Why might the effects of stress last longer with age?

HERETICAL, provisional and conditional: In a subset of aging humans, excessive muscle-to-liver lactate-to-glucose recycling could become a primary generator of post-stress metabolic demand. Muscle glycolysis supplies lactate; hepatic gluconeogenesis returns glucose while consuming more ATP equivalents than the preceding muscle glycolysis generated. This energetic accounting is a background assumption unverified by the supplied archive, and its quantitative contribution in the proposed subgroup is unknown. Increased hepatic energetic demand is hypothesized to sustain cardiovascular and endocrine mobilization, which renews muscle glycolytic production and supplies further lactate. The frozen mechanism requires energetic demand to cause the return signal. A collateral hepatic output that changes independently of demand could establish a different liver-to-muscle mechanism but would leave this mechanism unresolved. Muscle glycolytic production, lactate formation, uptake, oxidation, accumulation and net release are distinct quantities. The strong claim remains that reciprocal coupling maintains an unnecessarily costly state after ordinary activity ends despite recovery of the constituent organs under otherwise matched uncoupled conditions. Excess recycling, demand-dependent transmission, causal primacy and autonomous persistence are unverified assumptions. Stable delayed recovery or transient amplification would establish a narrower result. Reducing excessive recycling while preserving glucose availability and necessary compensation could improve sustained muscle recovery without increasing intrinsic muscle respiratory capacity. Functional benefit, whole-person energetic benefit and radical slowing of human aging remain separate unestablished outcomes.

Combine complementary glucose and lactate tracers, regional muscle balance and an identifiable non-steady-state carbon model. Account for circulating glucose, glycogen, alternative lactate fates, other glucose sources and compartment exchange. For a defined muscle compartment, lactate-pool change equals local formation plus uptake minus oxidation, other disposal and export; net vascular release equals export minus uptake. Arteriovenous balance alone cannot identify production. Report uncertainty bounds where formation, uptake or disposal cannot be separated. Bound hepatic energetic expenditure using regional or independently validated mechanistic measurements; whole-body calorimetry, hepatic oxygen consumption alone and an ATP concentration alone cannot identify recycling-specific ATP demand. Define Q_H as hepatic ATP-equivalent turnover attributable to the relevant processing over a specified interval, R as a candidate demand-transmitting cardiovascular or endocrine relay, and Z as specified competing substrate, redox or secretory changes with a possible demand-independent action. These are experimental variables requiring identification, not observed pathways. Identify R's origin, muscle target, exposure-response relation, gain and delay. Validate a liver-restricted recycling perturbation and exclude direct muscle exposure to it or its active products. Match glucose concentration and delivery, insulin, oxygen, electrolytes, arterial lactate and independently imposed upstream inputs; independently replay altered substrate trajectories. Then add a demand-versus-collateral-output discrimination. In one experimental series, independently vary Q_H within the relevant hepatic compartment while matching the specified Z trajectories and allowing R to respond. The frozen mechanism predicts a change in R followed by a corresponding change in independently resolved muscle glycolytic production. In a complementary series, vary candidate Z trajectories while Q_H and upstream inputs are matched. A muscle response fully attributable to Z without an independently demonstrated Q_H-to-R effect supports collateral hepatic signaling rather than establishing the frozen mechanism. If Z is itself proposed to be R, first test whether changing Q_H causes its production; subsequent Z add-back at fixed Q_H tests downstream sufficiency only. Never clamp the actual mediator in a test intended to establish intact transmission. A validated restoration of the relevant hepatic demand during continued recycling reduction, at matched competing outputs, should restore R and muscle production if demand is the operative cause. Generic ATP consumption in another compartment, hypoxia, injury or altered ATP concentration does not qualify as equivalent demand restoration. Independent perturbations or a quantitatively identified dissociation must separate the effects; correlated measurements and statistical adjustment alone are insufficient. No qualifying manipulation is supplied. If separation is infeasible, even successful relay blockade and physiological add-back justify only liver-to-muscle signaling, leaving demand dependence unresolved. In separate mediation experiments, relay blockade should remove the transmitted muscle response and physiological R replacement should restore it despite reduced recycling. Independent glucose replacement should fail to restore the loop only if a non-glucose demand-dependent relay closes it; include the replacement's physiological production, delivery, endocrine and other-organ costs in whole-person accounting. A fall in net lactate release with unchanged formation does not confirm the mechanism. Compare complete coupled and uncoupled systems with matched substrates, hormones, oxygen, waste removal, tissue support and local history. Before testing persistence, specify individualized recovery ranges, required duration within those ranges, an observation horizon based on independently measured recovery distributions, and measurements of residual tissue injury and retained states. Estimate directed responses independently and predict held-out challenges. The strong claim additionally predicts a post-input state incompatible with measured stable-decay and local-memory alternatives, and termination of that state after specific return-path interruption. Longer recovery or a positive root of a local fitted model alone is insufficient; finite observation cannot prove indefinite persistence. Under common forcing, hepatic processing may fall while muscle production follows residual injury or upstream activation. Null results constrain only the tested subgroup and window when target engagement, specificity, sensitivity and pathway representation are adequate.

