Systemic Metabolic Bottlenecks as Constraints on Exercise Responsiveness and Functional Healthspan During Aging

Systemic Metabolic Bottlenecks as Constraints on Exercise Responsiveness and Functional Healthspan During Aging | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Systemic Metabolic Bottlenecks as Constraints on Exercise Responsiveness and Functional Healthspan During Aging Ruixiang Ren This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8629270/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Regular physical exercise is a cornerstone of healthy aging, yet its efficacy progressively diminishes with age, posing a central challenge to preventive medicine. This study leverages a data-driven conceptual integration of a whole-body metabolic atlas of aging to define the molecular constraints driving this decline. Our analysis reveals three core findings: (1) metabolic aging follows significant organ-specific trajectories with persistent sex dimorphism, most notably in cardiac redox metabolism, where females exhibit enhanced glutathione activity; (2) trans-4-hydroxyproline was identified as a pan-organ biomarker whose systemic decline indicates impaired collagen turnover and a systemic defect in extracellular matrix (ECM) remodeling; and (3) α-ketoglutarate (AKG) was independently identified as a core metabolic regulator linking cellular energy status to epigenetic maintenance. Our central hypothesis posits that the age-related attenuation of exercise benefits reflects the cumulative limitation of cellular adaptive capacity by these metabolic bottlenecks. Targeting these conserved molecular constraints represents a rational strategy to restore tissue responsiveness to exercise and promote a functional healthspan. Health sciences/Biomarkers Health sciences/Cardiology Health sciences/Medical research Biological sciences/Physiology Figures Figure 1 Figure 2 Figure 3 Figure 4 1.0 Introduction In an era of global aging and rising chronic disease, regular physical activity stands as one of the most effective non-pharmacological interventions for maintaining health. However, a critical paradox exists: the magnitude and durability of exercise-induced benefits decline in older populations. This phenomenon represents a major barrier to long-term efficacy and a formidable challenge for public health, rendering exercise recommendations increasingly ineffective for the world's fastest-growing demographic. This observation strongly suggests that intrinsic biological constraints, rather than factors like adherence or effort, progressively limit the body's ability to adapt to physiological stress. At the core of this challenge is the concept of Systemic Metabolic Aging —a progressive, asynchronous dysregulation of biochemical pathways governing energy production, redox balance, and tissue maintenance. This process unfolds heterogeneously across the body, creating a landscape of tissue-specific vulnerabilities and resiliencies that ultimately constrains the organism's capacity to remodel and strengthen in response to exercise. Despite extensive research, the specific molecular determinants that limit exercise responsiveness during aging remain incompletely understood. The primary objectives of this research are threefold: To analyze the organ- and sex-specific features of the systemic metabolic aging atlas to understand its inherent heterogeneity. To identify conserved molecular bottlenecks from the data that can be mechanistically targeted for intervention. To propose a conceptual framework and a multi-target metabolic intervention designed to synergize with physical activity, enhance cellular resilience, and extend functional healthspan. To dissect these constraints, we undertook a deep analytical integration of a landmark whole-body metabolic atlas, seeking to uncover the conserved molecular drivers of functional decline. 2.0 Analytical Approach and Data Framework The findings and hypotheses presented herein are derived from a deep, data-driven analysis and conceptual integration of a pre-existing, comprehensive dataset. This work is therefore positioned not as a primary data-generating study, but as one of hypothesis generation based on the synthesis of foundational molecular evidence. The analysis is grounded in the whole-body metabolic atlas of aging published by Tomas-Loba et al. in Cell Metabolism ( 6 ). This landmark dataset provides an unprecedented view of metabolic aging, encompassing an analysis of 169 core metabolites across 12 key organs in both male and female aging mice. This analytical framework allows for the identification of conserved patterns and core drivers of aging, providing the foundation for the key findings detailed below. 3.0 Results: Characterizing the Whole-Body Metabolic Atlas of Aging Analysis of the metabolic atlas revealed a complex, multi-layered picture of aging that challenges the notion of a uniform, systemic decline. Instead, the data point to specific, conserved molecular drivers that operate within a broader context of organ-specific aging trajectories. This complex "mosaic" of aging provides critical insights into the underlying mechanisms of functional decline. 