The selection of Vit B12 as the focus of this study was based on its emerging significance in PD pathophysiology and clinical manifestations. Given the vital role of Vit B12 in neural function, its deficiency may contribute to neurodegeneration and exacerbate motor and non-motor symptoms in PD patients. Vit B12 deficiency plays an important role in patients who chronically use oral Levodopa (LDopa) and those who undergo its intestinal formulation, contributing to the acceleration of damage to peripheral nerves. Therefore, a comprehensive exploration of the role of vitamin B12 in PD is warranted to understand its implications for disease management and potential therapeutic interventions.

Literature search

A systematic literature search was conducted using electronic databases including “PubMed” and “Scopus”. The search strategy included combinations of keywords such as "Parkinson's Disease", “Parkinsonism", “Vitamin B12", "Neuropathy", "Homocysteine", "Cognition", "Motor", "Non-motor", "therapeutic", “supplementation”, “treatment” and “outcome”.

Assessment of vitamin B12 status

The assessment of Vit B12 status in included studies varied with measurements typically conducted through serum or plasma levels of Vit B12. Additional biomarkers such as homocysteine and methylmalonic acid (MMA) were also considered to evaluate functional deficiency. The diagnostic criteria for Vit B12 deficiency followed standard clinical guidelines with levels below 200 pg/mL indicating its deficiency.

Analysis and interpretation

The extracted data were qualitatively synthesized to identify patterns, trends, and associations between Vitamin B12 status and PD clinical manisfestations. Findings were categorized into motor symptoms (such as gait disturbances and neuropathy), non-motor symptoms (focusing on cognitive decline), pathophysiological mechanisms (based on molecular pathways, neuropathology, metabolites and genes), and therapeutic implications (Vit B12 supplementation, clinical recommendations).

Clinical recommendations

Based on the synthesized evidence, recommendations for clinical practice were developed. These recommendations aimed to guide healthcare providers managing levodopa–carbidopa intestinal gel (LCIG)–treated patients in the assessment, monitoring, and management of Vit B12 deficiency.

The pathophysiological links between Vit B12 and PD: complex and robust dynamicsVit B12: from the gut to the brain

Vit B12, also known as cobalamin, is a water-soluble vitamin characterized by a remarkably intricate molecular structure. This structure reflects the complexity of the processes governing the absorption and transportation of this vitamin within the body and into the brain.

The sources of intake of VitB12 include meat, eggs, and dairy products with an average estimated daily consumption equivalent to 2.4 µg per day per person. Vit B12 is mainly bound to food proteins. Thus, it must be liberated to couple with the dedicated transport proteins.

Transcobalamin receptor CD320 can be found in endothelial cells at the blood–brain barrier. It's responsible for taking in and moving Vit B12 into the central nervous system (CNS) (Orozco-Barrios et al. 2009; Wu et al. 2023).

Once into the brain, Vit B12 interacts with genes (LRRK2), proteins (synuclein and Lewy bodies), neurotransmitters (dopamine, Acetylcholine (Ach)) and metabolites that do impact on CNS hemostasis.

Vit B12 pathophysiological fingerprints in PDVit B12 and neural pathways in PD

Vit B12 has been touted as a crucial component of the cholinergic pathway within the brain in several neurodegenerative diseases (El-Mezayen et al. 2022) and recently non-motor subtypes of PD including a cholinergic subtype in particular has been described (Aarsland et al. 2021; Bohnen et al. 2022; Sauerbier et al. 2016). Such vitamin has also been linked to the dopaminergic pathways. In Transcobalamin-Oleodin animal models, Vit B12 deficiency induced dopaminergic caspase-2 mediated cell-death in substantia Nigra (Orozco-Barrios et al. 2009).

The direct role of Vit B12 is to serve as a co-factor in the methylation reaction of Hcy to methionine acting in synergy with the enzyme: Methionine Synthetase (MS) which would integrate methionine via a series of reactions and serves to produce S-adenosyl-methionine (SAM), a major ‘methyl’ donor for several methylation pathways (Fig. 2). In the scenario of cobalamin deficiency, the methylation of Hcy to methionine will essentially lead to an excessive turnover of Choline resulting in a posited cholinergic deficiency.