Urinary net acid excretion is ammonium plus titratable acid minus bicarbonate, expressed as equivalent amounts over the same interval. Titratable acid is operationally the acid equivalents in urinary non-ammonium buffers determined by titration to a specified reference pH, with assay conventions reported. Whole-body acid retention requires an independent balance; buffering redistributes or binds retained acid equivalents rather than automatically eliminating them. Serial blood chemistry does not by itself establish the acid exposure at muscle. Identify the relevant acid-base species, pH and concentration-time trajectory reaching muscle using direct measurements where feasible or an independently validated transport-and-buffering model with uncertainty bounds. If the interval permits negligible or insufficient muscle exposure, the acute mechanism remains unsupported even with reduced ammonium excretion. Replay the participant-range muscle exposure at matched potassium, glucose, insulin, aldosterone and other relevant inputs, and measure regional uptake, release, dose-response and delay. Separately replay the resulting secondary potassium trajectory in renal preparations at matched acid exposure and other inputs; quantify total net acid disposal. Both independently estimated response functions and potassium and acid balances must predict the secondary coupled response on held-out ordinary challenges. A candidate muscle-specific intervention qualifies only if it selectively removes the acid-associated uptake defect while its independent effects on baseline transport, hormones and renal exposure are quantified. Test it in a crossed set of participant-relevant acid, potassium, hormonal and tissue states, including baseline-acid and open-path controls. Validate that direct-effect estimates transfer to the corresponding coupled states; do not subtract a single baseline effect from a nonlinear coupled response without demonstrating that transfer. Under the loop, interruption removes the predicted acid-mediated secondary potassium rise and renewed renal suppression beyond state-matched direct effects. Under primary renal or shared hormonal dysfunction, acid transmission is absent, too weak or too late, although nonspecific uptake enhancement or systemic alkali can still improve concentrations. If no selective interruption or defensible state-specific direct-effect bound is available, report the identified local responses and leave complete-loop rescue unresolved. Separately demonstrate that each protective function alone accommodates the same load over the same horizon and limits, with shared determinants and the other compartment's support specified, before interpreting joint indicator failure as failed redundancy. Follow later potassium release and eventual elimination. Acid exposure developing only after the relevant muscle response rejects the acute timing claim in the tested setting; a slower interaction remains a separate unestablished possibility. Null results require verified target engagement, adequate sensitivity, relevant exposures and represented pathways.