3.1 Metabolic Aging is an Asynchronous and Heterogeneous Process Our analysis reveals that aging is not a monolithic systemic decay but a complex "mosaic" of organ-specific trajectories. This metabolic asynchrony highlights the profound heterogeneity of the aging process across the body's tissues . Rapidly-Changing Organs : The thymus emerges as the "sentinel" of aging, exhibiting the most dramatic metabolic changes that directly mirror its structural involution and the sharp decline in new T-cell production. The quadriceps muscle also shows significant age-related alterations, particularly in energy pathways, consistent with the molecular hallmarks of sarcopenia and functional decline. Slowly-Changing Organs : In stark contrast, the brain acts as a "silent zone," displaying minimal metabolic drift. This remarkable stability highlights robust protective mechanisms that preserve neurometabolic homeostasis. The tongue , despite also being composed of striated muscle, shows far fewer metabolic changes than the quadriceps, an observation consistent with its well-preserved function throughout life. The key implication of this finding is that "one-size-fits-all" anti-aging interventions are likely to be insufficient. Effective strategies must instead target molecular pathways that are conserved across tissues, even as the rate of aging varies dramatically between them. 3.2 Sex is a Persistent Determinant of Cardiac Metabolism Beyond organ-specific aging rates, our analysis confirms that sex is a non-negotiable, lifelong determinant of metabolic fate. Of all organs examined, the heart exhibits the most pronounced and persistent differences between males and females. The critical molecular distinction lies in its protective pathways: female hearts consistently display enhanced antioxidant activity , specifically within glutathione metabolism . This inherent redox advantage provides a plausible molecular basis for the well-documented lower incidence and delayed onset of cardiovascular disease observed in females. 3.3 Impaired Extracellular Matrix (ECM) Turnover is a Conserved Hallmark of Aging Despite wide organ-specific variations, our analysis identified a conserved "common code" of aging that transcends tissue type. The metabolite trans-4-hydroxyproline emerged as the most consistent pan-organ biomarker of aging, showing a significant age-related decline in 11 out of the 12 organs examined . The biological significance of this finding is profound. The simultaneous observation of increased total collagen content in aging tissues with a systemic decrease in its breakdown product (hydroxyproline) provides unequivocal evidence of a fundamental defect in ECM remodeling. This failure to clear old collagen leads to its abnormal accumulation, a process that precedes and promotes tissue fibrosis, increased stiffness, and a systemic loss of both mechanical and metabolic flexibility. 3.4 Alpha-Ketoglutarate (AKG) is Independently Identified as a Key Metabolic Regulator Leveraging the descriptive atlas for mechanism discovery enabled the construction of organ-specific 'metabolic clocks' to predict biological age. When this predictive model was applied to plasma data, it successfully and independently "re-discovered" α-ketoglutarate (AKG) as a primary age-related regulator. The importance of AKG stems from its dual role as both a marker and a mediator of metabolic capacity: Energy Metabolism : As a critical intermediate in the tricarboxylic acid (TCA) cycle, AKG is essential for cellular energy production. Epigenetic Regulation : AKG serves as an essential cofactor for TET dioxygenase enzymes , which actively remove age-related DNA methylation marks. This function helps to resist epigenetic drift and restore more youthful gene expression patterns. These discrete findings—from organ asynchrony to conserved molecular markers—provide the building blocks for the unified theory of age-related functional decline presented in the following discussion. 4.0 Discussion: A Unifying Framework for Restoring Exercise Responsiveness This section synthesizes the preceding results into a cohesive framework that redefines age-related exercise decline as a problem of constrained metabolic capacity. This new perspective provides a clear, evidence-based rationale for a novel, targeted intervention strategy designed to restore physiological adaptability. 