Fig. 2

Pictorial illustration of the homocysteine methylation reaction into methionine (in purple) and its link with the cholinergic system. Illustrated in blue is the preferential pathway which is vitamin B-dependent (B12, B6, B9, B9) where the methylation process of homocysteine requires the synergic action of Vit B12 and MS. In Pink, if the secondary methylation pathway that is favored in case of Vit B12 deficiency pinpointing choline transformation to Betaine, the key ‘methyl’ donor for homocysteine. A joint pathway starts with the transformation of methionine into SAM. SAM will serve for the transformation of nicotinamide into N-methy-nicotinamide which inhibits competitively the efflux of choline out of the CNS. In yellow is the pathway of Levodopa transformation into dopamine and into 3-O-Methyl-Dopa. B6 vitamin B6, COMT Catheco-O-Methyl-transferase MAT methionine adenosyl-transferase, MS Methionine systhetase, PEMT Phosphatidyl-ethanolamine N-methyl-transferase, SAH S-adenosyl-homocysteine, SAM S-adenosyl- methionine, THF Tetrahydro-folate

Vit B12 and synucleinopathy

A recent study demonstrated that Vit B12 effectively impedes the formation of αSN fibrils in a manner dependent on dosage in-vitro experiments. Circular dichroism spectroscopy data indicated that Vit B12 delays the conformational transition of αSN into β-sheet-rich structures, with a particular influence on the parallel β-sheet conformation. Consequently, Vit B12 significantly alleviated the cytotoxic effects associated with αSN aggregates. Furthermore, it exhibited the ability to bind directly to αSN and to dismantle preexisting mature αSN fibrils and alleviate the ensuing cytotoxicity (Jia et al. 2019) (Fig. 3).

Fig. 3

Summary of the different mechanisms of how Vit B12 could potentially interact with the physiopathology of PD. The yellow section focuses on evidence regarding the impact of Vit b12 deficiency on both cholinergic and dopaminergic pathways. The pink section summarizes how Vit B12 deficiency aggravates αSN pathology. Illustrated in the green section Vit B12 deficiency interaction with PD genetic background. In blue, the mechanisms of HHcy induced neurotoxicity in case of Vit B12 deficiency. The dotted pink arrow implies the presence of interaction between the two specified elements. NLRP-3 Nod-like receptor pyrin 3, NF-B nuclear factor kappa B, LRRK2 Leucine-Rich Repeat Kinase 2, αSN alpha-synuclein

To our knowledge, only one study has been conducted among PD patients linking Vit B12 role, the methylation status and the neurodegenerative markers of PD including αSN (Obeid et al. 2009). In the study design, they tested serum concentrations of Vit B12, B6, SAM, Amyloid Beta1-41, Hcy and methylmalonic acid (MMA) among 87 PD patients. The results of the study did not suggest direct correlations between Vit B12 serum levels and αSN platelet levels.

Vit B12 and PD genetics

Growing evidence is pinpointing new facets of B12 in neurodegenerative diseases, such as AD and PD, as a gene and epigenetics regulator (El-Mezayen et al. 2022). In relation with PD, Vit B12 is considered as a natural Leucine-Rich Repeat Kinase 2 (LRRK2) kinase inhibitor. In fact, LRRK2 gene mutation (G2019S mutation) is known to cause the majority of autosomal dominant familial forms of PD and some of the sporadic cases (Rui et al. 2018). Such mutation induces a hyperactivity of the LRRK2 kinase, known to be highly neurotoxic. Unlike previously tested LRRK2 kinase inhibitors with significant side effects, Vit B12, derivative AdoCobalamin (AdoCb) specifically, exhibits a neuroprotective effect by regulating the activity of LRRK2 kinase (Schaffner et al. 2019) and has been shown to be effective in preventing neurotoxicity in cultured rodent neurons and able to mitigate dopaminergic deficit in these models.

Being a potential potent inhibitor of LRRK2 kinase activity with low risk for off-target side effects, Vit B12 supplementation in patients with PD (PwPD) seems appealing and encourages future clinical trials to investigate this potent effect as such evidence remains restrained to animal models (Green and Christine 2019) (Fig. 3).

Vit B12 and homocysteine

Vit B12 deficiency is linked to HHcy-induced-neurotoxicity. As shown in Fig. 2, HHcy is a surrogate marker of B12 deficiency. While there is a link of Hcy with neurodegeneration (Bonetti et al. 2016; Mattson and Shea 2003; Obeid and Herrmann 2006), little attention has been given to depict the mechanisms of how Hcy may impact on the neuropathological process in PD in particular. Recent evidence emerging from in-vitro studies, animal models and PD patients studies are starting to shed light on this matter (Al-Kuraishy et al. 2023).

HHcy aggravates the degeneration of dopaminergic neurons. Since Hcy can mimic the action of CNS excitatory neurotransmitters (McCully 2009), it produces an excess of intracellular influx of calcium which activates cellular apoptosis causing DNA cleavage and fragmentation (McCully 2009; Boldyrev 2009; Tyagi et al. 2005). Excitotoxicity, along with the increase of oxidative stress, embodies the major mechanisms of Hcy-induced neurodegeneration.

