Similarities in the pathophysiological and immunological mechanisms between MS and depressive disorder:

(1)

Dysfunction of HPA-axis activity.

SNfLs are recognized as biomarkers of CNS neuroaxonal damage and are referred to as the “neurologist’s troponin”. In the future, they will also make it easier for psychiatrists to classify neuropsychiatric symptoms. Increased sNfL as a neuron-specific marker can be observed in a variety of diseases of the CNS (and also in the peripheral nervous system) [45]. Patients with depression had significantly increased neurofilament light chain (NfL) values and these could serve as a biomarker in depression as in PwMS [46, 47].

However, significantly lower cerebrospinal fluid (CSF) NfL values were seen in psychiatric disorders than in neurological diseases [48]. At the same time, the TNF-α levels were also increased in depression. There was a positive relationship between sNfL and TNF-α levels. This points to the important role of neuroaxonal injury by proinflammatory cytokines in depression [49]. SNfLs as an indication of axonal damage are also increased in cognitive dysfunction [50].

In a study, significantly elevated sNfL values were observed in 41 patients with depression, which were above the age-specific cut-off values and corresponded to reduced cognitive performance [tested using Digit Symbol Substitution Test (DSST) scores (cognitive processing speed)] [51]. The increase in sNfL levels also correlated with neuroimaging studies showing subtle, widespread white matter changes and reduced brain volumes in patients with depression [51, 52].

Treatment resistance in major depression was associated with elevated sNfL levels [53].

Studies in patients with bipolar diseases showed a complex interaction of increased sNfL and VitD deficiency [49, 54].

A considerable problem, therefore, is the evaluation of sNfL in patients suffering from both MS and depression as a comorbidity. This marker can be used to assess the severity of depression but can indicate an acute MS flare.

For the early detection of suicidal behavior, the determination of inflammatory markers (IL-2, IL-4, IL-6, TGF-β) and the increase in sNfL as a result of complex neuroinflammatory processes could be used in the future. The sNfL values were significantly higher in patients with suicide attempts than in the controls (40.52 ng/mL ± 33.54 ng/mL vs. 13.73 ng/mL ± 5.11 ng/mL) [55].

In addition to activation of the HPA-axis and MG during suicidal behavior, there is another chain of biochemical and neuropathological changes resulting in neuroaxonal damage [55].

SNfLs were documented in over 200 studies as an indication of activity in MS [56].

Longitudinal studies of PwMS have shown that sNfL levels rise 5 months before the acute MS flare, peak at the clinical onset of the flare, and recover within 4–5 months [57]. In the preclinical phase of MS, sNfL increases could be verified 6 years before the onset of clinical symptoms [58]. Premorbid cognitive deficits and frequent physician visits and hospitalizations, including psychiatric consultations, demonstrate the early onset of neurodegenerative disease prior to the definitive diagnosis of MS [59, 60]. As early as 2016, an inverse association between s25(OH)D and CSF NfL levels could be demonstrated. 25-hydroxyvitamin D [25(OH)D] levels above 40 ng/mL were associated with lower CSF NfL levels regardless of ongoing MS treatment [61]. High sNfL levels within the first year of illness were associated with a long-term deterioration of the disability in PwMS [62].

With a 96-week VitD supplementation in relapsing-remitting MS (RRMS) with weekly administration of 20,000 IU, a decrease in the sNfL level by 32.6% was shown even without disease-modifying therapy (DMT) [63]. However, in a small number (n = 24) of PwMS with RRMS, 48 weeks of VitD supplementation (14,000 IU/day) failed to lower the sNfL levels [64].

In the future, the determination of sGFAP, an intermediate filament of astrocytes in serum and a marker of astrocyte activation, will play an important role in the assessment of the clinical picture and monitoring of both depression and MS [65–67]. The level of sGFAP was linked with depression severity and age [65, 68], while, in MS, sGFAP will be a biomarker to assess a chronic progression independent of relapse activity (PIRA) [66, 69]. With the simultaneous determination of sNfL as a biomarker for neuronal damage, the activity of both diseases can be better assessed, which is of great importance for therapy management [70].

It is known from experimental studies that increased vitD intake in neuronal, axonal, and glial damage causes a reduction in sGFAP [71] (Table 2).

The neuroprotective properties of VitD work through several mechanisms [97]. The direct neuroprotective effect of VitD is linked to the regulation of neurotrophic factors and the reduction of oxidative stress. Only adequate s25(OH)D levels (> 30 ng/mL) down-regulate oxidative stress, suboptimal VitD levels lead to the opposite effect [98]. Neurotrophic factors are crucial for the differentiation, survival, and maintenance of nerve and glial cells. VitD stimulates the expression of the glial cell line-derived neurotrophic factor (GDNF), nerve growth factor (NGF), BDNF, NT3 [98, 99], and the p75 NT receptor (p75^NTR) in neurons, glial cells, and Schwann cells [97]. Reduction in vitD promoted neurotrophic factor expression due to vitD deficiency may result in neurons becoming more susceptible to damage [97, 98, 100, 101].

It is evident that in PwMS, the psychological burden and its consequences can be alleviated by slowing down the progression of MS. But also the fear and risk of side effects from various pharmaceuticals (DMTs) become a burden in the course of their lives.

