Vitamin B12 and Affective Disorders: A Focus on the Gut-Brain Axis

Several clinical studies have suggested that low serum VitB12 levels and high Hcy concentrations are associated with affective disorders in both adolescents and adults [32, 33, 34, 35, 36, 37, 38, 39], supporting the hypothesis that VitB12 deficiency may lead to Hcy accumulation [7]. Individuals with depression have been found to exhibit significantly higher serum Hcy levels compared to healthy controls, and hyperhomocysteinemia (defined as serum Hcy level >15 µmol/L) has been linked to an increased risk of developing depressive symptoms [32]. In patients with BD, Hcy levels may serve as a potential predictor of mood states, with elevated Hcy levels observed during manic and euthymic phases compared to healthy individuals. Notably, Hcy levels peaked during manic episodes, correlating with greater symptom severity [38]. Furthermore, elevated Hcy levels were linked to poorer clinical outcomes, including higher rates of mixed affective states [33], increased relapse risk, and pronounced cognitive deficits [37]. Collectively, these findings underscore the potential role of Hcy in both the pathophysiology and progression of mood disorders.

Despite growing evidence, the causal relationship between serum VitB12 levels and affective disorders remains unclear. It has been hypothesized that low VitB12 and elevated Hcy levels may contribute to the onset of affective symptoms. Alternatively, the progress of affective disorder and associated pharmacological treatments may negatively impact dietary intake or intestinal absorption of VitB12, leading to secondary deficiency [35]. Sex- and age-related variations further complicate this relationship. For instance, men generally present with higher Hcy levels than women, possibly due to estrogen's regulatory effects on Hcy metabolism [40, 41]. Observational studies also indicate that deficiencies in both VitB12 and folate contribute to elevated Hcy levels, suggesting that VitB12 insufficiency alone may not fully explain the risk of mood disorders. Mechanistically, VitB12 serves as a coenzyme for methionine synthase, an enzyme essential for the remethylation of Hcy to methionine using 5-methyltetrahydrofolate as a methyl donor. Thus, both VitB12 and folate deficiencies would hamper this process, leading to the accumulation of Hcy and subsequent disruptions in neurotransmitter synthesis, DNA methylation, myelin maintenance, and other processes critical to neurological and mental health [42].

Epidemiological data suggest that approximately 5% of general psychiatric inpatients present with low serum VitB12 levels, with this figure rising to 10–20% among elderly individuals. Notably, chronic VitB12 deficiency spanning over 40 years may contribute to the development of Alzheimer's disease or multiple sclerosis, suggesting that the neurological consequences of VitB12 deficiency may develop progressively over long-term exposure [43]. Furthermore, subclinical VitB12 deficiency (blood level less than 200 µmol/L), defined as a state of inadequate VitB12 in the absence of overt clinical symptoms or with only mild symptoms [44], has also been implicated in affective disorders. These findings suggest that even mild or asymptomatic deficiencies could induce subtle physiological impairments, potentially influencing mental health outcomes. Nevertheless, the mechanisms by which subclinical VitB12 deficiency contributes to affective disorders, as well as its broader systemic effects, call for more comprehensive investigation.

Emerging evidence reveals neuroinflammation as a critical mediator of neural impairment in mood disorders [45]. Chronic neuroinflammatory processes, driven by dysregulated immune signaling, disrupt synaptic plasticity, impair neurogenesis, and damage neuronal networks, particularly in brain regions like the prefrontal cortex, hippocampus, and amygdala, which govern emotional regulation and cognitive function [45]. Activated microglia,as well as pro-inflammatory cytokines (e.g., IL-6, TNF-α), participate in perpetuating oxidative stress, mitochondrial dysfunction, and excitotoxicity, further compromising neuronal integrity [46]. Notably, sustained neuroinflammation correlates with treatment resistance and core symptoms of mood disorders, including anhedonia and cognitive deficits [47]. While anti-inflammatory therapies show promise in preclinical models, the bidirectional relationship between neuroinflammation and neural dysfunction, as well as its clinical implications, remains an area highlighting the need for more investigations.

