pain and KLG status. Among those with KOA, MFI and oxidative phosphorylation were associated with thigh FFM volume, leg power, and specific power. CONCLUSION:Muscle health is associated with the presence and severity of KOA and differs by sex. Although muscle composition and power are lower in both male and female patients with KOA, regardless of pain status, mitochondrial energetics is reduced only in female patients.

Cardiorespiratory fitness and mitochondrial oxidative capacity are associated with reduced walking speed in older adults. The impact of cardiorespiratory fitness and mitochondrial oxidative capacity on walking speed in older adults with diabetes has not been clearly defined. We examined differences in cardiorespiratory fitness and skeletal muscle mitochondrial oxidative capacity between older adults with and without diabetes as well as determine their relative contribution to slower walking speed in older adults with diabetes. Participants with diabetes (n=159) had lower cardiorespiratory fitness and mitochondrial respiration in permeabilized fiber bundles when compared to those without diabetes (n=717), following adjustments for covariates including BMI, chronic comorbid health conditions, and physical activity. 4-m and 400-m walking speeds were slower in those with diabetes. Mitochondrial oxidative capacity alone or combined with cardiorespiratory fitness mediated ~20-70% of the difference in walk speed between older adults with and without diabetes. Additional adjustments with BMI and co-morbidities further explained the group differences in walk speed. Cardiorespiratory fitness and skeletal muscle mitochondrial oxidative capacity contribute to slower walking speeds in older adults with diabetes.

ABSTRACTRationaleCardiorespiratory fitness and mitochondrial energetics are associated with reduced walking speed in older adults. The impact of cardiorespiratory fitness and mitochondrial energetics on walking speed in older adults with diabetes has not been clearly defined.ObjectiveTo examine differences in cardiorespiratory fitness and skeletal muscle mitochondrial energetics between older adults with and without diabetes. We also assessed the contribution of cardiorespiratory fitness and skeletal muscle mitochondrial energetics to slower walking speed in older adults with diabetes.FindingsParticipants with diabetes had lower cardiorespiratory fitness and mitochondrial energetics when compared to those without diabetes, following adjustments for covariates including BMI, chronic comorbid health conditions, and physical activity. 4-m and 400-m walking speeds were slower in those with diabetes. Mitochondrial oxidative capacity alone or combined with cardiorespiratory fitness mediated ∼20-70% of the difference in walk speed between older adults with and without diabetes. Further adjustments of BMI and co-morbidities further explained the group differences in walk speed.ConclusionsSkeletal muscle mitochondrial energetics and cardiorespiratory fitness contribute to slower walking speeds in older adults with diabetes. Cardiorespiratory fitness and mitochondrial energetics may be therapeutic targets to maintain or improve mobility in older adults with diabetes.ARTICLE HIGHLIGHTSWhy did we undertake this study?To determine if mitochondrial energetics and cardiorespiratory fitness contribute to slower walking speed in older adults with diabetes.What is the specific question(s) we wanted to answer?Are mitochondrial energetics and cardiorespiratory fitness in older adults with diabetes lower than those without diabetes? How does mitochondrial energetics and cardiorespiratory fitness impact walking speed in older adults with diabetes?What did we find?Mitochondrial energetics and cardiorespiratory fitness were lower in older adults with diabetes compared to those without diabetes, and energetics, and cardiorespiratory fitness, contributed to slower walking speed in those with diabetes.What are the implications of our findings?Cardiorespiratory fitness and mitochondrial energetics may be key therapeutic targets to maintain or improve mobility in older adults with diabetes.Graphical Abstract

Abstract An age-related decline in muscle mitochondrial energetics is known to contribute to the loss of muscle function in older adults. Women experience a higher prevalence of mobility impairment compared to men, but it is unknown whether gender-specific differences in muscle energetics underlie this disparity. In the Study of Muscle, Mobility and Aging (SOMMA), muscle energetics were characterized using in vivo 31-Phosphorus Magnetic Resonance Spectroscopy and High-Resolution Respirometry of the vastus lateralis. A Short Physical Performance Battery score ≤ 8 was used to define lower-extremity mobility impairment. In this analysis of 773 participants age 70-94, women had greater odds (OR=1.83, p=0.03) of mobility impairment compared to men, which was largely explained by significantly lower muscle energetics (Maximal OXPHOS in Women= 55.06 +/- 15.95; Men= 65.80 +/- 19.74; p<0.0001) using mediation modeling (adjusted OR=1.25, p=0.45). The mobility impairment disparity could be further explained by BMI, race, physical activity, and number of comorbidities (adjusted OR=0.98, p=0.96). However, none of these covariates, including age, fully explained gender differences in muscle mitochondrial energetics (e.g. Adjusted Maximal OXPHOS in Women= 56.5, 95%CI=54.8, 58.1; Men= 64.4, 95%CI=62.5, 66.2; p<0.0001). Notably, high BMI and low gait speed were associated with lower muscle energetics, though BMI slightly more so in women (ATPmax BMI/gender interaction, p= 0.02) than men, and age was only significantly negatively associated with muscle energetics in men (e.g. Maximal ETS capacity age/gender interaction, p=0.04). In conclusion, women had lower muscle mitochondrial energetics compared to men, which largely explained their greater odds of lower-extremity mobility impairment.