4.1 Central Hypothesis: Metabolic Capacity Constrains Exercise Responsiveness We formally articulate the central hypothesis of this work: the age-related decline in exercise responsiveness is not primarily limited by behavior or mechanical load, but is fundamentally constrained by the progressive limitation of systemic metabolic capacity. The evidence from the metabolic atlas provides a firm molecular foundation for this hypothesis. The ability of tissues to adapt and remodel in response to the physiological stress of exercise is fundamentally limited by the accumulation of metabolic bottlenecks, including: Impaired Energy Buffering : Manifested in the metabolic shifts observed within the quadriceps muscle. Reduced Epigenetic Plasticity : Directly linked to the decline of key cofactors like AKG, which are necessary to reset gene expression patterns required for tissue repair and growth. Defective ECM Remodeling : The failure to clear old collagen results in tissue stiffening that physically impedes effective remodeling in response to mechanical stress. These limitations are not independent; they create a vicious cycle that functionally uncouples effort from adaptation. The epigenetic inability to activate repair pathways (AKG decline) is compounded by a physical matrix (ECM fibrosis) that resists remodeling, all within a system of diminishing energy reserves (quadriceps). 4.2 Metabolic Nodes Potentially Amenable to Targeted Modulation The framework proposed here naturally points toward a limited set of conserved metabolic nodes that may be amenable to nutritional or pharmacological modulation as an adjunct to physical exercise. Importantly, this study does not evaluate interventions directly, nor does it advocate any specific formulation for clinical use. Instead, it outlines mechanistically grounded targets whose modulation could, in principle, alleviate the metabolic constraints identified. First, α-ketoglutarate (AKG) emerges as a central node linking mitochondrial energy metabolism to epigenetic maintenance. As a core tricarboxylic acid (TCA) cycle intermediate and an obligate cofactor for TET dioxygenases, AKG occupies a unique position at the intersection of bioenergetics and transcriptional plasticity. Age-associated reductions in AKG availability may therefore restrict both ATP production and the epigenetic reprogramming required for effective tissue adaptation following exercise ( 5 ). Second, the systemic decline of trans-4-hydroxyproline highlights impaired extracellular matrix (ECM) turnover as a conserved hallmark of aging. Strategies aimed at reducing non-enzymatic collagen cross-linking or enhancing physiological collagen renewal could, in theory, restore mechanical compliance and metabolic flexibility across multiple tissues, thereby improving their responsiveness to mechanical loading. Third, metabolites involved in intracellular buffering and redox homeostasis —particularly those contributing to carnosine synthesis and glutathione metabolism—represent additional candidates for intervention. The preservation of redox balance and pH buffering capacity is essential for sustaining repeated bouts of exercise-induced stress and for preventing the accumulation of molecular damage that further limits adaptability ( 4 ). Together, these nodes define a rational, systems-level target space for future intervention studies. Critically, any attempt to translate this framework into clinical or nutritional strategies will require rigorous experimental validation, ideally through randomized controlled trials integrating exercise interventions with longitudinal metabolomic and epigenetic readouts. This evidence-based approach is essential for moving from hypothesis to validated therapeutic strategy. 5.0 Conclusion and Future Directions By reframing age-related exercise decline as a direct function of systemic metabolic capacity, this work shifts the interventional focus from behavioral science to molecular biology . The comprehensive metabolic atlas of aging revealed conserved, druggable bottlenecks—including impaired epigenetic regulation and deficient matrix remodeling—that represent rational targets for intervention. We advocate that combining scientifically designed metabolic support with physical activity offers a potent, synergistic strategy. The ultimate goal is not merely to extend lifespan, but to enhance the physiological resilience and functional vitality that define a robust healthspan . Declarations Author Contributions: R.R. conceived the study, performed data interpretation, and wrote the manuscript. Corresponding Author: Correspondence and requests for materials should be addressed to R.R. Funding: Not applicable. Competing Interests: The author declares no competing interests. Acknowledgements: The author thanks Parx Materials NV，Rotterdam Netherlands and China Institute of Applied Sciences，NIngbo, China for research support. References Blancquaert, L., et al. Effects of Histidine and β-alanine Supplementation on Carnosine Concentration in Unstimulated Human Muscle Biopsies. PLoS One 12(1), e0170429 (2017). Cesàk, O., et al. Carnosine and Beta-Alanine Supplementation in Human Nutrition, Health, and Disease—An Overview of the Current Evidence. Nutrients 15(14), 3122 (2023). Demidenko, O., et al. Rejuvant®, a potential life-extending compound formulation with alpha-ketoglutarate and vitamins, conferred an average 8.0 year reduction in biological aging, after an average of 7 months of use in the TruAge DNA methylation test. Aging 13, 24485–24499 (2021). Rezende, N.S., et al. The Muscle Carnosine Response to Beta-Alanine Supplementation Is Dependent on Nutrition and Training. Front. Physiol. 