The Vitamin D-Gut-Brain Axis in Autism Spectrum Disorder: A Bidirectional Crosstalk

Abstract

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder arising from the interaction of genetic and environmental triggers. In recent years, vitamin D deficiency and gut microbiota dysbiosis have emerged as two pivotal environment-related factors in ASD pathogenesis. Rather than acting independently, these factors jointly participate in the onset and progression of ASD through intricate bidirectional interaction mechanisms. This article presents a narrative review that summarizes the current research progress regarding the association between vitamin D deficiency and gut microbiota dysbiosis in ASD. Mechanistically, vitamin D modulates the gut-brain axis across multiple levels, including gut microbiota composition, intestinal barrier function, intestinal inflammation, bacterial translocation, microbial metabolites, blood-brain barrier integrity and neuroinflammation. In turn, the gut microbiota influences vitamin D metabolism and bioactivity through short-chain fatty acids, establishing a bidirectional vitamin D-gut-brain axis. This axis likely contributes to ASD phenotypes by regulating neuroinflammation, gut-brain crosstalk and neurodevelopmental processes. Notably, most available evidence in this field is derived from preclinical studies, while relevant clinical evidence remains limited. Future studies should focus on dissecting the underlying molecular crosstalk and developing personalized intervention strategies targeting vitamin D status and microbial signatures.

Graphical Abstract

Introduction

Autism spectrum disorder (ASD) is a group of neurodevelopmental disorders characterized by core features including social interaction deficits, communication impairments and restricted repetitive behaviors. Recent studies have shown that, beyond social communication and behavioral impairments, children with ASD often exhibit visual perceptual and oculomotor deficits that can affect daily functioning.¹ The global prevalence and mortality rates of ASD have been steadily increasing.^2–6 Current estimates indicate that roughly 1 in 127 people globally are on the autism spectrum. The global age-standardized prevalence is 788.3 per 100,000 population and the disability-adjusted life year (DALY) rate is 147.6 per 100,000 population, which decreases with age.⁷ ASD exhibits a significantly higher risk of suicidal ideation and suicidal behaviors,⁸ posing an escalating challenge to global public health. Although the genetic basis of ASD has been extensively established, genetic factors alone account for only a subset of cases. Environmental factors and their interplay with genetic predispositions play a significant role in the pathogenesis of ASD.

Among numerous environmental factors, vitamin D deficiency and gut microbiota dysbiosis have emerged as major research hotspots in recent years, holding significant value for both research and clinical translation. Globally, the prevalence of vitamin D deficiency in children ranges from 30% to 60%,^9–12 and a similar prevalence is observed among children with ASD.¹³ Meanwhile, the gut microbiota influences brain development and behavior through the gut-brain axis. Patients with ASD commonly exhibit alterations in gut microbial composition. Fecal microbiota transplantation has been shown to ameliorate core symptoms of ASD.^14–17 Both factors converge on the gut-brain axis through shared biological pathways, including immune regulation, neuroinflammation and neurotransmitter metabolism. Moreover, they are closely intertwined physiologically: vitamin D can modulate gut microbial composition and intestinal barrier integrity, while gut microbial metabolites in turn influence the expression and activity of the vitamin D receptor. This interplay positions the vitamin D-gut-brain axis as a systemic framework warranting a dedicated review, which may jointly contribute to the pathophysiology of ASD.

In recent years, several reviews have separately addressed vitamin D in ASD and the gut-brain axis in ASD, but few have systematically examined the two as a unified, bidirectionally interactive regulatory axis. The contribution of this review lies in its focus on the complete vitamin D-gut-brain axis loop, integrating the synergistic effects and feedback regulation between vitamin D signaling and the gut microbiome in ASD, thereby filling the gap in existing reviews at the level of an integrative “axis” perspective.

This review aims to synthesize the progress on the interplay between vitamin D and the gut-brain axis in ASD, integrate the evidence for their synergistic effects and bidirectional regulation, and identify current research gaps.

Vitamin D Deficiency and ASD

Vitamin D Metabolism

In humans, approximately 80–90% of vitamin D is synthesized endogenously in the skin upon ultraviolet exposure (as vitamin D₃). The majority of vitamin D is bound to vitamin D-binding protein (DBP) in the circulation. Vitamin D needs to be converted by 25-hydroxylase (CYP2R1) to 25-hydroxyvitamin D (25(OH)D) in the liver, then by 1α-hydroxylase (CYP27B1) to 1,25-dihydroxyvitamin D (1,25(OH)₂D) in the kidney. The active vitamin D (1,25(OH)₂D) binds to the vitamin D receptor (VDR), then translocates to the cell nucleus to regulate the target genes. VDR is expressed across a wide range of tissues throughout the body, including intestinal epithelial cells, immune cells, neurons and microglia. This broad expression profile provides the molecular basis for the pleiotropic, non-classical functions of vitamin D.¹⁸

Vitamin D Pathway and ASD

Vitamin D Levels and ASD

Recent studies have shown that children with ASD have lower serum 25(OH)D levels,^13,19–21 and lower 25(OH)D levels are negatively correlated with the risk of ASD.^22–24 Beyond conventional detection methods, one study employing functionalized graphene quantum dots to measure serum 25(OH)D₃ levels in children with ASD also validated the same results.²⁵ Although a few studies have failed to identify the association between vitamin D and ASD,^26–28 the negative correlation remains stable across different geographical regions. Vitamin D deficiency may represent a cross-regional, universal phenomenon associated with ASD, rather than being attributable to specific geographic locations or dietary patterns.

Lower vitamin D levels, more severe core symptoms of ASD.²⁹ Vitamin D deficiency is a significant risk factor affecting multidimensional neurodevelopment in children with ASD, closely associated with abnormalities in motor function, personal-social skills developmental quotient (DQ), listening-language DQ and eye-hand coordination DQ.³⁰ Different vitamin D subtypes exhibit distinct effects on neurodevelopment in ASD. 25(OH)D₂ demonstrates selective impairment of adaptive behavior and personal-social behavior, whereas 25(OH)D₃ shows a strong negative correlation with symptom severity and positive correlations with adaptive behavior, fine motor skills and personal-social functioning.²⁰

Vitamin D deficiency during pregnancy is associated with an elevated risk of ASD in offspring. Each 25 nmol/L (or 10 ng/mL) increment in maternal serum vitamin D levels corresponds to a 19% reduction in the risk of ASD in offspring.³¹ Offspring of mothers with prenatal 25(OH)D at 25–50 nmol/L have a 1.58-fold higher risk of developing ASD compared with those whose mothers’ 25(OH) D ≥ 50 nmol/L. Similarly, newborns with 25(OH)D < 25 nmol/L exhibit a 1.33-fold increased risk of ASD compared with those having 25(OH)D ≥ 50 nmol/L.³² Offspring of dams with vitamin D deficiency during pregnancy exhibit typical ASD-like behaviors.^33–36 Vitamin D homeostasis during early life in mitigating neurodevelopmental deficits associated with ASD.