VitB12 exerts neuroprotective effects on neural tissues through modulating DNA methylation and bioactive factors metabolism [48, 49, 50]. For instance, de Queiroz et al. [48] revealed that VitB12 counteracted hippocampal neuronal damage in a rat model of bacterial meningitis by modulating DNA methylation, stabilizing genomic integrity, and upregulating anti-inflammatory gene expression. S-adenosyl-l-methionine (SAMe), a methyl donor engaging in DNA methylation for genomic instability [48], is not only synthesized systemically through VitB12-engaged one-carbon metabolism [51] but also is derived from gut microbiota in the intestinal lumen [52]. Notably, SAMe itself exhibits antidepressant-like properties via promoting serotonin synthesis and activating the serotonin 1A receptor activation [53]. Similarly, Suryavanshi et al. [50] reported that VitB12 supplementation in diabetic models reduced neuronal apoptosis and restored levels of brain-derived neurotrophic factor (BDNF), a key mediator of nerve development, survival, and synaptic plasticity [54]. Reduced levels of BDNF were strongly linked to depression, and its upregulation was a recognized mechanism of antidepressant treatment [54, 55]. Collectively, this evidence highlights the potential of VitB12 in enhancing resilience against affective disorders by preserving neuronal stability through epigenetic, anti-inflammatory, and neurotrophic pathways.

VitB12 alleviates neuroinflammation through multiple mechanisms. For example, Jonnalagadda et al. [22] reported that neuroinflammation suppressed the VitB12-transcobalamin 2-CD320 pathway in an animal model of multiple sclerosis, and fingolimod, a therapeutic agent for multiple sclerosis, exerted its anti-inflammatory effects via restoring this pathway. Cassiano et al. [56] uncovered that VitB12 attenuated inflammatory infiltrate in the central nervous system, thereby mitigating hippocampal damage and preventing neurological dysfunction in pneumococcal meningitis [57]. These findings collectively implied that the VitB12-mediated modulation of neuroinflammatory processes in the central nervous system was intricately linked to the pathophysiology of neuropsychiatric diseases. In addition, peripheral inflammation alone can trigger functional and structural dysconnectivity in frontostriatal, amygdala-prefrontal, and interoceptive circuits. Connectomic analysis showed that dysregulation within these neuronal dynamic networks underlay mood fluctuations, a core clinical manifestation of BD [58]. Systemic inflammation further disrupted the blood-brain barrier (BBB) and up-regulated reactive oxygen species (ROS), thus creating a neurotoxic milieu that exacerbates affective disorders [57, 59, 60, 61]. This also implied the potential of peripheral inflammatory profiles as predictive biomarkers for affective disorders.

Taken together, VitB12 deficiency leads to the accumulation of ROS, neuroinflammation, and demyelination, contributing to a vicious circle in the central nervous system [57] and potentially involving in the pathogenesis of neuropsychiatric disorders. Conversely, VitB12 supplementation demonstrates neuroprotective properties by mitigating neuronal damage, stabilizing neural integrity, and suppressing inflammatory pathways. However, clinical evidence is lacking to validate this assumption, especially in patients with affective disorders.