11, 607 (2020). Sandalova, E., et al. Alpha-ketoglutarate supplementation as a potential therapeutic intervention against biological aging in humans: a narrative review of the current evidence. Aging Med. 6(3), 213–222 (2023). Tomas-Loba, A., et al. A metabolic atlas of mouse aging. Cell Metab. 36, 1–18 (2024). Additional Declarations No competing interests reported. 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However, a critical paradox exists: the magnitude and durability of exercise-induced benefits decline in older populations. This phenomenon represents a major barrier to long-term efficacy and a formidable challenge for public health, rendering exercise recommendations increasingly ineffective for the world's fastest-growing demographic. This observation strongly suggests that intrinsic biological constraints, rather than factors like adherence or effort, progressively limit the body's ability to adapt to physiological stress.\u003c/p\u003e \u003cp\u003eAt the core of this challenge is the concept of \u003cb\u003eSystemic Metabolic Aging\u003c/b\u003e\u0026mdash;a progressive, asynchronous dysregulation of biochemical pathways governing energy production, redox balance, and tissue maintenance. This process unfolds heterogeneously across the body, creating a landscape of tissue-specific vulnerabilities and resiliencies that ultimately constrains the organism's capacity to remodel and strengthen in response to exercise. Despite extensive research, the specific molecular determinants that limit exercise responsiveness during aging remain incompletely understood.\u003c/p\u003e \u003cp\u003eThe primary objectives of this research are threefold:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eTo analyze the organ- and sex-specific features of the systemic metabolic aging atlas to understand its inherent heterogeneity.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eTo identify conserved molecular bottlenecks from the data that can be mechanistically targeted for intervention.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eTo propose a conceptual framework and a multi-target metabolic intervention designed to synergize with physical activity, enhance cellular resilience, and extend functional healthspan.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eTo dissect these constraints, we undertook a deep analytical integration of a landmark whole-body metabolic atlas, seeking to uncover the conserved molecular drivers of functional decline.\u003c/p\u003e"},{"header":"2.0 Analytical Approach and Data Framework","content":"\u003cp\u003eThe findings and hypotheses presented herein are derived from a deep, data-driven analysis and conceptual integration of a pre-existing, comprehensive dataset. This work is therefore positioned not as a primary data-generating study, but as one of hypothesis generation based on the synthesis of foundational molecular evidence.\u003c/p\u003e \u003cp\u003eThe analysis is grounded in the whole-body metabolic atlas of aging published by Tomas-Loba et al. in \u003cem\u003eCell Metabolism\u003c/em\u003e (\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e). This landmark dataset provides an unprecedented view of metabolic aging, encompassing an analysis of 169 core metabolites across 12 key organs in both male and female aging mice.\u003c/p\u003e \u003cp\u003eThis analytical framework allows for the identification of conserved patterns and core drivers of aging, providing the foundation for the key findings detailed below.\u003c/p\u003e"},{"header":"3.0 Results: Characterizing the Whole-Body Metabolic Atlas of Aging","content":"\u003cp\u003eAnalysis of the metabolic atlas revealed a complex, multi-layered picture of aging that challenges the notion of a uniform, systemic decline. Instead, the data point to specific, conserved molecular drivers that operate within a broader context of organ-specific aging trajectories. This complex \"mosaic\" of aging provides critical insights into the underlying mechanisms of functional decline.\u003c/p\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Metabolic Aging is an Asynchronous and Heterogeneous Process\u003c/h2\u003e \u003cp\u003eOur analysis reveals that aging is not a monolithic systemic decay but a complex \"mosaic\" of organ-specific trajectories. This metabolic asynchrony highlights the profound heterogeneity of the aging process across the body's tissues .