Muscle-brain endocrine axis in autism spectrum disorder: Mechanisms, evidence, and research priorities

Table 1 Evidence map and translational gaps across proposed muscle-brain mechanisms in autism spectrum disorder

Mechanistic domain	ASD-specific evidence	Transdiagnostic mechanistic evidence	Human/clinical evidence	Typical methods represented	Current interpretation and key translational gap	Ref.
Synaptic plasticity and BDNF/TrkB signaling	Altered synaptic structure and function are reported in ASD-relevant models and subgroups. Peripheral BDNF findings are heterogeneous; MDGA2 deficiency can produce excessive BDNF/TrkB activation and autism-relevant phenotypes in mice. An ASD exercise trial reported concurrent serum BDNF and executive-function changes	Exercise-related PGC-1α/FNDC5/irisin signaling can influence hippocampal BDNF-associated pathways in non-ASD systems. Mechanistic studies also show that precursor and mature BDNF can engage distinct signaling programs	Peripheral BDNF differs between some ASD and comparison cohorts, but serum/plasma measures are strongly influenced by platelet storage and release and by pre-analytical handling. Parallel changes in biomarkers and behavior do not establish mediation	Genetic and pathway-manipulation models; systematic review/meta-analysis; peripheral biomarker assays; exercise intervention trial	Biological plausibility is substantial, but ASD-specific causal mediation remains unproven. The BDNF paradox requires baseline-, compartment-, time-, and molecular-form-specific testing; peripheral BDNF should remain an exploratory marker of response	[3,11,55-65]
Neuroinflammation and microglial states	Innate immune dysfunction, neuroinflammation, and altered microglial programs have been implicated in subsets of ASD. Causal role, developmental timing, and subgroup specificity remain unresolved	Exercise-related IL-6 responses are context dependent. Human experimental studies show that IL-6 can increase IL-1ra/IL-10 and that exercise or IL-6 infusion can attenuate endotoxin-induced TNF-α. Single-cell studies demonstrate multidimensional microglial heterogeneity beyond a binary M1/M2 model	There is no direct demonstration that exercise-induced, muscle-derived IL-6 crosses the blood-brain barrier and reprograms human microglia in ASD. Clinical ASD evidence is therefore indirect	ASD neuroimmune reviews; human cytokine infusion/exercise experiments; single-cell transcriptomics; observational immune profiling	The pathway is best treated as an indirect, hypothesis-level model. A demonstrated M1-to-M2 conversion should not be inferred. ASD studies need time-resolved peripheral mediator measures linked to central neuroimmune readouts and functional outcomes	[4,12,66-70]
Energy metabolism, mitochondrial dysfunction, and lactate	Mitochondrial abnormalities and mitochondrial-dysfunction biomarkers have been reported in ASD subsets, with substantial biological heterogeneity. A single cerebral-energy-deficit model should not be generalized across the spectrum	Exercise improves insulin signaling and systemic glucose regulation. Lactate acts as a metabolic substrate and signaling molecule; the human brain can increase lactate uptake during exercise. Sustained oxidative use of lactate depends on downstream mitochondrial capacity	Direct human evidence that exercise-derived lactate corrects ASD-associated mitochondrial dysfunction or mediates behavioral improvement is insufficient	Systematic reviews/meta-analyses; metabolic physiology; brain substrate-uptake studies; mechanistic and integrative reviews	Metabolic support is plausible, but lactate is not a myokine and should not be presented as a generic bypass for mitochondrial dysfunction. ASD-specific mediation requires metabolic phenotyping linked to brain measures and outcomes	[5,13,71-79]
HPA axis and stress response	Meta-analytic and review evidence indicates altered cortisol patterns and stress responsivity in ASD, but findings vary by developmental stage, sampling method, stress paradigm, and subgroup. Exercise studies report selected improvements in emotional and behavioral outcomes	In broader populations, higher physical activity is associated with a steeper diurnal cortisol slope, although effects are not uniform across HPA indices. IL-6- and irisin-related immune/neurotrophic pathways provide only indirect links to stress-regulatory systems	No direct evidence establishes that a specific muscle-derived myokine recalibrates HPA-axis function in ASD. Behavioral change can arise through multiple nonexclusive mechanisms	Cortisol meta-analysis; stress-system review; physical-activity meta-analysis; ASD behavioral intervention studies	The muscle-HPA link is a testable indirect hypothesis, not an established mechanism. Future trials require repeated cortisol sampling, standardized stress tasks, mediator kinetics, and formal mediation analyses	[6,53,54,60,68,80-82]
Cross-cutting biomarkers and stratified exercise translation	ASD exercise outcomes are heterogeneous. Broad symptom severity and stereotyped behaviors show null or inconsistent effects in some syntheses. Peripheral BDNF and irisin profiles vary across cohorts, and an aquatic-exercise trial showed concurrent, but not necessarily mediated, changes in BDNF and executive function	Exercise-responsive BDNF, IL-6, and irisin vary with intervention characteristics, participant characteristics, sampling conditions, and assay methodology. Multiple non-muscle factors influence circulating BDNF levels; human irisin studies show methodological heterogeneity	No validated ASD-specific biomarker thresholds, response-prediction rules, or genotype- or omics-based exercise decision algorithms currently exist. Pediatric BDNF RCT evidence is not uniformly positive and is not ASD-specific	Clinical-trial syntheses; systematic reviews/meta-analyses; biomarker studies; exercise RCTs; assay-methodology reviews	Myokines and related mediators should be treated as candidate research measures, not validated clinical biomarkers. Priorities include assay standardization, prespecified subgroup analyses, mediation testing, external validation, and comparison with standard clinical assessment	[6,12,44,45,57,59,62-64,86-90,97,98,102]