Gut–liver feedback may prolong cortisol exposure by suppressing its breakdown predicts instead: First separate adrenal secretion, reversible tissue regeneration, binding, distribution, extrahepatic metabolism and irreversible disposal using specialist isotope kinetics, serial free and total cortisol, binding measurements and steroid metabolomics over repeated circadian cycles. Establish identifiability; report bounds when these determinants cannot be separated. Specify whether clearance is referenced to free or total concentration. Establish a hepatic contribution independently through regional or enzyme-pathway measurements. E_H denotes an unidentified candidate hepatic disposal pathway, not an identified enzyme. Before claiming an enzyme-specific defect, identify the enzyme or defined reaction sequence, labeled products, irreversible fate, hepatic contribution and response to participant-range inflammatory exposure. Measure substrate-to-product conversion and candidate enzyme abundance or activity under matched substrate conditions; a changed expression marker alone is insufficient. A validated pathway-specific loss-and-restoration experiment would need to change irreversible disposal as predicted while alternative pathways and tissue viability are characterized. Until these steps identify E_H, the parent remains a conditional pathway-class hypothesis. Next replay participant-measured free-cortisol concentrations, durations and recovery periods in human enteric preparations containing glia and necessary immune cells, distinguishing glial responses from direct immune glucocorticoid suppression. M_G denotes an unidentified liver-active gut output. Determine its cellular source, production rate, molecular identity or explicitly characterized active fraction, transport, binding, degradation and removal, and the resulting hepatic concentration-time profile. Local gut abundance or culture-supernatant concentration cannot substitute for hepatic exposure. If direct hepatic exposure measurements are unavailable, use a prospectively specified transport model with independently constrained parameters and uncertainty bounds. Test whether the supported exposure range overlaps the measured hepatic inhibitory range. Suppression only at higher concentrations demonstrates pharmacological sufficiency, not operation of the human return arm. At fixed free cortisol and non-gut inflammatory inputs, selective gut interruption should increase pathway-resolved hepatic inactivation after direct hepatocyte exposure to the intervention and active products is excluded. Add-back using the supported hepatic exposure profile should restore suppression. This establishes a return arm only if specificity and physiological delivery are adequate. In a separate unclamped condition, the initial clearance improvement should lower free cortisol, reduce gut output and yield additional clearance recovery. Cortisol replacement should remove the exposure-mediated secondary component; the initial direct consequence of gut interruption can remain. Hepatic restoration followed by replacement alone fits a one-way downstream model. Estimate both tissue response functions, mediator transport, cortisol production and disposal, and adrenal feedback with delays. Test whether their independently measured ranges predict a persistent post-stress state and its interruption, versus stable delayed recovery, on new observations. Include compensatory secretion after increased clearance rather than treating it as zero. A persistent state may coexist with negative feedback without requiring that one local positive gain exceed every opposing process; the complete physiological dynamics must determine the outcome. If compensation restores exposure, predict reduced or absent exposure-mediated functional benefit rather than inferring benefit from clearance alone. Fixed apparatus cortisol tests exposure-response behavior only. Null results constrain adequately represented, specifically perturbed pathways within the tested physiological range. Sleep, activity, host defense and the response to a subsequent safe stress require separate assessment.

CROSS-DOMAIN TRANSFER, conditional: In older humans with limited renal reserve, potassium-acid interactions could cause harmful renal-muscle feedback after an ordinary meal or a separately evaluated tolerable activity challenge. Increased extracellular potassium is hypothesized to suppress renal ammonium excretion sufficiently to reduce total net acid disposal. The resulting retained acid burden must generate a sufficiently large and timely exposure at skeletal muscle to reduce potassium uptake; the secondary extracellular potassium rise then further impairs renal acid disposal. Urinary ammonium, whole-body retained acid and muscle extracellular exposure are distinct quantities. Every directed effect, its magnitude and its timing remain unverified. Renal elimination and temporary muscle sequestration provide nominally redundant one-out-of-two protection only if either independently accommodates the same load within the same horizon and physiological limits under matched shared hormonal conditions. Under that condition, reciprocal chemistry could make otherwise sufficient protective functions jointly unavailable. If the functions are sequentially necessary, the redundancy interpretation fails while the chemical-loop question remains open. The target is harmful amplification with useful coordination preserved. Preventing repeated reserve loss and slowing human aging remain additional hypotheses, including the possibility that temporary sequestration merely postpones the burden.