\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eRapidly-Changing Organs\u003c/b\u003e: The \u003cb\u003ethymus\u003c/b\u003e emerges as the \"sentinel\" of aging, exhibiting the most dramatic metabolic changes that directly mirror its structural involution and the sharp decline in new T-cell production. The \u003cb\u003equadriceps\u003c/b\u003e muscle also shows significant age-related alterations, particularly in energy pathways, consistent with the molecular hallmarks of sarcopenia and functional decline.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eSlowly-Changing Organs\u003c/b\u003e: In stark contrast, the \u003cb\u003ebrain\u003c/b\u003e acts as a \"silent zone,\" displaying minimal metabolic drift. This remarkable stability highlights robust protective mechanisms that preserve neurometabolic homeostasis. The \u003cb\u003etongue\u003c/b\u003e, despite also being composed of striated muscle, shows far fewer metabolic changes than the quadriceps, an observation consistent with its well-preserved function throughout life.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eThe key implication of this finding is that \"one-size-fits-all\" anti-aging interventions are likely to be insufficient. Effective strategies must instead target molecular pathways that are conserved across tissues, even as the rate of aging varies dramatically between them.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Sex is a Persistent Determinant of Cardiac Metabolism\u003c/h2\u003e \u003cp\u003eBeyond organ-specific aging rates, our analysis confirms that sex is a non-negotiable, lifelong determinant of metabolic fate. Of all organs examined, the \u003cb\u003eheart\u003c/b\u003e exhibits the most pronounced and persistent differences between males and females. The critical molecular distinction lies in its protective pathways: \u003cb\u003efemale hearts consistently display enhanced antioxidant activity\u003c/b\u003e, specifically within \u003cb\u003eglutathione metabolism\u003c/b\u003e. This inherent redox advantage provides a plausible molecular basis for the well-documented lower incidence and delayed onset of cardiovascular disease observed in females.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Impaired Extracellular Matrix (ECM) Turnover is a Conserved Hallmark of Aging\u003c/h2\u003e \u003cp\u003eDespite wide organ-specific variations, our analysis identified a conserved \"common code\" of aging that transcends tissue type. The metabolite \u003cb\u003etrans-4-hydroxyproline\u003c/b\u003e emerged as the most consistent \u003cb\u003epan-organ biomarker\u003c/b\u003e of aging, showing a significant age-related decline in \u003cb\u003e11 out of the 12\u003c/b\u003e organs examined .\u003c/p\u003e \u003cp\u003eThe biological significance of this finding is profound. The simultaneous observation of \u003cem\u003eincreased\u003c/em\u003e total collagen content in aging tissues with a \u003cem\u003esystemic decrease\u003c/em\u003e in its breakdown product (hydroxyproline) provides unequivocal evidence of a fundamental defect in ECM remodeling. This failure to clear old collagen leads to its abnormal accumulation, a process that precedes and promotes tissue fibrosis, increased stiffness, and a systemic loss of both mechanical and metabolic flexibility.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Alpha-Ketoglutarate (AKG) is Independently Identified as a Key Metabolic Regulator\u003c/h2\u003e \u003cp\u003eLeveraging the descriptive atlas for mechanism discovery enabled the construction of organ-specific 'metabolic clocks' to predict biological age. When this predictive model was applied to plasma data, it successfully and independently \"re-discovered\" \u003cb\u003eα-ketoglutarate (AKG)\u003c/b\u003e as a primary age-related regulator. The importance of AKG stems from its dual role as both a marker and a mediator of metabolic capacity:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eEnergy Metabolism\u003c/b\u003e: As a critical intermediate in the tricarboxylic acid (TCA) cycle, AKG is essential for cellular energy production.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eEpigenetic Regulation\u003c/b\u003e: AKG serves as an essential cofactor for \u003cb\u003eTET dioxygenase enzymes\u003c/b\u003e, which actively remove age-related DNA methylation marks. This function helps to resist epigenetic drift and restore more youthful gene expression patterns.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eThese discrete findings\u0026mdash;from organ asynchrony to conserved molecular markers\u0026mdash;provide the building blocks for the unified theory of age-related functional decline presented in the following discussion.\u003c/p\u003e\u003c/div\u003e"},{"header":"4.0 Discussion: A Unifying Framework for Restoring Exercise Responsiveness","content":"\u003cp\u003eThis section synthesizes the preceding results into a cohesive framework that redefines age-related exercise decline as a problem of constrained metabolic capacity. This new perspective provides a clear, evidence-based rationale for a novel, targeted intervention strategy designed to restore physiological adaptability.