The impact of dietary derived VitB12 on the gut microbiota remains a subject of debate, owing to inconsistent findings across in vitro and in vivo studies. In vitro experiments suggested that VitB12 supplementation enhanced α-diversity (a measure of microbial community richness and functional redundancy) and altered β-diversity (an indicator of shifts in gut microbiome composition). It also promoted the growth of specific bacteria taxa and boosted the production of short-chain fatty acids (SCFAs). However, these findings have not been consistently replicated in animal studies or population-based research, with variations observed in microbial composition at both phylum and genus levels and no consensus on overarching patterns. Discrepancies may arise from differences in VitB12 formulation, dosage, interactions with co-administered nutrients or medications, and host-specific variables [8, 62, 63]. Recent studies in Caenorhabditis elegans [9, 64] and zebrafish [16] have illuminated the role of VitB12 in modulating the GBA, affecting balanced gut microbial networks and cholinergic signalling. In humans, Oliphant et al. [5] found that gut microbial function in VitB12 synthesis was positively associated with cognitive performance and stress regulation in infants, suggesting its critical role in neurodevelopment. Given that the gut microbiome stabilizes by around three years of age [65], further research is warranted to explore the benefits of VitB12 intervention in infants and pregnant women, potentially harnessing early developmental plasticity to strengthen resilience against affective disorders later in life.

Within the gut microenvironment, VitB12 serves as an essential nutrient for microbial communities. About 20% of gut bacteria synthesize VitB12 de novo, while 80% possess receptors for absorbing VitB12, highlighting its important role in microbial survival and cooperation [66]. There are two major routes (aerobic and anaerobic) by certain gut microbes to synthesize VitB12 de novo. The two routes diverge in their cobalt chelation mechanisms and oxygen dependencies, as the aerobic pathway requires oxygen to catalyze ring-contraction, whereas the anaerobic pathway proceeds independently of oxygen [67]. VitB12 is involved in cross-feeding interactions in microbe-microbe and microbe-host patterns [68], and microbe-derived VitB12 is self-sufficient in healthy adult gut [69]. For Escherichia coli , the presence of VitB12 suppresses the expression of BtuB, a gene encoding a transportor responsible for active VitB12 intake [70]. This feedback mechanism illustrates how commensal bacteria dynamically regulate VitB12 utilization to adapt to its availability, optimizing metabolic efficiency within the gut ecosystem.

VitB12 supplementation has limited effects on the gut environment in both in vitro and in vivo studies [8, 69], suggesting the existence of regulatory mechanisms that stabilize VitB12 levels in the gut lumen. For example, Kelly et al. [71] research indicated that high-dose VitB12 supplementation suppressed the endogenous VitB12 production by gut microbiota, suggesting that gut microbiota can sense VitB12 concentrations and regulate its production. Similarly, Li et al. [72] confirmed that in Propionibacterium strain UF1, a high dose (750 µM) of VitB12 completely inhibited gene expression responsible for VitB12 biosynthesis. These findings highlight how VitB12-dependent microbiota and their metabolic activities are integrated into a homeostatic system. Notably, while VitB12 supplementation depleted Bacteroides populations, it failed to change SCFAs concentration or microbial diversity in the distal gut, despite depleting, implying the intrinsic resilience in the gut lumen [71]. Further complexity arises from interactions between gut microbiota and corrinoids, a class of compounds including cobalamin and cobalamin analogs. High-dose (exceeding the recommended levels by 1000-fold) VitB12 supplementation temporarily disrupted corrinoid profiles, but it returned to the baseline level within 10 days [4, 73]. Degnan et al. [73] even proposed an inspiring idea to use non-metabolizable corrinoids that can evade microbial remodelling mechanisms to manipulate the composition and function of gut microbiota. However, major questions remain regarding how microbes discriminate among corrinoids, the dynamics of cobalamin-to-analog conversion, and the precise molecular mechanisms that govern microbial remodeling processes. Altogether, these findings challenge the assumption that dietary VitB12 supplementation has a straightforward or broadly beneficial impact on gut microbiota. Instead, they underscore the complexity and resilience of microbial ecosystems and highlight the need for deeper investigation into how VitB12 and related compounds functionally reshape gut microbial ecology and, by extension, influence mental health outcomes.