VDR and ASD

The VDR is the primary receptor mediating the biological functions of vitamin D. VDR gene polymorphisms are key determinants of genetic susceptibility to ASD. The VDR rs7975232 is a protective genetic site for ASD,³⁷ whereas the VDR rs731236 and rs2228570 loci are genetic risk sites.³⁸ The rs731236 T/C genotype exhibits a synergistic effect with vitamin D deficiency. Their combination further increases the risk of ASD in individuals.¹⁹

Notably, the aforementioned common VDR gene polymorphism loci are almost directly uncorrelated with serum vitamin D concentrations. Current clinical studies have shown no significant statistical difference in VDR expression levels between ASD patients and healthy controls.³⁹ This finding suggests that VDR gene polymorphisms do not regulate ASD risk by affecting basal vitamin D metabolic levels but rather may do so by altering VDR protein structure, receptor activity and downstream signaling pathway transduction efficiency.

CYP2R1, CYP27B1 and ASD

CYP2R1 and CYP27B1 are the key functional enzymes mediating vitamin D hydroxylation and maintaining vitamin D homeostasis. Compared with healthy controls, the expression levels of CYP2R1 and CYP27B1 are upregulated in individuals with ASD.^39,40 No significant association has been found between polymorphisms in the CYP27B1 gene (rs4646536) or the CYP2R1 gene (rs10741657) and genetic susceptibility to ASD.³⁷ The elevated expression of these activating enzymes is not attributable to genetic mutations but rather represents a secondary compensatory metabolic alteration in response to vitamin D deficiency, neurodevelopmental abnormalities, or gut microbiota dysbiosis.

DBP and ASD

DBP is a key carrier protein that regulates the transport, storage and bioavailability of vitamin D in the body. DBP is encoded by the GC gene located in the 4q11-q13 region of chromosome 4. GC isoforms, particularly GC1f, are significantly more prevalent in ASD compared with healthy controls.⁴¹ GC gene isoforms and polymorphisms influence the clinical phenotypic presentation and degree of functional impairment in individuals with ASD.⁴¹

Plasma DBP levels are lower in individuals with ASD compared with healthy controls,²¹ which is one of the contributors to vitamin D deficiency.²⁴ DBP structural and functional abnormalities mediated by GC gene polymorphisms may indirectly induce relative vitamin D deficiency by reducing vitamin D bioavailability, thereby synergistically exacerbating neurodevelopmental impairments and clinical symptoms in ASD.

Vitamin D in ASD: Potential Pathophysiological Mechanisms

The relationship between vitamin D deficiency and ASD is underpinned by multiple potential mechanisms, including genetic mutations, oxidative stress, inflammatory responses, immune regulation, excessive neuronal proliferation and aberrant neurotransmitter levels.⁴² Recent investigations have primarily focused on two major mechanistic domains: neurodevelopmental regulation and immune regulation.

Neurodevelopmental Regulation

Multiple studies using mouse models of ASD have demonstrated that vitamin D supplementation can ameliorate core ASD-like phenotypes, including stereotypic behaviors and social deficits.^36,43,44 Imaging and fundamental research have further confirmed that vitamin D participates in the onset and progression of ASD by regulating neuronal proliferation, neurotransmitter function, synaptic function, brain structural development and brain functional synchronization.

From a structural brain perspective, maternal vitamin D deficiency during pregnancy and lactation promotes abnormal proliferation of cortical intermediate progenitor cells by suppressing the WNT/β-catenin signaling pathway in the offspring’s cerebral cortex.³⁶ In children with ASD, vitamin D levels are significantly correlated with gray matter volume in the frontal and cerebellar regions. These structural brain abnormalities are closely associated with symptom severity and long-term developmental outcomes in affected children.⁴⁵ Vitamin D deficiency induces structural brain defects, providing an organic basis for subsequent functional abnormalities in ASD.

From a functional brain perspective, vitamin D regulates hippocampal axon guidance development,⁴⁴ upregulates brain levels of 5-Hydroxytryptamine (5-HT),⁴³ protects synaptic structure and function,⁴⁶ restoring balanced neural signal transmission. Resting-state functional magnetic resonance imaging (rs-fMRI) studies have further corroborated the regulatory role of vitamin D in brain functional activity in ASD. Vitamin D deficiency disrupts local neural synchronization and functional connectivity across multiple brain regions in children with ASD, leading to impairments in social interaction, emotional regulation and cognitive function, exacerbating core ASD symptoms.⁴⁷

Vitamin D deficiency leads to abnormal proliferation and differentiation of neural cells during the embryonic period, resulting in structural developmental defects in key brain regions. Concurrently, it disrupts hippocampal axon guidance development and induces synaptic dysfunction and neurotransmitter imbalances. Ultimately, these pathological processes manifest as multi-regional abnormalities in neural synchronization and functional connectivity across the entire brain, contributing to the onset and progression of core ASD symptoms.

Immune Regulation

Vitamin D alleviates the characteristic immune dysregulation in ASD by modulating systemic inflammation, central neuroinflammation and intestinal adaptive immune homeostasis. Patients with ASD exhibit elevated inflammatory markers, which are negatively correlated with serum 25(OH)D levels.⁴⁸ Vitamin D supplementation downregulates the expression of various pro-inflammatory cytokines, alleviates systemic chronic inflammation in ASD and inhibits excessive central neuroinflammation.^49–52 High maternal vitamin D levels ameliorate the aberrant immune phenotype of intestinal CD4⁺ T cells in offspring exposed to maternal inflammation, preventing and attenuating ASD-related immune and neurodevelopmental abnormalities from the earliest stages.⁵³

Gut Microbiota Dysbiosis and ASD

Composition and Function of the Gut Microbiota

The human intestine is colonized by trillions of microorganisms, collectively termed the gut microbiota, including bacteria, viruses, fungi and other microbes. The gut microbiota is dominated by six major bacterial phyla: Firmicutes, Bacteroidetes, Proteobacteria, Actinobacteria, Verrucomicrobia and Fusobacteria. Under healthy conditions, the gut microbiota is primarily composed of Firmicutes and Bacteroidetes.