\u003c/p\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Central Hypothesis: Metabolic Capacity Constrains Exercise Responsiveness\u003c/h2\u003e \u003cp\u003eWe formally articulate the central hypothesis of this work: the age-related decline in exercise responsiveness is not primarily limited by behavior or mechanical load, but is fundamentally constrained by the progressive limitation of systemic metabolic capacity.\u003c/p\u003e \u003cp\u003eThe evidence from the metabolic atlas provides a firm molecular foundation for this hypothesis. The ability of tissues to adapt and remodel in response to the physiological stress of exercise is fundamentally limited by the accumulation of metabolic bottlenecks, including:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eImpaired Energy Buffering\u003c/b\u003e: Manifested in the metabolic shifts observed within the quadriceps muscle.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eReduced Epigenetic Plasticity\u003c/b\u003e: Directly linked to the decline of key cofactors like AKG, which are necessary to reset gene expression patterns required for tissue repair and growth.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eDefective ECM Remodeling\u003c/b\u003e: The failure to clear old collagen results in tissue stiffening that physically impedes effective remodeling in response to mechanical stress.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eThese limitations are not independent; they create a vicious cycle that functionally uncouples effort from adaptation. The epigenetic inability to activate repair pathways (AKG decline) is compounded by a physical matrix (ECM fibrosis) that resists remodeling, all within a system of diminishing energy reserves (quadriceps).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e4.2 Metabolic Nodes Potentially Amenable to Targeted Modulation\u003c/h2\u003e \u003cp\u003eThe framework proposed here naturally points toward a limited set of conserved metabolic nodes that may be amenable to nutritional or pharmacological modulation as an adjunct to physical exercise. Importantly, this study does not evaluate interventions directly, nor does it advocate any specific formulation for clinical use. Instead, it outlines mechanistically grounded targets whose modulation could, in principle, alleviate the metabolic constraints identified.\u003c/p\u003e \u003cp\u003eFirst, \u003cb\u003eα-ketoglutarate (AKG)\u003c/b\u003e emerges as a central node linking mitochondrial energy metabolism to epigenetic maintenance. As a core tricarboxylic acid (TCA) cycle intermediate and an obligate cofactor for TET dioxygenases, AKG occupies a unique position at the intersection of bioenergetics and transcriptional plasticity. Age-associated reductions in AKG availability may therefore restrict both ATP production and the epigenetic reprogramming required for effective tissue adaptation following exercise (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSecond, the systemic decline of \u003cb\u003etrans-4-hydroxyproline\u003c/b\u003e highlights impaired extracellular matrix (ECM) turnover as a conserved hallmark of aging. Strategies aimed at reducing non-enzymatic collagen cross-linking or enhancing physiological collagen renewal could, in theory, restore mechanical compliance and metabolic flexibility across multiple tissues, thereby improving their responsiveness to mechanical loading.\u003c/p\u003e \u003cp\u003eThird, metabolites involved in \u003cb\u003eintracellular buffering and redox homeostasis\u003c/b\u003e\u0026mdash;particularly those contributing to carnosine synthesis and glutathione metabolism\u0026mdash;represent additional candidates for intervention. The preservation of redox balance and pH buffering capacity is essential for sustaining repeated bouts of exercise-induced stress and for preventing the accumulation of molecular damage that further limits adaptability (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eTogether, these nodes define a rational, systems-level target space for future intervention studies. Critically, any attempt to translate this framework into clinical or nutritional strategies will require rigorous experimental validation, ideally through randomized controlled trials integrating exercise interventions with longitudinal metabolomic and epigenetic readouts. This evidence-based approach is essential for moving from hypothesis to validated therapeutic strategy.\u003c/p\u003e\u003c/div\u003e"},{"header":"5.0 Conclusion and Future Directions","content":"\u003cp\u003eBy reframing age-related exercise decline as a direct function of systemic metabolic capacity, this work shifts the interventional focus from behavioral science to \u003cb\u003emolecular biology\u003c/b\u003e. The comprehensive metabolic atlas of aging revealed conserved, druggable bottlenecks\u0026mdash;including impaired epigenetic regulation and deficient matrix remodeling\u0026mdash;that represent rational targets for intervention.