Evidence categories reflect study context and narrative interpretation, not formal risk-of-bias grading.

ASD: Autism spectrum disorder; BDNF: Brain-derived neurotrophic factor; TrkB: Tropomyosin receptor kinase B; PGC-1α: Peroxisome proliferator-activated receptor γ coactivator-1α; FNDC5: Fibronectin type III domain-containing protein 5; IL: Interleukin; TNF-α: Tumor necrosis factor-α; HPA: Hypothalamic-pituitary-adrenal; RCT: Randomized controlled trial.

Table 2 Autism spectrum disorder-specific intervention evidence and transdiagnostic mechanistic evidence relevant to the proposed muscle-brain axis

Evidence tier	Model/population	Intervention/manipulation	Main findings	Relevance to the proposed muscle-brain axis and major limitation	Ref.
ASD-specific exercise evidence	BTBR mice (ASD-relevant model)	5 weeks of voluntary wheel running	Attenuated nociceptive abnormalities; no significant improvement in social interaction deficits	Shows outcome-specific effects of physical activity in an ASD-relevant model. Does not establish myokine mediation or broad behavioral efficacy	[83]
ASD-specific multimodal contextual evidence	Juvenile BTBR mice	Semi-natural housing/environmental enrichment with social and physical components	Improved social behavior and reduced repetitive exploratory behavior	Relevant to multimodal interventions, but not an isolated exercise manipulation; effects cannot be attributed specifically to physical activity	[84]
ASD-specific exercise evidence	Male and female rats with VPA-induced autism-like phenotype	Low- and moderate-intensity interval training	Improved selected anxiety-like and social outcomes; spatial learning and memory did not significantly improve	Supports outcome-specific rather than uniform efficacy. Myokine mediation was not demonstrated	[85]
ASD-specific clinical intervention evidence	Children with ASD	Aquatic exercise intervention	Improved selected executive-function outcomes; serum BDNF increased concurrently	Provides preliminary concurrent human evidence. Co-occurrence does not establish BDNF mediation, skeletal muscle origin, or central target engagement	[62]
ASD-specific clinical synthesis	Autistic children, adolescents, and broader autistic samples across exercise trials	Multiple exercise modalities	Syntheses report benefits for selected social, cognitive, sleep, anxiety, and functional outcomes, but effects are heterogeneous, and some domains show null or inconsistent results	Supports continued clinical investigation while arguing against broad efficacy claims or inference of a single biological mechanism	[6,87-88,102]
ASD-specific mechanistic synthesis	Autistic individuals/ASD-relevant literature	Mechanistic review of exercise interventions	Summarizes candidate neuronal, glial, and gut microbiota-related pathways	Useful for hypothesis generation; mechanistic review evidence does not demonstrate direct causal mediation by myokines	[89,103]
ASD-specific biomarker evidence	Children with ASD	Cross-sectional serum BDNF assessment	Elevated or altered peripheral BDNF has been reported; some studies associate higher levels with symptom severity	Central to the BDNF paradox: Higher peripheral BDNF is not equivalent to beneficial central signaling. Cross-sectional data cannot establish cause, compensation, or tissue source	[57,59]