Urinary net acid excretion is ammonium plus titratable acid minus bicarbonate, expressed as equivalent amounts over the same interval. Titratable acid is operationally the acid equivalents in urinary non-ammonium buffers determined by titration to a specified reference pH, with assay conventions reported. Whole-body acid retention requires an independent balance; buffering redistributes or binds retained acid equivalents rather than automatically eliminating them. Serial blood chemistry does not by itself establish the acid exposure at muscle. Identify the relevant acid-base species, pH and concentration-time trajectory reaching muscle using direct measurements where feasible or an independently validated transport-and-buffering model with uncertainty bounds. If the interval permits negligible or insufficient muscle exposure, the acute mechanism remains unsupported even with reduced ammonium excretion. Replay the participant-range muscle exposure at matched potassium, glucose, insulin, aldosterone and other relevant inputs, and measure regional uptake, release, dose-response and delay. Separately replay the resulting secondary potassium trajectory in renal preparations at matched acid exposure and other inputs; quantify total net acid disposal. Both independently estimated response functions and potassium and acid balances must predict the secondary coupled response on held-out ordinary challenges. A candidate muscle-specific intervention qualifies only if it selectively removes the acid-associated uptake defect while its independent effects on baseline transport, hormones and renal exposure are quantified. Test it in a crossed set of participant-relevant acid, potassium, hormonal and tissue states, including baseline-acid and open-path controls. Validate that direct-effect estimates transfer to the corresponding coupled states; do not subtract a single baseline effect from a nonlinear coupled response without demonstrating that transfer. Under the loop, interruption removes the predicted acid-mediated secondary potassium rise and renewed renal suppression beyond state-matched direct effects. Under primary renal or shared hormonal dysfunction, acid transmission is absent, too weak or too late, although nonspecific uptake enhancement or systemic alkali can still improve concentrations. If no selective interruption or defensible state-specific direct-effect bound is available, report the identified local responses and leave complete-loop rescue unresolved. Separately demonstrate that each protective function alone accommodates the same load over the same horizon and limits, with shared determinants and the other compartment's support specified, before interpreting joint indicator failure as failed redundancy. Follow later potassium release and eventual elimination. Acid exposure developing only after the relevant muscle response rejects the acute timing claim in the tested setting; a slower interaction remains a separate unestablished possibility. Null results require verified target engagement, adequate sensitivity, relevant exposures and represented pathways.

Combine complementary glucose and lactate tracers, regional muscle balance and an identifiable non-steady-state carbon model. Account for circulating glucose, glycogen, alternative lactate fates, other glucose sources and compartment exchange. For a defined muscle compartment, lactate-pool change equals local formation plus uptake minus oxidation, other disposal and export; net vascular release equals export minus uptake. Arteriovenous balance alone cannot identify production. Report uncertainty bounds where formation, uptake or disposal cannot be separated. Bound hepatic energetic expenditure using regional or independently validated mechanistic measurements; whole-body calorimetry, hepatic oxygen consumption alone and an ATP concentration alone cannot identify recycling-specific ATP demand. Define Q_H as hepatic ATP-equivalent turnover attributable to the relevant processing over a specified interval, R as a candidate demand-transmitting cardiovascular or endocrine relay, and Z as specified competing substrate, redox or secretory changes with a possible demand-independent action. These are experimental variables requiring identification, not observed pathways. Identify R's origin, muscle target, exposure-response relation, gain and delay. Validate a liver-restricted recycling perturbation and exclude direct muscle exposure to it or its active products. Match glucose concentration and delivery, insulin, oxygen, electrolytes, arterial lactate and independently imposed upstream inputs; independently replay altered substrate trajectories. Then add a demand-versus-collateral-output discrimination. In one experimental series, independently vary Q_H within the relevant hepatic compartment while matching the specified Z trajectories and allowing R to respond. The frozen mechanism predicts a change in R followed by a corresponding change in independently resolved muscle glycolytic production. In a complementary series, vary candidate Z trajectories while Q_H and upstream inputs are matched. A muscle