Review Papers Claim by Claim

What the paper claims

• The review compiles evidence that mature human erythrocytes release or present multiple bioactive signals during exercise: ATP, nitric oxide and cGMP, sphingosine-1-phosphate (S1P), lactate, reactive oxygen species (ROS), extracellular vesicles and microRNAs, and act as carriers for chemokines and mtDNA via surface receptors such as DARC and TLR9 Erythrocyte exerkine inventory

Strengths

• Comprehensive synthesis across molecular classes (small metabolites, lipids, gases, EVs, nucleic acids) emphasizing integrative physiology and plausible mechanisms connecting RBC physiology to vascular and metabolic responses Comprehensive molecule coverage
• Explicit discussion of uncertainties, methodological variability, and needed standardization strengthens the review as an agenda-setting piece rather than an overclaiming manifesto Uncertainties and methods

Critical limitations and caveats

1. Narrative review, not primary data — the paper synthesizes disparate primary studies of varying quality; causal claims require targeted interventional or cell-specific tracing experiments which are absent Narrative review status
2. Attribution problem — for many measured circulating factors (eg S1P, lactate, NO metabolites, EVs), the relative quantitative contribution of erythrocytes versus endothelium, muscle or platelets is not resolved; the review acknowledges this but sometimes implies erythrocyte primacy where evidence is incomplete Source attribution uncertainty
3. Methodological heterogeneity across cited studies — miRNA and EV studies are prone to pre-analytical and analytical biases (plasma vs whole blood, isolation kits, hemolysis, sequencing depth); the review notes this but does not provide a standardized evidence-weighting framework EV miRNA methodological bias
4. Functional proof largely indirect — for many candidate erythrokines (eg RBC-derived lactate, RBC miRNA delivered via EVs), direct demonstrations that RBC-origin molecules reach target cells in vivo and change function after exercise remain limited or absent; mechanistic transfer experiments are needed Speculative cargo transfer

Technical and interpretive blindspots

• Quantitation: few cited studies provide absolute flux or mass balance measurements (molecules per RBC, export rates per minute) across exercise intensities — without that, physiological magnitude is uncertain Lack of flux quantitation
• Interindividual variability and population context: most mechanistic studies derive from small human cohorts or animal models; the review recommends broader population and longitudinal studies but the current synthesis cannot establish generalizable effect sizes Population gaps
• Confounding by hemolysis: many assays of plasma miRNA or metabolites are sensitive to erythrocyte lysis; the review discusses RBC-origin signals but measurement artifacts (hemolysis increasing apparent RBC-derived miRNA in plasma) are a perennial confounder that complicates interpretation Hemolysis confounding

High-priority experiments to resolve open questions

1. Direct tracer experiments: stable-isotope or uniquely labelled RBC-specific tracers (eg 13C-labelled RBC glycolytic products, or RBC-specific membrane tags on EVs) during controlled exercise to quantify RBC contribution to plasma pools and uptake by target tissues.
2. Cell-specific blockade or depletion in animals: conditional erythrocyte manipulations (eg Mfsd2b KO, RBC-targeted MCT1 knockdown, or conditional DARC deletion) combined with exercise challenge to determine functional importance of RBC-derived S1P, lactate, or chemokine carriage Model evidence for S1P
3. Standardized EV and miRNA pipelines: agreed pre-analytic handling, hemolysis metrics, spike-in controls, and cross-lab proficiency testing to distinguish true RBC-EV signals from contamination.
4. Physiological dose-response: measure RBC exerkine export kinetics across graded exercise intensities and recovery, with simultaneous endothelial/muscle sampling where feasible (eg microdialysis) to partition sources.

Conclusion and confidence

Overall the review is a thoughtful, well-referenced synthesis that reframes erythrocytes as active participants in exercise physiology and usefully lists candidate molecules and mechanisms; however, the evidence base is mixed: some pathways (ATP release, S1P export) have solid mechanistic support, while others (functional delivery of RBC EV miRNA to distant tissues; RBC as dominant source of lactate signaling) remain speculative and require targeted quantitative and causal studies Overall balanced synthesis

Actionable takeaways for researchers

• Prioritize tracer and genetic perturbation experiments to move from correlation to causation.
• Create community standards for EV and miRNA measurement in exercise studies.
• Quantify per-cell release rates and tissue uptake to assess physiological relevance.