Gut Microbiota Dysbiosis in ASD

ASD exhibits significant structural disturbances of the gut microbiota and functional dysbiosis of the microecosystem. Evidence from recent meta-analyses and systematic reviews indicates that there is no universally consistent pattern of gut microbial alterations in ASD (Table 1), which may be attributable to substantial differences in sample characteristics and methodological approaches. Across comparative studies, the decrease in Bifidobacterium abundance emerges as the most consistent finding in the ASD gut microbiome, with all but one of the six studies reporting this decrease (the remaining study did not report this genus). Dorea, Blautia, Lachnospira and Streptococcus also demonstrate relatively high consistency, each supported by at least two to three studies with no contradictory reports. In contrast, although Clostridium tends to increase in most studies, one study reported a decrease, suggesting that alterations in this genus remain somewhat heterogeneous.

Table 1.

Gut Microbiota Dysbiosis in ASD

Study	Year	Gut Microbiota Alterations in ASD	References
Meta-analysis	2024	↑Parabacteroides, Anaerostipes, Faecalibacterium, Clostridium, Dorea, Phascolarctobacterium, Lachnoclostridium, Catenibacterium, Collinsella, ↓Barnesiella, Odoribacter, Paraprevotella, Blautia, Turicibacter, Lachnospira, Pseudomonas, Parasutterella, Haemophilus, Bifidobacterium	[54]
Meta-analysis	2021	↓Streptococci, Bifidobacterium	[55]
Meta-analysis	2022	↓Bacteroides stercoris t__190463, Granulicatella, Massilioclostridium coli	[56]
Meta-analysis	2025	↑Clostridium ↓Bifidobacterium	[57]
Systematic review	2023	↑Proteobacteria, Actinobacteria, Sutterella	[58]
Systematic review	2023	↑Firmicutes, Pseudomonadota, ↓Bacteroidetes. The Bacteroidetes to Firmicutes ratio tends to be lower	[59]
Systematic review	2025	↑Oscillospira, Dorea, Collinsella. Enriched trends of Oscillospira, Dorea, and Collinsella. Depleted trends of Streptococcus, Akkermansia, Coprococcus, and Dialister.	[60]
Systematic review	2024	↓Bifidobacterium, Bacteroides, Prevotella, Ruminococcus, Lachnospira, Clostridium	[61]
Systematic review	2024	↑Faecalibacterium, Bacteroides, Clostridium, ↓Bifidobacterium, Parabacteroide	[62]
Systematic review	2022	↑Bilophila, Clostridium, Dorea, Lactobacillus, ↓Blautia	[63]

Note: ↑ = increased, ↓ = decreased.

Abbreviations: VD, vitamin D; VDR, vitamin D receptor; ASD, autism spectrum disorder; HPA, hypothalamic-pituitary-adrenal; IPA, indole-3-propionic acid; 5-HT, 5-Hydroxytryptamine; KP, kynurenine pathway; GABA, γ-aminobutyric acid; Glu, glutamate; PPA, propionic acid; CRH, corticotropin-releasing hormone; BBB, blood-brain barrier; ↑, indicates upgrade; ↓, indicates degrade.

Two systematic reviews have specifically summarized the between-group characteristics of alpha diversity and beta diversity of the gut microbiota in ASD. No significant statistical difference in alpha diversity has been observed between ASD and healthy controls, whereas a significant difference in beta diversity has been observed.^58,60 This further suggests that microbial abnormalities in ASD are primarily manifested as a global restructuring of the microbial community architecture, rather than simply alterations in community richness. Notably, common confounding variables such as age, sex, dietary patterns and comorbidities remain insufficient to fully account for the heterogeneity observed in current microbiome studies.⁵⁸

Gut-Brain Axis in ASD

The gut microbiota regulates central nervous system development and function through a complex communication network, collectively termed the gut-brain axis. The gut-brain axis encompasses neuroanatomical, immunological, neuroendocrine and metabolic pathways. This article systematically reviews recent advances in the pathophysiological mechanisms of the gut-brain axis in ASD (Figure 1).

Figure 1.

Gut-brain axis in ASD.

Notes: ↑ indicates upgrade; ↓ indicates degrade.

Abbreviations: VD, vitamin D. VDR, vitamin D receptor. ASD, autism spectrum disorder. HPA, hypothalamic-pituitary-adrenal. IPA, indole-3-propionic acid. 5-HT, 5-Hydroxytryptamine. KP, kynurenine pathway. GABA, γ-aminobutyric acid. Glu, glutamate. PPA, propionic acid. CRH, corticotropin-releasing hormone. BBB, blood-brain barrier.

Neuronal Pathways

The gut microbiota can directly transmit signals to the brain via the vagus nerve. Vagus nerve stimulation (VNS) suppresses the pro-inflammatory activation phenotype of microglia, regulates neurogenesis, maintains synaptic plasticity homeostasis and inhibits abnormal neuronal apoptosis.^64–67 Electroacupuncture-mediated VNS ameliorates social deficits, anxiety-like and depression-like behaviors in ASD mouse models. Transplantation of fecal microbiota from ASD mice into VNS-treated mice attenuated the neuroprotective effects conferred by VNS.⁶⁸ Furthermore, gut bacteria can translocate to the brain via the vagus nerve.⁶⁹ The gut microbiota dysbiosis in ASD may directly compromise central nervous system homeostasis through a dual mechanism involving the vagus nerve pathway: interference with neural signaling and direct bacterial translocation into the brain.

Immunological Pathways

Gut microbiota dysbiosis increases intestinal permeability and inflammatory levels, which may lead to the translocation of gut microbes into the bloodstream, subsequently triggering neuroinflammation by crossing the compromised BBB.