ASD-specific biomarker evidence	Children with ASD vs healthy controls	Cross-sectional serum irisin assessment	Lower circulating irisin was reported in ASD	Suggests a potential association with physical inactivity or metabolic phenotype but does not establish a causal deficiency or muscle-to-brain mediation	[86]
ASD-specific synaptic mechanism	Mdga2-deficient mice with autism-relevant phenotype	Genetic disruption of a synaptic regulator	Aberrant/excessive BDNF-TrkB signaling was linked to synaptic and autism-relevant behavioral changes	Demonstrates that greater TrkB signaling is not uniformly beneficial and supports caution regarding BDNF/TrkB-targeted strategies. This was not an exercise study	[58]
Transdiagnostic foundational mechanism	Non-ASD mice	Exercise-induced PGC-1α/FNDC5 pathway activation	Exercise-induced hippocampal BDNF through a PGC-1α/FNDC5-related pathway	Provides foundational biological plausibility for an exercise-responsive FNDC5/BDNF pathway. Does not establish ASD-specific mediation	[11]
Transdiagnostic mechanistic evidence	Non-ASD mice; Alzheimer’s disease model	Whole-body FNDC5 loss of function; experimental elevation of circulating irisin	Loss of FNDC5 abolished selected exercise-related cognitive effects; elevated circulating irisin improved selected cognitive and neuropathological outcomes in an Alzheimer’s disease model	Supports pathway dependence or sufficiency within those models. Neurodegenerative findings cannot be treated as causal evidence in ASD	[42]
Transdiagnostic peripheral mechanism	Aging-associated sarcopenia mouse model	Muscle-specific FNDC5 deletion during aerobic exercise	Deletion impaired exercise-related benefits in muscle function and mass	Supports a role for muscle-derived FNDC5/irisin in peripheral exercise adaptation. Does not establish brain or ASD mediation	[50]
Transdiagnostic mechanistic evidence	Non-ASD mouse model	Irisin-neutralizing antibody during exercise	Attenuated selected cognitive and mood-related effects and reduced exercise-associated hippocampal BDNF upregulation and cell proliferation	Supports pathway dependence in a non-ASD setting. Generalization to ASD requires direct validation	[51]
Transdiagnostic molecular mimicry evidence	Non-ASD experimental context; quadriceps muscle	Recombinant irisin administration	Induced skeletal muscle proteomic, metabolic, and microstructural changes resembling aspects of exercise adaptation	Supports partial molecular mimicry in muscle. Does not demonstrate CNS effects, behavioral benefits, or ASD relevance on its own	[52]
Non-ASD pediatric biomarker synthesis	Children aged 5-12 years across randomized exercise trials; not restricted to ASD	Exercise interventions with peripheral BDNF measurement	Only two of five included randomized trials reported significant exercise-related increases in BDNF	Demonstrates that BDNF responses are not uniformly positive even in pediatric exercise studies. Not ASD-specific and cannot establish mediation	[90]

Evidence categories reflect study context, not formal risk-of-bias grading.

ASD: Autism spectrum disorder; VPA: Valproic acid; BDNF: Brain-derived neurotrophic factor; TrkB: Tropomyosin receptor kinase B; PGC-1α: Peroxisome proliferator-activated receptor γ coactivator-1α; FNDC5: Fibronectin type III domain-containing protein 5; CNS: Central nervous system.