Abstract We investigated the association of diabetes with cardiorespiratory fitness and skeletal muscle mitochondrial energetics in older adults (N=876, mean age ± SD: 76.3 ± 5.0 yrs.; 59% females) from the Study of Muscle, Mobility and Aging (SOMMA). Participants were grouped by self-reported diabetes status (N=131, with diabetes). Cardiorespiratory fitness (modified Balke protocol), mitochondrial energetics (31P magnetic resonance spectroscopy [31P-MRS] and high resolution respirometry from skeletal muscle biopsy), physical activity (wearable accelerometer) and body composition (magnetic resonance imaging) were measured. Generalized linear regression models were used to calculate means adjusted by age, race, gender, BMI, and co-morbidities. Thigh fat-free muscle volume (p=0.12) and abdominal subcutaneous adipose tissue depot (p=0.15) were similar between those with and without diabetes. However, visceral (18.84%, p>0.01) and intramuscular adipose tissue (6.57%, p=0.02) were higher in older adults with diabetes. VO2peak (-4.79%, p>0.01), and mitochondria energetics measured by ATPmax (-5.10%, p=0.04), maximum complex I+II carbohydrate (maxOXPHOSCHO) (-6.67%, p=0.02) and fatty acid (maxOXPHOSFAO) (-8.70%, p=0.01) stimulated respiration were lower in those with diabetes compared to those without. Following further adjustments for both physical activity and visceral adiposity, revealed that only VO2peak (-4.65%, p>0.01) and maxOXPHOSFAO (-7.38%, p=0.02) remained significantly lower in older adults with diabetes. Overall, our data suggests that cardiorespiratory fitness and skeletal muscle mitochondria energetics are reduced in older adults with diabetes, independent of visceral adiposity and physical activity.

Separate lines of investigation indicate reduced mitochondrial energetics in skeletal muscle and cardiorespiratory fitness in diabetes and with advanced age. This evidence comes from smaller studies that often don’t account for confounding factors including physical activity and adiposity levels. PURPOSE: To determine if older adults with diabetes have reduced muscle energetics and fitness while controlling for objectively assessed physical activity levels and adiposity. METHODS: Data from 876 older participants (mean ± stdev: 76.3 ± 5.0 yrs.; 59% females) in the Study of Muscle, Mobility and Aging (SOMMA) was used for this analysis. Participants were grouped by self-reported diabetes status (with diabetes, n = 131 and without diabetes, n = 745). A modified Balke protocol was used to assess cardiorespiratory fitness (VO2peak). Mitochondrial energetics was assessed by 31P magnetic resonance spectroscopy (ATPmax) and high resolution respirometry of fiber bundles from vastus lateralis biopsies. Physical activity was measured with a wearable accelerometer, and body composition by magnetic resonance imaging. RESULTS: Older adults with diabetes had lower VO2peak (-6.6%, p < 0.001), ATPmax (-7.7%, p < 0.05), and maximum complex I + II respiration supported by pyruvate/malate (maxOXPHOSCHO) (-7.0%, p < 0.05) or palmitoyl carnitine/malate (maxOXPHOSFAO) (-8.9%, p < 0.05) compared to those without diabetes, when adjusted for age, race, clinical site, gender, BMI, and co-morbidities. However, older adults with diabetes also had lower moderate-to-vigorous physical activity (p < 0.01), greater sedentary behavior (p < 0.01), and greater visceral (p < 0.01) and intramuscular adipose tissue (p = 0.02). Following adjustments for both physical activity and visceral adiposity, we found that VO2peak (-6.0%, p < 0.05), ATPmax (-7.7%, p < 0.05) and maxOXPHOSFAO (-9.3%, p < 0.05) remained significantly lower in older adults with diabetes. Interestingly, adjusting for visceral adiposity and physical activity reduced the group difference in maxOXPHOSCHO (-6.0, p = 0.06). CONCLUSIONS: Older adults with diabetes have lower cardiorespiratory fitness and skeletal muscle mitochondrial energetics that is largely independent of body fat composition and physical activity. SOMMA is funded by the NIA R01AG059416.