Gut microbiota dysbiosis leads to increased intestinal barrier permeability and intestinal inflammation. Mice transplanted with fecal microbiota from children with ASD exhibit reduced exploratory and social behaviors.^70,71 The intestinal tissues show decreased levels of E-cadherin, claudin-3 and immunoglobulin A (IgA), resulting in increased intestinal barrier permeability and exacerbated intestinal inflammation.^70–72 Intestinal immaturity and intestinal inflammation may jointly contribute to intestinal barrier injury in ASD.^73–75

Impairment of the intestinal barrier structure allows bacterial components or products to translocate into the bloodstream, from where they may reach the brain and trigger aberrant immune responses. Children with ASD exhibit elevated plasma levels of lipopolysaccharide-binding protein (LBP),⁷⁵ a marker of lipopolysaccharide (LPS) translocation from Gram-negative bacteria into the bloodstream. Mice treated with Enterococcus faecium, a Gram-positive bacterium that does not produce LPS, also showed increased serum LPS levels.⁷² This observation suggests that colonization with Enterococcus faecium disrupts microbial balance, promoting the translocation of Gram-negative bacteria into the systemic circulation.

Gut microbiota dysbiosis impairs BBB integrity. Mice colonized with human fecal microbiota from ASD exhibit reduced expression of claudin-5, a key tight junction protein, in cortical tissue, which leads to BBB dysfunction and exacerbates neuroinflammatory responses.^70,72 In addition, gut microbiota dysbiosis modulates the functional activity and resident phenotype of brain-resident T cells, leading to the pro-inflammatory conversion of microglia and further promoting neuroinflammation.⁷⁶

Immune phenomena regulated by gut microbiota dysbiosis have also been observed in the maternal immune activation (MIA) mouse model. Previous studies have demonstrated that the maternal IL-17A pathway contributes to autism-like phenotypes in offspring exposed to MIA.⁷⁷ Transfer of fecal microbiota from pregnant mice exhibiting enhanced IL-17A responses into germ-free mice primes the naive CD4⁺ T cells of the offspring toward a sensitized state, triggering intestinal inflammation.⁷⁸ Recipient mice colonized with microbiota from MIA dams exhibited reduced social ability, decreased gut microbial diversity and increased neuroinflammation.⁷⁹

Neuroendocrine Pathways

The gut microbiota influences host neuroendocrine function through its metabolites. Butyrate, a short-chain fatty acid produced by bacterial fermentation, inhibits histone deacetylases (HDACs) and promotes the expression of corticotropin-releasing hormone receptor 2 (CRHR2), thereby alleviating hypothalamic-pituitary-adrenal (HPA) axis hyperactivity and improving ASD-like symptoms.⁸⁰ Corticotropin-releasing hormone (CRH) is a key regulator of the HPA axis. The release of CRH induces mast cell degranulation, which in turn disrupts BBB integrity.⁸¹

Metabolic Pathways

Children with ASD consistently exhibit abnormal metabolite concentrations. Dysregulated metabolites derived from the gut microbiota serve as key mediators, exacerbating ASD symptoms by affecting BBB permeability, neuroinflammation and neurotransmitter balance. In this section, we review recent advances over the past five years, focusing on short-chain fatty acids (SCFAs), amino acids, phenolic compounds and discuss their alterations in ASD as well as the mechanisms by which they influence ASD pathogenesis.

SCFAs

Disruption of SCFAs produced by the gut microbiota represents an important gut-brain axis mechanism mediating brain dysfunction and behavioral impairments in ASD. The primary SCFA subtypes (acetate, propionate and butyrate) exhibit distinct regulatory effects on neurodevelopment, inflammatory responses and synaptic homeostasis.

Acetate

Acetate concentrations in infants with ASD increase with the severity of autistic symptoms, suggesting that aberrant acetate accumulation is closely associated with early pathological progression of ASD.⁸² Acetate ameliorates social deficits in ASD mice (Shank3-deficient) and remodels transcriptional regulation patterns in the prefrontal cortex, thereby ameliorating central nervous system dysfunction to a certain extent.⁸³

Propionic Acid (PPA)

PPA is a key harmful metabolite mediating neurotoxic injury in ASD. PPA levels are elevated in individuals with ASD. PPA levels are negatively correlated with the abundance of intestinal Lactobacillus and positively correlated with the severity of core ASD symptoms.⁸⁴ Excessive accumulation of PPA induces central nervous system injury through multiple mechanisms: it significantly increases oxidative stress, promotes neuronal apoptosis, activates widespread neuroinflammation, downregulates brain-derived neurotrophic factor (BDNF) and inhibitory γ-aminobutyric acid (GABA) levels, up regulates excitatory glutamate (Glu) levels, subsequently induces excessive astrocyte activation, gliosis and neuronal loss in the hippocampus and cerebellum, thereby disrupting structural integrity in key brain regions.^85–102 PPA induces cerebral demyelination.^87,103 Furthermore, PPA disrupts intestinal barrier integrity.^89,93

Butyrate

ASD mouse models generally exhibit significantly reduced butyrate levels, which is a protective SCFA.¹⁰⁴ Butyrate exerts multiple neuroprotective effects by reversing cerebellar cortical hypertrophy, Purkinje cell firing abnormalities and synaptic plasticity deficits.¹⁰⁵ Butyrate alleviates HPA axis hyperactivity in ASD models, stabilizing central nervous system homeostasis.⁸⁰

Amino Acid Metabolism

Glu and GABA

Glu is the key excitatory neurotransmitter in the central nervous system, whereas GABA is the primary inhibitory neurotransmitter. Glutamine (Gln), a non-toxic and readily transported neutral amino acid, serves as the primary storage and transport form of Glu in the brain. Excitation/inhibition imbalance mediated by Glu and GABA represents a core pathological feature of ASD.

In fecal samples, patients with ASD exhibit decreased levels of Gln and Glu, alongside elevated GABA levels, which are closely associated with overgrowth of Escherichia coli.^106,107 Bifidobacterium species possess the capacity for GABA synthesis, whereas Clostridium species can consume GABA.¹⁰⁸

In plasma samples, patients with ASD exhibit decreased levels of Gln and GABA.^109–112 GABA levels are negatively correlated with the severity of ASD,¹¹³ whereas Glu levels are positively correlated with ASD severity.¹¹⁴

In the ASD brain, the expression of Gln, Glu and GABA exhibits marked brain region specificity. The cerebral cortex (including the temporal, parietal and frontal cortices), thalamus and cerebellum exhibit a hyperexcitable state characterized by elevated levels of Gln and Glu, as well as reduced levels of GABA, which contribute to social dysfunction, sensory abnormalities, and difficulties in emotion regulation.^115–120 In contrast, one study found decreased Gln levels in the cerebrospinal fluid of ASD patients.¹²¹ The left dorsolateral prefrontal cortex and the right hemisphere of the brain exhibit a high-GABA, low-Glu expression profile.^122,123