response fully attributable to Z without an independently demonstrated Q_H-to-R effect supports collateral hepatic signaling rather than establishing the frozen mechanism. If Z is itself proposed to be R, first test whether changing Q_H causes its production; subsequent Z add-back at fixed Q_H tests downstream sufficiency only. Never clamp the actual mediator in a test intended to establish intact transmission. A validated restoration of the relevant hepatic demand during continued recycling reduction, at matched competing outputs, should restore R and muscle production if demand is the operative cause. Generic ATP consumption in another compartment, hypoxia, injury or altered ATP concentration does not qualify as equivalent demand restoration. Independent perturbations or a quantitatively identified dissociation must separate the effects; correlated measurements and statistical adjustment alone are insufficient. No qualifying manipulation is supplied. If separation is infeasible, even successful relay blockade and physiological add-back justify only liver-to-muscle signaling, leaving demand dependence unresolved. In separate mediation experiments, relay blockade should remove the transmitted muscle response and physiological R replacement should restore it despite reduced recycling. Independent glucose replacement should fail to restore the loop only if a non-glucose demand-dependent relay closes it; include the replacement's physiological production, delivery, endocrine and other-organ costs in whole-person accounting. A fall in net lactate release with unchanged formation does not confirm the mechanism. Compare complete coupled and uncoupled systems with matched substrates, hormones, oxygen, waste removal, tissue support and local history. Before testing persistence, specify individualized recovery ranges, required duration within those ranges, an observation horizon based on independently measured recovery distributions, and measurements of residual tissue injury and retained states. Estimate directed responses independently and predict held-out challenges. The strong claim additionally predicts a post-input state incompatible with measured stable-decay and local-memory alternatives, and termination of that state after specific return-path interruption. Longer recovery or a positive root of a local fitted model alone is insufficient; finite observation cannot prove indefinite persistence. Under common forcing, hepatic processing may fall while muscle production follows residual injury or upstream activation. Null results constrain only the tested subgroup and window when target engagement, specificity, sensitivity and pathway representation are adequate.

Gut–liver feedback may prolong cortisol exposure by suppressing its breakdown predicts instead: First separate adrenal secretion, reversible tissue regeneration, binding, distribution, extrahepatic metabolism and irreversible disposal using specialist isotope kinetics, serial free and total cortisol, binding measurements and steroid metabolomics over repeated circadian cycles. Establish identifiability; report bounds when these determinants cannot be separated. Specify whether clearance is referenced to free or total concentration. Establish a hepatic contribution independently through regional or enzyme-pathway measurements. E_H denotes an unidentified candidate hepatic disposal pathway, not an identified enzyme. Before claiming an enzyme-specific defect, identify the enzyme or defined reaction sequence, labeled products, irreversible fate, hepatic contribution and response to participant-range inflammatory exposure. Measure substrate-to-product conversion and candidate enzyme abundance or activity under matched substrate conditions; a changed expression marker alone is insufficient. A validated pathway-specific loss-and-restoration experiment would need to change irreversible disposal as predicted while alternative pathways and tissue viability are characterized. Until these steps identify E_H, the parent remains a conditional pathway-class hypothesis. Next replay participant-measured free-cortisol concentrations, durations and recovery periods in human enteric preparations containing glia and necessary immune cells, distinguishing glial responses from direct immune glucocorticoid suppression. M_G denotes an unidentified liver-active gut output. Determine its cellular source, production rate, molecular identity or explicitly characterized active fraction, transport, binding, degradation and removal, and the resulting hepatic concentration-time profile. Local gut abundance or culture-supernatant concentration cannot substitute for hepatic exposure. If direct hepatic exposure measurements are unavailable, use a prospectively specified transport model with independently constrained parameters and uncertainty bounds. Test whether the supported exposure range overlaps the measured hepatic inhibitory range. Suppression only at higher concentrations demonstrates pharmacological sufficiency, not operation of the human return arm. At fixed free cortisol and non-gut inflammatory inputs, selective gut