Clinical evidence highlights the role of VitB12 in mediating the interactions between GBA and affective disorders. Both dietary and gut microbiota-derived VitB12 help to keep a complex, balanced, and stable gut microbial network [9, 16]. Studies consistently indicated discrepancies in gut microbiota composition, stability index, and disrupted metabolite profiles in individuals with affective disorders compared to healthy controls. For example, in patients with MDD, fecal samples mostly showed decreased α-diversity, increased relative abundance of Actinobacteria (phylum), Bifidobacteriaceae (family), and Bacteroides (genus), alongside decreased abundance of Ruminococcaceae (family), Faecalibacterium, and Roseburia (genus) [74]. Strikingly, fecal microbiota transplantation from MDD patients to germ-free mice can induce depression-like behaviors [75], while antidepressant (R)-ketamine treatment converted the gut microbial dysbiosis and behavioral deficits [76]. Similarly, unpredictable chronic mild stress in rats led to depressive-like behaviors and altered metabolomic profiles in the hippocampus and jejunum [77], further implicating gut-brain crosstalk. Likewise, Bacteroidetes was the predominant gut phylum in BD patients, while Firmicutes predominated in healthy controls, and quetiapine treatment can alter gut microbial composition in patients with BD [78, 79, 80]. Moreover, serum metabolomic analysis in BD patients revealed distinct signatures compared to healthy controls, with gut microbe-derived "neuroactive metabolites" correlating with symptom severity [81]. These findings implicate VitB12 as a potential modulator of GBA dysfunction in affective disorders, linking microbial stability and host metabolic activities.

VitB12 plays a significant role in modulating inflammation implicated in MDD and BD [82, 83]. It activates gene expression in ileal epithelial cells by sustaining cellular methylation programs, thereby stimulating cell proliferation, enhancing SCFAs metabolism, and suppressing pro-inflammatory pathways [51]. SCFAs (e.g., acetate, propionate, and butyrate) are produced by gut microbiota through the fermentation of dietary fibres [84]. These metabolites can strengthen gut mucosal barrier [85], exert anti-inflammatory effects [12, 51], maintain enteric serotonin production [86], up-regulate BDNF [87], and reduce ROS accumulation [88]. In addition to dietary VitB12 [69], microbe-derived pseudo-VitB12 can promote propionate production independent of diet [89]. Meanwhile, butyrate, propionate, as well as VitB12 itself, serve as protectors to the gut barrier [16, 51, 89]. Food-derived SCFAs can normalize the immunodeficiency of germ-free mice via promoting microglia maturation, suggesting their role in maintaining immune homeostasis [90, 91]. Furthermore, SCFAs can cross the gut-blood barrier and exert an anti-inflammatory effect via binding to G-protein-coupled receptor 43 in systematic circulation [92]. Microbe-derived propionate protects BBB against ROS-induced damage [93]. Moreover, emerging research found that the vagus nerve acted as a key regulator in the gut-brain axis, bidirectionally mediating inflammatory signals between the brain and peripheral immune response [94]. The vagus nerve also transmits brain stress signals to regulate gut microbiota and immune activity [95]. This intricate crosstalk positions VitB12 and SCFAs as critical regulators of neuroimmune interactions in affective disorders.

Under pathological conditions of affective disorders, gut microbial composition exhibits a transdiagnostic pattern, characterized by a reduction in the abundance of butyrate-producing bacteria with anti-inflammatory properties and an increase in pro-inflammatory genera [12, 96]. Disturbance of the gut microbiome disrupts the permeability of the intestinal barrier, allowing the translocation of inflammatory molecules, bacterial endotoxins, and neuroactive metabolites into systemic circulation. Through bidirectional gut-brain crosstalk [81], these mediators propagate chronic systemic inflammation, which compromises the BBB and drives neuroinflammation [97, 98]. Collectively, VitB12, gut microbiota, SCFAs, gut mucosal defenses, and vagal neurotransmission interact synergistically within the GBA to maintain gut and neuroimmune homeostasis and enhance resilience to affective disorders (Fig. 3).