Different ASD animal models also exhibit inconsistent patterns of neurotransmitter abnormalities. In BTBR mice, GABA levels are reduced in the prefrontal cortex, hippocampus, and amygdala, accompanied by increased Glu levels in the amygdala.¹²⁴ Prenatal valproic acid (VPA)-exposed ASD mice show elevated Glu levels in the prefrontal cortex.¹²⁵ BTBR mice show increased Glu and decreased GABA levels in the auditory cortex.¹²⁶ In contrast, Cntnap2⁻/⁻ mice display low Glu and high GABA levels in the prefrontal cortex.¹²⁷

Tryptophan

Tryptophan metabolism proceeds primarily through three pathways: the 5-HT pathway, the kynurenine pathway (KP) and the indole pathway. Plasma tryptophan levels are negatively correlated with ASD severity.¹¹⁴ Specific tryptophan-related metabolites are decreased in the feces of children with ASD.

5-HT: 5-HT is extensively involved in multiple core neurobiological processes, including neurite outgrowth, dendritic spine remodeling, neuronal circuit formation, synaptic transmission and synaptic plasticity. It plays a critical regulatory role in ameliorating social deficits, empathic dysfunction and learning and memory impairments in ASD.^128–130

Peripheral 5-HT levels in individuals with ASD exhibit characteristic abnormalities with significant sex differences. Children with ASD generally show elevated serum 5-HT levels,¹¹³ a phenomenon that is more pronounced in males.¹³¹ More severe ASD symptoms associated with greater elevations in peripheral 5-HT.^131,132 In central 5-HT depletion mouse models, male individuals exhibit more severe ASD-like social behavioral deficits.¹³³

Brain 5-HT levels in ASD animal models exhibit brain region-specific differences. In the prenatal VPA-exposed autism mouse model, 5-HT levels are decreased in the prefrontal cortex. This abnormality is closely associated with an imbalance in the gut microbiota Bacteroidetes/Firmicutes ratio.^125,134 However, in the thalamus of the same model, 5-HT levels are elevated.¹³⁵ In the maternal LPS-induced ASD-like pup model, 5-HT levels are abnormally increased in the medial prefrontal cortex, accompanied by shortened neuronal dendrite length and reduced dendritic spine density.¹³⁶

Modulating gut microbiota composition upregulates 5-HT levels in both the frontal cortex and intestinal tissues of mice, reversing synaptic dysfunction in autism models.¹³⁷ Enterococcus faecium elevates 5-HT levels,⁷² whereas Ruminococcaceae suppresses 5-HT production.¹³⁸

Kynurenine (KYN): KYN gives rise to two primary downstream metabolites: kynurenic acid (KA), which exerts neuroprotective effects; quinolinic acid (QUIN), which is neurotoxic. Individuals with ASD exhibit KP disturbances, characterized by elevated serum KYN levels and decreased KA levels.^139–142 Reduced KA expression is closely associated with an increased risk of suicidal tendencies in ASD patients.¹⁴³ Considerable heterogeneity exists in KP findings across studies. Some studies have reported concurrent elevations of both serum KYN and KA levels in ASD patients, with no significant difference in QUIN levels compared with healthy controls.¹⁴⁴ Cerebrospinal fluid KYN levels are decreased in ASD patients.¹²¹

The gut microbiota regulates KP homeostasis, modulates central neurotransmitter levels and brain inflammation, ultimately ameliorating abnormal ASD phenotypes. Bifidobacterium longum comprehensively regulates KP metabolism in the peripheral gut, bloodstream and central brain. It modulates cerebral levels of the neurotoxic metabolite QUIN and regulates brain Glu and GABA levels, the Glu/GABA balance. This strain inhibits excessive microglial activation in the cerebellum, alleviating central neuroinflammatory injury.¹⁴⁵

Indole-3-propionic acid (IPA): The third branch of tryptophan metabolism is the gut-specific indole metabolic pathway, in which IPA is a core protective metabolite synthesized exclusively by the gut microbiota. IPA production is reduced in individuals with ASD.¹⁴⁶ Insufficient IPA synthesis leads to decreased IPA entry into the hippocampus via the peripheral circulation, which in turn triggers aberrant activation of the hippocampal IPA/AhR/NF-κB signaling pathway. This ultimately results in excessive microglial activation and neuronal synaptic over-pruning in the hippocampus.¹⁴⁶ Exogenous IPA supplementation upregulates ERK1 phosphorylation levels in the hippocampus and repairs inhibitory synaptic transmission deficits.¹⁴⁷ Microbial correlation studies have further confirmed that the abundances of Anaeromyces and Romboutsia are negatively correlated with IPA levels,¹⁴⁸ indicating that gut microbiota dysbiosis is a core trigger of IPA metabolic disturbance.

Phenolic Compounds

p-Cresol: Fecal levels of p-cresol are elevated in individuals with ASD. Excessive exposure to p-cresol directly triggers typical ASD-like abnormal behaviors, establishing it as an important risk factor driving ASD pathological phenotypes.¹⁴⁹ p-Cresol disrupts central nervous system homeostasis through multiple pathways. At the neuroimmune level, p-cresol downregulates the expression and maturation of ADAM17 and ADAM10, specifically impairing normal neuroimmune responses and disrupting the central nervous system microenvironment.¹⁵⁰ At the synaptic development level, p-cresol inhibits dendritic growth, synaptogenesis and synaptic function in hippocampal neurons, directly impairing hippocampal neuroplasticity.¹⁵¹ P-cresol targets and damages the brain’s social reward circuitry, mediating social deficits in ASD. The ventral tegmental area of the midbrain is a core catecholaminergic region in the social reward circuit, governing normal social behavior and emotional reward processing. p-Cresol exposure reduces dopaminergic neuron activity in the ventral tegmental area and impairs the biosynthesis and normal metabolism of catecholamine neurotransmitters. This results in insufficient neurotransmitter synthesis in the reward pathway and reduced neuronal activity.^152,153

Disrupted SCFA metabolism, central excitation/inhibition imbalance, disturbances in the three major tryptophan metabolic pathways and abnormal accumulation of the gut-derived toxic metabolite p-cresol collectively constitute the core pathological basis of neurometabolic disturbances in ASD. Gut-brain axis dysregulation serves as a key upstream trigger, sequentially leading to abnormalities in neurotransmitter metabolism in body fluids, region-specific neurotransmitter imbalances across multiple brain regions, neuroimmune activation, synaptic structural and functional damage and reward circuit dysfunction.