interruption should increase pathway-resolved hepatic inactivation after direct hepatocyte exposure to the intervention and active products is excluded. Add-back using the supported hepatic exposure profile should restore suppression. This establishes a return arm only if specificity and physiological delivery are adequate. In a separate unclamped condition, the initial clearance improvement should lower free cortisol, reduce gut output and yield additional clearance recovery. Cortisol replacement should remove the exposure-mediated secondary component; the initial direct consequence of gut interruption can remain. Hepatic restoration followed by replacement alone fits a one-way downstream model. Estimate both tissue response functions, mediator transport, cortisol production and disposal, and adrenal feedback with delays. Test whether their independently measured ranges predict a persistent post-stress state and its interruption, versus stable delayed recovery, on new observations. Include compensatory secretion after increased clearance rather than treating it as zero. A persistent state may coexist with negative feedback without requiring that one local positive gain exceed every opposing process; the complete physiological dynamics must determine the outcome. If compensation restores exposure, predict reduced or absent exposure-mediated functional benefit rather than inferring benefit from clearance alone. Fixed apparatus cortisol tests exposure-response behavior only. Null results constrain adequately represented, specifically perturbed pathways within the tested physiological range. Sleep, activity, host defense and the response to a subsequent safe stress require separate assessment.

SCOUT 2, from steroid biotransformation and endogenous pharmacokinetics, conditional: In a subset of aging humans, inflammatory suppression of a specific hepatic cortisol-inactivation pathway could prolong biologically active free-cortisol exposure after the initiating external stress ends and adrenal secretion begins to normalize. Sustained exposure could activate an enteric-glial and immune inflammatory relay whose liver-reaching output further suppresses hepatic inactivation. The frozen retained state is reciprocal gut-liver control of endogenous steroid exposure. Systemic clearance alone cannot localize the defect to liver. The enzyme pathway, inflammatory suppression, older-human glial response and physiologically sufficient liver-active mediator remain unknown. The contextual chronic-glucocorticoid mouse mechanism supplies no numerical human dose or duration equivalence. Strong persistence requires sufficient coupling under physiological hormone production, transport and hypothalamic-pituitary-adrenal feedback; tissue reciprocity might instead be stable and merely alter recovery time. Continuing psychosocial stress, administered steroids and learned neural memory are not necessary components, but they, residual infection, injury and retained inflammatory memory remain alternatives or coexisting causes. Restoring the clearance arm could reduce harmful SPV_12 coupling and permit sleep and ordinary-activity recovery while preserving the necessary initial stress response. This consequence remains conditional on preserved useful glucocorticoid actions, host defense and stress competence, and on compensatory secretion not canceling the exposure change.

Establish identifiability; report bounds when these determinants cannot be separated. Specify whether clearance is referenced to free or total concentration. Establish a hepatic contribution independently through regional or enzyme-pathway measurements. E_H denotes an unidentified candidate hepatic disposal pathway, not an identified enzyme. Before claiming an enzyme-specific defect, identify the enzyme or defined reaction sequence, labeled products, irreversible fate, hepatic contribution and response to participant-range inflammatory exposure. Measure substrate-to-product conversion and candidate enzyme abundance or activity under matched substrate conditions; a changed expression marker alone is insufficient. A validated pathway-specific loss-and-restoration experiment would need to change irreversible disposal as predicted while alternative pathways and tissue viability are characterized. Until these steps identify E_H, the parent remains a conditional pathway-class hypothesis. Next replay participant-measured free-cortisol concentrations, durations and recovery periods in human enteric preparations containing glia and necessary immune cells, distinguishing glial responses from direct immune glucocorticoid suppression. M_G denotes an unidentified liver-active gut output. Determine its cellular source, production rate, molecular identity or explicitly characterized active fraction, transport, binding, degradation and removal, and the resulting hepatic concentration-time profile. Local gut abundance or culture-supernatant concentration cannot substitute for hepatic exposure. If direct hepatic exposure measurements are unavailable, use a prospectively specified transport model with independently constrained parameters