Furthermore, elevated Hcy can cause mitochondrial dysfunction, facilitates pathological aggregation of proteins such as SN and induces a toxic effect on dopaminergic neurons (Kumar et al. 2022; Bonetti et al. 2016). A recent study conducted on mice models of PD established that the N-homocysteinylation of α-SN enhances its aggregation potential and increases its neurotoxicity and that higher Hcy levels correlated with more severe α-synuclein deposition within mouse brains (Zhou et al. 2023). Finally, Hcy is responsible of neuro-inflammation in PwPD (Grotemeyer et al. 2022; Singh et al. 2020) (Fig. 3).

Dopamine and VIT B12: what’s the link?

The incidence of Vit B12 deficiency increases with advancing age and it is more prevalent among the LDopa treated subjects (Zhao et al. 2019). Many reports have found that total LDopa daily dose negatively correlated with Vit B12 levels (Romagnolo et al. 2019; Zis et al. 2017). PD patients who received long-term treatment with LDopa also exhibited notably lower levels of serum Vit B12 and folate when compared to age-matched controls (Ceravolo et al. 2013; Lizárraga and Lang 2022; Mancini et al. 2014). Both oral-treated and LCIG patients could exhibit alterations in serum B12 levels, HHcy and increased MMA (Romagnolo et al. 2019; Mancini et al. 2014; Comi et al. 2014; Rispoli et al. 2017; Toth et al. 2010; Triantafyllou et al. 2007).

Potential interactions between dopamine and B-group vitamins have been particularly investigated regarding their link with the greater incidence of peripheral neuropathy (PN) among PwPD (Ceravolo et al. 2013; Mancini et al. 2014). This yielded to conflicting results, raising questions about whether PN could be a direct consequence of their interaction or an additional systemic feature of PD on which B12 deficiency can act as a negative predictive factor (Comi et al. 2014).

Prior to the LDopa/carbidopa intestinal gel (LCIG) era, subclinical or clinical mild PN was observed in up to 50% of PD patients receiving oral LDopa. The presence of PN in PD subjects has been found to be associated with various factors in two large studies, including the total dosage of LDopa, low serum Vit B12 levels, and elevated Hcy and MMA levels (Ceravolo et al. 2013; Mancini et al. 2014). Subclinical signs of peripheral PN could be found in oral LDopa treated patients during the course of the disease, and low Vit B12 levels may facilitate the development of a clinically manifest PN.

However, several research papers did not establish significant correlations between mean LDopa daily dose, Vit B12 levels and PD-related PN (Corrà et al. 2023; Lamberti et al. 2005; Rajabally and Martey 2013). Some of these reports are limited by the small sample size, the small amount of LDopa administered as well as a large variability of age and disease duration in the populations of study (Corrà et al. 2023; Lamberti et al. 2005). In line with these findings, a study group found phosphorylated αSN deposits in proximal peripheral nerves of PD patients with small nerve fiber PN compared to atypical parkinsonism and healthy controls, with no significant differences between LDopa exposure or Vit B12 deficiency (Donadio et al. 2014).

Levodopa and cobalamin-related neurotoxic metabolites and benefits of COMT inhibitors

The breakdown of LDopa through COMT requires methyl group transfer, leading to the formation of the stable compound 3-O-MethylDopa (Müller and Riederer 2023). When high doses of LDopa are administered chronically, there is an increased demand for SAM as a methyl-group donor (Uncini et al. 2015). This could lead to Vit B12 depletion and transforms SAM into S-adenosyl-homocysteine and then into Hcy, the latter serving as a marker for methylation capacity (Müller and Riederer 2023). Consequently, Vit B12 levels may decrease in PwPD due to heightened methylation requirements associated with LDopa therapy and alter myelin synthesis throughout carbohydrate and fat metabolism contributing to the development of NP (Uncini et al. 2015). COMT inhibitors (COMT-I) can rebalance this abnormal metabolic loop by suppressing the overproduction of SAH by COMT after LDopa administration and lowering Hcy levels (Cossu et al. 2016; Zoccolella et al. 2005) (Fig. 2). Recent evidence indicates that Opicapone's greater bioavailability of LDopa likely accounts for the absence of a significant decrease in Hcy levels in chronic LDopa users, in contrast to the reduction observed with Entacapone. Nevertheless, both medications effectively prevent its elevation, highlighting HHcy as a reliable marker of methylation dysfunction (Müller et al. 2022).