Potential Interactions Between Vitamin D and Gut-Brain Axis in ASD

Gut microbiota dysbiosis comprehensively participates in the onset and progression of ASD through the gut-brain axis by mediating neurotransmitter imbalances, tryptophan metabolic abnormalities, disturbances in SCFAs and toxic metabolites, neuroinflammation and brain network abnormalities. Vitamin D regulates the neuropathological progression of ASD through multiple dimensions, including brain structure, brain function, neurotransmitters and signaling pathways. On one hand, vitamin D deficiency or vitamin D supplementation modulates gut microbiota composition and diversity, microbial metabolic profiles, intestinal immune responses and intestinal epithelial barrier integrity in both humans and experimental animals, comprehensively improving gut microecological health. On the other hand, gut microbial metabolites reciprocally regulate the expression of the intestinal VDR, mediating the biological effects of vitamin D.¹⁵⁴ This bidirectional interaction jointly establishes a vitamin D-gut-brain axis, which influences ASD pathological progression and core symptomatic phenotypes at multiple levels.

Vitamin D Modulates Gut Microbiota, Intestinal Barrier, Gut Immunity and Bacterial Translocation

The vitamin D/VDR signaling pathway serves an upstream initiating role in the gut-brain axis regulatory network by modulating gut microbiota composition, intestinal epithelial barrier function, innate and adaptive immune responses, intestinal inflammation and bacterial translocation.^154–156

Vitamin D reshapes gut microbiota composition and abundance characteristics. In the gut microecology of healthy individuals, the abundances of Coprococcus and Bifidobacterium are negatively correlated with vitamin D levels.¹⁵⁷ These genera have been confirmed to be closely associated with ASD pathogenesis. Multiple disease model studies have demonstrated that vitamin D ameliorates gut microbiota dysbiosis and restores normal microecological structure.^158,159 Animal experiments further show that vitamin D intervention alters the abundance of specific bacterial taxa. Vitamin D downregulates Verrucomicrobia abundance in wild-type mice.¹⁶⁰ Vitamin D downregulates Akkermansia abundance in Cyp27b1⁻/⁻ mice.¹⁶¹ Maternal vitamin D deficiency shapes an ASD-susceptible gut microbiota structure in offspring. The offspring of vitamin D-deficient dams exhibited increased abundances of Akkermansia and Turicibacter, along with decreased abundances of Fusicatenibacter and Allobaculum.³⁴

Intestinal VDR deficiency triggers gut microbiota dysbiosis.¹⁶² Intestinal epithelium-specific VDR deficiency alters gut microbial community structure, leading to depletion of Lactobacillus, Alistipes and Odoribacter, while enriching Clostridium, Bacteroides and Eggerthella.¹⁶³ In tumor disease models, intestinal epithelial VDR deficiency shifts the gut microbiota from a healthy homeostatic state toward a disease-susceptible state, accompanied by exacerbated intestinal inflammation and accelerated disease progression.^164,165

Multiple animal models of various diseases have consistently demonstrated that vitamin D ameliorates intestinal permeability abnormalities and intestinal inflammation, thereby slowing disease progression.^{160,166–174} Mechanistically, vitamin D upregulates the expression of key tight junction proteins, repairing the intestinal epithelial tight junction structure. Vitamin D elevates the levels of antioxidant factors such as superoxide dismutase, reduces the expression of inflammatory cytokines, alleviates intestinal oxidative stress and inflammatory infiltration, maintaining intestinal barrier integrity.

The vitamin D/VDR signaling pathway regulates intestinal innate and adaptive immune homeostasis. At the level of intestinal innate immunity, vitamin D targets Paneth cells, group 3 innate lymphoid cells (ILC3s) and macrophages. Vitamin D deficiency leads to thinning of the colonic mucus layer and substantially increases the risk of bacterial translocation to mesenteric lymph nodes.¹⁶¹ Intestinal epithelial VDR deletion impairs the antimicrobial function of Paneth cells,¹⁷⁵ whereas vitamin D/VDR signaling promotes the synthesis and secretion of antimicrobial peptides such as defensins and lysozyme by Paneth cells, strengthening the intestinal antimicrobial defense system and stabilizing intestinal tight junction structures.^176–179 ILC3s are key cells maintaining the intestinal mucosal barrier and protective immunity. Vitamin D deficiency or VDR knockout results in a reduction in colonic ILC3 numbers and immune defense responses.^180,181 Vitamin D inhibits hyperactivation of the JAK-STAT pathway through a VDR-dependent mechanism, reversing macrophage polarization imbalance.¹⁷³

At the level of intestinal adaptive immunity, vitamin D regulates the differentiation and functional activation of dendritic cells (DCs), T cells and B cells, maintaining intestinal immune tolerance. VDR deficiency leads to a reduction in the number of tolerogenic DCs,¹⁸² indicating that adequate vitamin D promotes DCs to remain in a tolerogenic state through VDR activation, reducing the risk of intestinal inflammation caused by excessive pro-inflammatory cytokine release. DCs regulate T cell activation and homing properties. Vitamin D synergizes with plasmacytoid dendritic cells (pDCs) to promote the generation of gut-homing α4β7⁺ CX3CR1⁺ regulatory B cells and α4β7⁺ regulatory T cells, which migrate to the gastrointestinal mucosa.¹⁸³ CX3CR1⁺ regulatory B cells suppress excessive activation of effector CD4⁺ T cells, mitigating overactive intestinal adaptive immune responses.¹⁸⁴ Vitamin D promotes the differentiation and generation of IgA⁺ B cells and increases intestinal IgA secretion. IgA specifically binds to gut microbiota, maintains microecological homeostasis, exerting anti-inflammatory protective effects. Additionally, sufficient maternal vitamin D limits the programming and differentiation of offspring CD4⁺ T cells into inflammatory effector T cells induced by maternal inflammation in ASD.⁵³

Gut microbiota dysbiosis, intestinal barrier impairment and intestinal inflammation form a vicious cycle that promotes bacterial translocation, contributing to peripheral inflammation and central nervous system injury. Vitamin D or VDR deficiency promotes bacterial translocation, leading to elevated LPS levels.^157,165 Once in the circulation, LPS can cross the BBB, triggering chronic neuroinflammation. Notably, vitamin D intervention inhibits bacterial translocation and blocks the transmission of peripheral inflammation to the central nervous system.¹⁷¹

Vitamin D Regulates the BBB and Neuroinflammation

BBB integrity impairment and excessive central neuroinflammation are core central mechanisms underlying neurodevelopmental injury in ASD. The vitamin D/VDR signaling pathway stabilizes the BBB structure, inhibits neuroinflammatory cascades and blocks peripheral inflammation from invading the central nervous system.