and uncertainty bounds. Test whether the supported exposure range overlaps the measured hepatic inhibitory range. Suppression only at higher concentrations demonstrates pharmacological sufficiency, not operation of the human return arm. At fixed free cortisol and non-gut inflammatory inputs, selective gut interruption should increase pathway-resolved hepatic inactivation after direct hepatocyte exposure to the intervention and active products is excluded. Add-back using the supported hepatic exposure profile should restore suppression. This establishes a return arm only if specificity and physiological delivery are adequate. In a separate unclamped condition, the initial clearance improvement should lower free cortisol, reduce gut output and yield additional clearance recovery. Cortisol replacement should remove the exposure-mediated secondary component; the initial direct consequence of gut interruption can remain. Hepatic restoration followed by replacement alone fits a one-way downstream model. Estimate both tissue response functions, mediator transport, cortisol production and disposal, and adrenal feedback with delays. Test whether their independently measured ranges predict a persistent post-stress state and its interruption, versus stable delayed recovery, on new observations. Include compensatory secretion after increased clearance rather than treating it as zero. A persistent state may coexist with negative feedback without requiring that one local positive gain exceed every opposing process; the complete physiological dynamics must determine the outcome. If compensation restores exposure, predict reduced or absent exposure-mediated functional benefit rather than inferring benefit from clearance alone. Fixed apparatus cortisol tests exposure-response behavior only. Null results constrain adequately represented, specifically perturbed pathways within the tested physiological range. Sleep, activity, host defense and the response to a subsequent safe stress require separate assessment.

Combine complementary glucose and lactate tracers, regional muscle balance and an identifiable non-steady-state carbon model. Account for circulating glucose, glycogen, alternative lactate fates, other glucose sources and compartment exchange. For a defined muscle compartment, lactate-pool change equals local formation plus uptake minus oxidation, other disposal and export; net vascular release equals export minus uptake. Arteriovenous balance alone cannot identify production. Report uncertainty bounds where formation, uptake or disposal cannot be separated. Bound hepatic energetic expenditure using regional or independently validated mechanistic measurements; whole-body calorimetry, hepatic oxygen consumption alone and an ATP concentration alone cannot identify recycling-specific ATP demand. Define Q_H as hepatic ATP-equivalent turnover attributable to the relevant processing over a specified interval, R as a candidate demand-transmitting cardiovascular or endocrine relay, and Z as specified competing substrate, redox or secretory changes with a possible demand-independent action. These are experimental variables requiring identification, not observed pathways. Identify R's origin, muscle target, exposure-response relation, gain and delay. Validate a liver-restricted recycling perturbation and exclude direct muscle exposure to it or its active products. Match glucose concentration and delivery, insulin, oxygen, electrolytes, arterial lactate and independently imposed upstream inputs; independently replay altered substrate trajectories. Then add a demand-versus-collateral-output discrimination. In one experimental series, independently vary Q_H within the relevant hepatic compartment while matching the specified Z trajectories and allowing R to respond. The frozen mechanism predicts a change in R followed by a corresponding change in independently resolved muscle glycolytic production. In a complementary series, vary candidate Z trajectories while Q_H and upstream inputs are matched. A muscle response fully attributable to Z without an independently demonstrated Q_H-to-R effect supports collateral hepatic signaling rather than establishing the frozen mechanism. If Z is itself proposed to be R, first test whether changing Q_H causes its production; subsequent Z add-back at fixed Q_H tests downstream sufficiency only. Never clamp the actual mediator in a test intended to establish intact transmission. A validated restoration of the relevant hepatic demand during continued recycling reduction, at matched competing outputs, should restore R and muscle production if demand is the operative cause. Generic ATP consumption in another compartment, hypoxia, injury or altered ATP concentration does not qualify as equivalent demand restoration. Independent perturbations or a quantitatively identified dissociation must separate the effects; correlated measurements and statistical adjustment alone are insufficient. No qualifying manipulation is supplied. If separation is infeasible, even successful relay blockade and physiological add-back justify only liver-to-muscle