HHcy generally correlated with cumulative LDopa dose without significant association with vitamin levels (Mancini et al. 2014; Comi et al. 2014; Mathukumalli et al. 2020; Miller et al. 2003; Müller 2008; Müller and Kuhn 2009). Consequently, it has been proposed that LDopa itself could drive the increase of Hcy, even without Vit B12 deficiency (Miller et al. 2003). HHcy is also considered as an independent factor of peripheral nerve damage (Uncini et al. 2015; Merola et al. 2016). In fact, electrophysiological studies have linked elevated Hcy levels in LDopa-treated patients to axonal loss in the sural nerve and a tendency towards weight loss among these patients (Cossu et al. 2016; Zoccolella et al. 2005; Kim et al. 2023). Using a combined COMT-I/LDopa treatment in PD patients has also shown to be effective in lowering Hcy levels and PN incidence when compared to oral LDopa alone (Andréasson et al. 2017). A single-nucleotide polymorphism (sA158G rs4680) in the COMT gene also led to a greater risk of LDopa-related PN in PwPD through low enzymatic activity (Toth et al. 2010).

Measuring MMA levels improves diagnostic sensitivity and specificity for cobalamin deficiency not only because Hcy elevation can occur in various conditions, but also because approximately 50% of cobalamin-deficient patients may have normal serum cobalamin levels leading to under-diagnosis (Romagnolo et al. 2019; Rajabally and Martey 2013; Taher et al. 2022; Toth et al. 2010). MMA could serve like an early biomarker of functional B12 deficiency. It's also worth noting that almost all patients with PD-associated PN have shown elevated MMA levels. The cumulative lifetime intake of LDopa and serum MMA levels might correlate with the extent of PN progression and severity (Müller 2008).

Does the route of administration matter?

To date, whether the route of LDopa administration impacts Vit B12 profile and PN risk or not remains a controversial matter (Romagnolo et al. 2019; Jugel et al. 2013). The global adoption of LCIG therapy, led to some reports of potential complications related to peripheral nerve toxicity and abnormal vitamin metabolism (Taher et al. 2022; Antonini et al. 2007; Loens et al. 2017; Santos-García et al. 2011). Recent global registry based on post marketing analysis of LDopa infusion use in real life population pointed out that discontinuation rates due to the occurrence of PN was only noted in two cases (Chaudhuri et al. 2023). Yet, it's important to underline that the study did not include a blood assessment of B12 or Hcy levels, nor involved nerve conduction studies. In the study conducted by Mancini et al., PN incidence was higher among patients treated with LCIG and/or oral LDopa in comparison with alternative dopaminergic treatments: emphasizing further the link between LDopa and PN. Simultaneously, a prospective investigation proposed that LCIG patients exhibited similar PN patterns compared to those seen in oral LDopa patients (Loens et al. 2017; Lehnerer et al. 2014).

A robust link between higher LCIG doses and reduced B12 levels in PwPD with PN has been pointed out with beneficial effect of vitamin B supplementation. Several mechanisms have been implicated leading to Vit B12 deficiency, HHcy and increased levels of MMA (Fig. 4) (Mancini et al. 2014; Merola et al. 2016). As a matter of fact, a large amount of LDopa is delivered in that specific part of the small intestine in a methylcellulose gel form and this might hinder vitamin absorption interfering with its receptor coupling (Lehnerer et al. 2014; Aasheim et al. 2008). This phenomenon appeared to be dose-dependent and influenced by the infusion rate. Faster infusion rates have been also associated with weight loss majoring, as a consequence, the risk factor for PN (Merola et al. 2016; Klostermann et al. 2012; Pauls et al. 2021). Thus, weight loss seems to be an additional factor linked to Vit B12 deficiency and PN. In the DUOGLOBE study previously mentioned (Chaudhuri et al. 2023), approximately 20% of the participants witnessed a reduction in weight by 7% or greater but most of the individuals retained their initial BMI ultimately. Lastly, intrajejunal continuous infusion provides a greater systemic bioavailability of LDopa compared to the same equivalent oral daily dose. This could interfere more intensively with Vit B12 intra-cellular metabolism nullifying the "physiological metabolic rest" of these components (Merola et al. 2016; Santos-García et al. 2012).

Fig. 4

Potential causes of vitamin B12 deficiency in PD patients undergoing LCIG therapy. MMA: methylmalonic acid, LCIG levodopa–carbidopa intestinal gel; PEG percutaneous endoscopic gastrostomy, SIBO small intestinal bacterial overgrowth

Regarding subcutaneous administration of LDopa, there is currently not enough data regarding its interaction with Vit B12 neither regarding potential sides effects such as PN. Thus, it is mandatory for post marketing surveillance to tackle this issue.

Clinical peculiarities of VIT B12 deficiency in PDMotor clinical correlates of Vit B12 deficiency in PD

To our knowledge, data to address the impact of Vit B12 deficiency on the motor aspect of PD is mostly lacking. In one study, authors