Vitamin D is a key protective factor for maintaining BBB structural integrity and regulating cerebral vascular permeability. Vitamin D deficiency leads to increased immunoglobulin extravasation in brain tissue, increasing cerebral vascular permeability.¹⁸⁵ Vitamin D reduces the abnormal accumulation of exogenous substances in brain tissue, restricting the entry of harmful substances into the brain.^186,187 Multiple disease and intervention models have confirmed that vitamin D upregulates the expression of key BBB integrity markers, including occludin-1, claudin-3, claudin-5, occludin, annexin A1, VE-cadherin and osteopontin.^188–191 VDR deficiency promotes excessive secretion of pro-inflammatory cytokines such as TNF-α and IFN-γ by microglia/macrophages, which in turn induces endothelial cells to release CXCL10, continuously exacerbating BBB disruption and central inflammatory infiltration.¹⁹²

Vitamin D comprehensively ameliorates neuroinflammation. The neuroprotective effects of vitamin D encompass multiple dimensions, including inhibition of pro-inflammatory cytokine release, suppression of microglial M1 pro-inflammatory polarization, reduction of oxidative stress injury, attenuation of neuronal apoptosis and pyroptosis and modulation of neurotransmitter synthesis.^{188,190,193–205} Studies in ASD animal models have demonstrated that vitamin D during pregnancy reduces the overexpression of typical pro-inflammatory cytokines such as IL-6 and IL-17A in the offspring’s brain, effectively ameliorating ASD-like neuropathological phenotypes.⁵²

Vitamin D Regulates the HPA Axis Functional Homeostasis

The HPA axis dysfunction is an important contributor to neurodevelopmental abnormalities and behavioral dysregulation in ASD. Vitamin D enhances corticosterone-mediated negative feedback inhibition, calming the hyperactivated HPA axis, blocking secondary neurological, immune and intestinal functional injuries.¹⁶⁷

Vitamin D Regulates Gut Metabolites

Vitamin D Regulates Glu/GABA

Vitamin D alleviates neurotoxicity and neuronal injury induced by glutamate metabolic disturbances. VD deficiency reduces the levels of Gln and Glu in brain tissue,^206,207 whereas VD intervention upregulates Gln and Glu levels.²⁰⁸ In Glu toxicity injury models, excessive monosodium glutamate (MSG) intake leads to abnormal accumulation of Glu in both the peripheral blood and brain, inducing central excitotoxicity. Vitamin D attenuates MSG-induced neuronal injury by upregulating VDR expression.^197,209 Vitamin D activates the mTOR signaling pathway to counteract neuronal apoptosis and central neuroinflammation induced by excessive Glu stimulation.¹⁹⁸

Studies on vitamin D regulation of GABA have shown brain region-specific differences. One study reported that vitamin D deficiency increases baseline GABA levels in the striatum.²¹⁰ In contrast, Pei Jiang et al found that vitamin D elevates GABA levels in the prefrontal cortex and hippocampus.²⁰⁸ Vitamin D deficiency disrupts the exocytotic release and uptake of Glu and GABA, resulting in central Glu/GABA neurotransmitter imbalance and synaptic transmission dysfunction.²¹¹

Vitamin D Regulates 5-HT and KYN

Vitamin D deficiency leads to insufficient central 5-HT synthesis.^207,212 Vitamin D upregulates the expression of tryptophan hydroxylase 2 (TPH2) in the brain by activating the VDR, promoting brain 5-HT production.^213,214 These findings have been validated in the VPA-induced rat model of ASD, where vitamin D intervention significantly increases brain 5-HT levels and alleviates ASD-like neuropathological abnormalities.⁴³

Vitamin D participates in regulating the homeostasis of the KP, exerting both neuroprotective and psychological risk-modulating effects. In a clinical study involving deep brain stimulation in Parkinson’s disease patients, vitamin D was found to elevate KYN concentrations and increase the levels of the neuroprotective metabolite KA.²¹⁵ Reduced KA levels in ASD are closely associated with an increased risk of suicidal tendencies.¹⁴³ Vitamin D elevates protective KA metabolite levels, which are considered a potential bioactive factor for reducing the risk of suicide and suicide attempts.²¹⁶ The VPA-induced rat model of ASD further clarified the inhibitory regulatory effect of vitamin D on the KP, demonstrating that vitamin D intervention reduces the accumulation of abnormal intermediate metabolites in the KP and decreases neurotoxic metabolite accumulation.⁵¹ Notably, no published studies to date have identified a clear regulatory association between vitamin D and IPA.

Vitamin D Regulates Acetate, Propionate and Butyrate

Vitamin D positively regulates intestinal acetate synthesis and gut microbiota homeostasis, while acetate, in turn, mediates immune repair under vitamin D-deficient conditions. Vitamin D directly promotes the proliferation of the acetate-producing dominant bacterium Cetobacterium somerae and enhances its acetate synthesis capacity.²¹⁷ Acetate alleviates the suppression of β-defensin expression in vitamin D-deficient zebrafish, restoring innate immune function.²¹⁷ Acetate metabolic imbalance exerts toxic effects. In an acetate-induced colitis model, excessive acetate elevates colonic levels of pro-inflammatory cytokines, downregulates the tight junction protein occludin, disrupts the intestinal barrier and induces severe intestinal inflammation. Vitamin D counteracts acetate-induced intestinal toxicity.²¹⁸

Vitamin D deficiency leads to elevated ileal PPA levels in ASD mice.³⁴ In mice with short bowel syndrome, PPA upregulates intestinal VDR expression by activating Yes-associated protein (YAP) signaling, thereby activating the vitamin D signaling pathway.²¹⁹ However, this regulatory mechanism has not yet been validated in ASD models.