signaling, leaving demand dependence unresolved. In separate mediation experiments, relay blockade should remove the transmitted muscle response and physiological R replacement should restore it despite reduced recycling. Independent glucose replacement should fail to restore the loop only if a non-glucose demand-dependent relay closes it; include the replacement's physiological production, delivery, endocrine and other-organ costs in whole-person accounting. A fall in net lactate release with unchanged formation does not confirm the mechanism. Compare complete coupled and uncoupled systems with matched substrates, hormones, oxygen, waste removal, tissue support and local history. Before testing persistence, specify individualized recovery ranges, required duration within those ranges, an observation horizon based on independently measured recovery distributions, and measurements of residual tissue injury and retained states. Estimate directed responses independently and predict held-out challenges. The strong claim additionally predicts a post-input state incompatible with measured stable-decay and local-memory alternatives, and termination of that state after specific return-path interruption. Longer recovery or a positive root of a local fitted model alone is insufficient; finite observation cannot prove indefinite persistence. Under common forcing, hepatic processing may fall while muscle production follows residual injury or upstream activation. Null results constrain only the tested subgroup and window when target engagement, specificity, sensitivity and pathway representation are adequate.

Potassium-driven acid retention may weaken muscle potassium uptake and amplify kidney stress predicts instead: Measure dietary potassium entry separately from activity-related redistribution, regional muscle potassium uptake and release, urinary potassium, urinary ammonium, titratable acid, bicarbonate excretion, acid production, nonrenal acid-base gains and losses, relevant extracellular volumes and buffering. Urinary net acid excretion is ammonium plus titratable acid minus bicarbonate, expressed as equivalent amounts over the same interval. Titratable acid is operationally the acid equivalents in urinary non-ammonium buffers determined by titration to a specified reference pH, with assay conventions reported. Whole-body acid retention requires an independent balance; buffering redistributes or binds retained acid equivalents rather than automatically eliminating them. Serial blood chemistry does not by itself establish the acid exposure at muscle. Identify the relevant acid-base species, pH and concentration-time trajectory reaching muscle using direct measurements where feasible or an independently validated transport-and-buffering model with uncertainty bounds. If the interval permits negligible or insufficient muscle exposure, the acute mechanism remains unsupported even with reduced ammonium excretion. Replay the participant-range muscle exposure at matched potassium, glucose, insulin, aldosterone and other relevant inputs, and measure regional uptake, release, dose-response and delay. Separately replay the resulting secondary potassium trajectory in renal preparations at matched acid exposure and other inputs; quantify total net acid disposal. Both independently estimated response functions and potassium and acid balances must predict the secondary coupled response on held-out ordinary challenges. A candidate muscle-specific intervention qualifies only if it selectively removes the acid-associated uptake defect while its independent effects on baseline transport, hormones and renal exposure are quantified. Test it in a crossed set of participant-relevant acid, potassium, hormonal and tissue states, including baseline-acid and open-path controls. Validate that direct-effect estimates transfer to the corresponding coupled states; do not subtract a single baseline effect from a nonlinear coupled response without demonstrating that transfer. Under the loop, interruption removes the predicted acid-mediated secondary potassium rise and renewed renal suppression beyond state-matched direct effects. Under primary renal or shared hormonal dysfunction, acid transmission is absent, too weak or too late, although nonspecific uptake enhancement or systemic alkali can still improve concentrations. If no selective interruption or defensible state-specific direct-effect bound is available, report the identified local responses and leave complete-loop rescue unresolved. Separately demonstrate that each protective function alone accommodates the same load over the same horizon and limits, with shared determinants and the other compartment's support specified, before interpreting joint indicator failure as failed redundancy. Follow later potassium release and eventual elimination. Acid exposure developing only after the relevant muscle response rejects the acute timing claim in the tested setting; a slower interaction remains a separate unestablished possibility. Null results require verified target engagement, adequate sensitivity, relevant exposures and represented pathways.