\u003c/p\u003e \u003cp\u003eWe advocate that combining scientifically designed metabolic support with physical activity offers a potent, synergistic strategy. The ultimate goal is not merely to extend lifespan, but to enhance the physiological resilience and functional vitality that define a robust \u003cb\u003ehealthspan\u003c/b\u003e.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor Contributions:\u003c/strong\u003e R.R. conceived the study, performed data interpretation, and wrote the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding Author:\u003c/strong\u003e Correspondence and requests for materials should be addressed to R.R.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests:\u003c/strong\u003e The author declares no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements:\u003c/strong\u003e The author thanks Parx Materials NV，Rotterdam Netherlands and China Institute of Applied Sciences，NIngbo, China for research support.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBlancquaert, L., et al. Effects of Histidine and β-alanine Supplementation on Carnosine Concentration in Unstimulated Human Muscle Biopsies. \u003cem\u003ePLoS One\u003c/em\u003e 12(1), e0170429 (2017).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCes\u0026agrave;k, O., et al. Carnosine and Beta-Alanine Supplementation in Human Nutrition, Health, and Disease\u0026mdash;An Overview of the Current Evidence. \u003cem\u003eNutrients\u003c/em\u003e 15(14), 3122 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDemidenko, O., et al. Rejuvant\u0026reg;, a potential life-extending compound formulation with alpha-ketoglutarate and vitamins, conferred an average 8.0 year reduction in biological aging, after an average of 7 months of use in the TruAge DNA methylation test. \u003cem\u003eAging\u003c/em\u003e 13, 24485\u0026ndash;24499 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRezende, N.S., et al. The Muscle Carnosine Response to Beta-Alanine Supplementation Is Dependent on Nutrition and Training. \u003cem\u003eFront. Physiol.\u003c/em\u003e 11, 607 (2020).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSandalova, E., et al. Alpha-ketoglutarate supplementation as a potential therapeutic intervention against biological aging in humans: a narrative review of the current evidence. \u003cem\u003eAging Med.\u003c/em\u003e 6(3), 213\u0026ndash;222 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTomas-Loba, A., et al. A metabolic atlas of mouse aging. \u003cem\u003eCell Metab.\u003c/em\u003e 36, 1\u0026ndash;18 (2024).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":false,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8629270/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8629270/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eRegular physical exercise is a cornerstone of healthy aging, yet its efficacy progressively diminishes with age, posing a central challenge to preventive medicine. This study leverages a data-driven conceptual integration of a whole-body metabolic atlas of aging to define the molecular constraints driving this decline. Our analysis reveals three core findings: (1) metabolic aging follows significant organ-specific trajectories with persistent sex dimorphism, most notably in cardiac redox metabolism, where females exhibit enhanced glutathione activity; (2) \u003cem\u003etrans-4-hydroxyproline\u003c/em\u003e was identified as a pan-organ biomarker whose systemic decline indicates impaired collagen turnover and a systemic defect in extracellular matrix (ECM) remodeling; and (3) α-ketoglutarate (AKG) was independently identified as a core metabolic regulator linking cellular energy status to epigenetic maintenance. Our central hypothesis posits that the age-related attenuation of exercise benefits reflects the cumulative limitation of cellular adaptive capacity by these metabolic bottlenecks. Targeting these conserved molecular constraints represents a rational strategy to restore tissue responsiveness to exercise and promote a functional healthspan.\u003c/p\u003e","manuscriptTitle":"Systemic Metabolic Bottlenecks as Constraints on Exercise Responsiveness and Functional Healthspan During Aging","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-20 04:57:12","doi":"10.21203/rs.3.rs-8629270/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ff4521c1-133a-430a-95ba-c9f1f43569a5","owner":[],"postedDate":"January 20th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":61309692,"name":"Health sciences/Biomarkers"},{"id":61309693,"name":"Health sciences/Cardiology"},{"id":61309694,"name":"Health sciences/Medical research"},{"id":61309695,"name":"Biological sciences/Physiology"}],"tags":[],"updatedAt":"2026-02-18T02:54:26+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-20 04:57:12","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8629270","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8629270","identity":"rs-8629270","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}