Vitamin D and butyrate also exhibit a potent bidirectional synergistic relationship. Vitamin D ameliorates gut dysbiosis by inhibiting Enterococcus overgrowth and enriching butyrate-producing bacteria, indirectly increasing butyrate levels and alleviating ASD pathological damage.^72,173,220 Intestinal VDR deletion exacerbates butyrate deficiency by downregulating the abundance of the butyrate-producing genus Butyricimonas and the expression of fecal butyryl-CoA transferase, a key enzyme for butyrate synthesis.^164,175 Butyrate upregulates VDR expression by activating the transforming TGF-β/Smad3 signaling pathway, exerting synergistic effects with vitamin D.^221,222 The combination of butyrate and vitamin D synergistically enhances intestinal inflammation suppression, barrier repair, and antioxidant capacity.^{175,223–226} Furthermore, butyrate and vitamin D synergistically regulate HPA axis homeostasis, jointly intervening in ASD pathological progression at the neuroendocrine level.^80,167

Considering the overall regulatory network of the gut-brain axis, with the exception of the vagus nerve pathway, the vitamin D/VDR signaling pathway is involved in almost all aspects of gut-brain axis-mediated ASD pathogenesis. It comprehensively regulates gut microbiota composition, intestinal barrier integrity, gut immunity, bacterial translocation, the BBB, neuroinflammation, gut metabolism, and neuroendocrine homeostasis. Current studies in ASD models have clearly demonstrated that vitamin D targets and regulates gut microbiota homeostasis, PPA metabolism, 5-HT and KYN metabolic pathways, central neuroinflammation and intestinal T-cell immune function. To date, there is a lack of human intervention studies investigating the modulation of gut microbiota through vitamin D supplementation in individuals with autism spectrum disorder.

Conclusion

This review systematically synthesizes the evidence linking vitamin D, gut microbiota and ASD, and integrates these findings to construct a potential regulatory framework for the vitamin D-gut-brain axis. Vitamin D acts as an upstream core regulator by remodeling gut microbiota composition and diversity, repairing intestinal epithelial barrier damage, balancing gut immune homeostasis, inhibiting aberrant bacterial translocation, stabilizing central BBB integrity, suppressing excessive central neuroinflammation, modulating HPA axis neuroendocrine hyperactivity and balancing the homeostasis of multiple key metabolites. Through these actions, vitamin D comprehensively regulates gut-brain axis function. Concurrently, the gut microbiota, through its derived metabolites, reciprocally regulates intestinal VDR expression and vitamin D/VDR signaling pathway activity, forming a closed-loop bidirectional interaction system between vitamin D and the gut microbiota that jointly influences ASD disease progression and symptom severity.

However, the construction of the above integrative framework must be grounded in a sober appraisal of the current evidence hierarchy. First, the evidence currently supporting the role of the vitamin D-gut-brain axis in ASD derives primarily from correlational human studies (cross-sectional and case-control designs) and rodent models, with causal inference fundamentally constrained. Second, some of the key mechanistic evidence originates from non-ASD disease models (eg, inflammatory bowel disease, short bowel syndrome), and extrapolation to the ASD pathological context warrants caution. Third, ASD gut microbiota research is generally beset by issues such as small sample sizes, geographic and dietary heterogeneity, variations in sequencing methodologies, and a lack of longitudinal follow-up, such that the reproducibility and generalizability of existing conclusions remain to be validated. Fourth, direct causal evidence for the bidirectional interaction between vitamin D and the gut microbiota in humans is extremely limited, and systematic studies that simultaneously integrate vitamin D status, gut microbiota profiles, host genomics, and ASD phenotypes are still lacking.

In light of the above limitations, the current evidence suggests that the vitamin D-gut-brain axis may be involved in the pathophysiology of ASD; however, its core role, causal direction and clinical operability in ASD still require further validation. Future research should be advanced in the following directions: (1) conducting prospective cohort studies stratified by baseline 25(OH)D levels and gut microbiota characteristics to track the effects of vitamin D supplementation on ASD symptoms and dynamic changes in the microbiome; (2) employing Mendelian randomization to circumvent confounding factors and enhance the causal inference regarding the influence of vitamin D and gut microbiota on ASD pathogenesis; (3) utilizing mediation analysis to quantify the mediating contribution of gut microbiota and their metabolites in vitamin D-regulated neurodevelopment; (4) establishing longitudinal mother-child cohorts from pregnancy through childhood to delineate the time windows and dose-dependent effects through which maternal vitamin D status influences offspring neurodevelopment via gut microbiota colonization; and (5) on this basis, exploring precision intervention strategies tailored to individual vitamin D status and microbiota profiles, and validating their efficacy and safety through rigorously designed randomized controlled trials. Only through the progressive refinement of the above evidence base can the vitamin D-gut-brain axis truly transition from concept to clinical application.

Abbreviations

ASD, autism spectrum disorder; DALY, disability-adjusted life year; DBP, vitamin D-binding protein; CYP2R1, 25-hydroxylase; 25(OH)D, 25-hydroxyvitamin D; CYP27B1, 1α-hydroxylase; 1,25(OH)₂D, 1,25-dihydroxyvitamin D; VDR, vitamin D receptor; DQ, developmental quotient; 5-HT, 5-Hydroxytryptamine; rs-fMRI, resting-state functional magnetic resonance imaging; CARS, Childhood Autism Rating Scale; VNS, vagus nerve stimulation; IgA, immunoglobulin A; LBP, lipopolysaccharide-binding protein; LPS, ipopolysaccharide; MIA, maternal immune activation; HDACs, histone deacetylases; CRHR2, orticotropin-releasing hormone receptor 2; HPA, ypothalamic-pituitary-adrenal; CRH, orticotropin-releasing hormone; SCFA, hort-chain fatty acid; PPA, ropionic acid; BDNF, rain-derived neurotrophic factor; GABA, γ-aminobutyric acid; Glu, glutamate; Gln, glutamine; VPA, alproic acid; KP, kynurenine pathway; KYN, ynurenine; KA, ynurenic acid; QUIN, quinolinic acid; IPA, indole 3-propionic acid; ILC3, group 3 innate lymphoid cell; DC, dendritic cell; pDC, plasmacytoid dendritic cell; MSG, monosodium glutamate; TPH2, tryptophan hydroxylase 2; YAP, Yes-associated protein.

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Disclosure

The authors report no conflicts of interest in this work, including disclosure of financial interests or other conflicts of interest.