VitD supplementation dampens the pathogenicity of the T helper 17 (Th17) cell IL-17 synthesis, increases sensitivity of effector CD4⁺ T cells to extrinsic cell death signals, and promotes CD4⁺CD25⁺forkhead/winged helix transcriptional factor P3 (FoxP3)⁺ Treg cell and CD4⁺IL-10⁺FoxP3⁻ type 1 regulatory T (Tr1) cell development [102]. High dose VitD supplementation reduces IL-17 producing CD4⁺ T cells and effector memory CD4⁺ T cells in PwMS when these s25(OH)D levels are significantly increased [103]. If 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] lowers the proinflammatory cytokines IL-1β, IL-6, IL-8, IL-12, IL-17, and IL-21 as well as TNF-α, promotes the ability of Tregs to migrate to the CNS, and antiinflammatory IL-10 production increases [103–105], the therapeutic efficiency is increased by additional therapy with VitD.

A recent finding is that a subset of Th17 cells, the Th17.1 cells, infiltrate the MS brain and are an important player in MS activity as they also produce a multidrug resistance protein 1 (MDR1) and can therefore be refractory to methylprednisolone (MP) in relapse therapy [106].

Simultaneous treatment with 1,25(OH)₂D₃ reduces both the MDR1 expression and the secretion of IFN-γ and TNF-α by Th17.1 cells.

Through the VitD supplementation the glucocorticoid reaction is improved in MP pulse therapy [106]. There is probably a therapeutic time window for a VitD supplementation in the earliest relapsing phases of MS [106]. S25(OH)D concentrations are inversely correlated with a risk of relapse, CNS lesions, and disability progression in MS. The number of new gadolinium-enhancing or new increasing T2 lesions is significantly reduced by VitD supplementation [107, 108].

The sealing of the BBB by MP in MS can be potentiated by VitD. The protective effect of 1,25(OH)₂D₃ (calcitriol) on the fragile BBB in MS can be explained by the up-regulation of tight junction proteins and the down-regulation of adhesion molecules [109, 110]. The migration of autoreactive CD4⁺ T cells into the CNS is blocked by calcitriol. The activation of macrophages (MK), MG, and astrocytes is regulated and reduces the passage of immune cells through the BBB. The differentiation of effector T and B cells is reduced while regulating subsets are supported [110]. The preventive administration of VitD is more successful the earlier it is used so that irreversible damage is reduced [111]. The aim of the therapy is to restore the integrity of the brain barrier and, thus, CNS homeostasis [111].

1,25(OH)₂D₃ also makes it possible to prevent the migration of Th cells into the CNS parenchyma on a second pathway. The autoimmune cells are kept in the periphery by slowing the migration from the lymph nodes into the CNS parenchyma [112].

There is ample evidence of a disruption of the BBB integrity in depression, with evidence of the presence of elevated inflammatory markers and immune cell dysfunction. Whether the disruption of the BBB primarily causes depression or is a secondary consequence of the disease cannot be answered at present. The comorbidity of depression in MS may indicate a common mechanism of BBB disruption. Stress and inflammation in parallel with MS disrupt the integrity of the BBB [113].

TNF, IFN-γ, Th17 cells, IL-17A, IL-23, IL-6, and lipopolysaccharides (LPS), for example, influence the permeability of the BBB [114–116]. T and B lymphocytes and monocytes have been found in the brain parenchyma of depressed patients (post-mortem studies) and in animals [114–116]. IL-17A expressing CD4 cells or Th cells accumulate in the hippocampus and promote susceptibility to depression-like behaviors [114–116].

The structure of the BBB differs significantly between healthy and depressed subjects at different cellular levels. Due to stress and inflammation, changes in the brain in depression include a reduction in the number of pericytes, loss and modification of the foot processes of astrocytes [117, 118], activated MG, and loss of tight junctions [113]. The tight junction claudin-5 expression is reduced in the hippocampus of patients with depression, the expression of claudin-5, claudin-12, and zona occludens-1 (ZO-1) correlates with the age of onset and the duration of the depressive episodes [119].

If the reduction in inflammation can lead to BBB integrity by reducing TNF and IL-6 and thereby achieve an antidepressant effect [113], VitD supplementation is a complementary therapeutic strategy.

In chronically active lesions that are responsible for insidious PIRA in MS, MG/MK mediated inflammation occurs [124]. During lesion development and MS progression, the MG activity may contribute to neurotoxicity through the release of proinflammatory cytokines, ROS, proteases, and glutamate [125].

MG activation is the earliest biomarker of inflammatory processes in the CNS in MS [126]. Because the proinflammatory MG1 type is particularly active in the early stages of MS, it is biologically plausible to use a VitD supplementation to start reducing inflammatory mediators to reduce myelin damage. Elevated levels of calcitriol lead to a reduction in MG activation, oxidative stress, and lower BBB permeability [98]. 1,25(OH)₂D₃ effectively shifts MG from a proinflammatory M1 phenotype to a reparative M2 phenotype, particularly in early MS, resulting in the limitation of inflammation and demyelination [98, 121].

Neuron-specific VitD receptor (VDR) signaling induces antiinflammatory molecules that protect the CNS from autoimmunity. 1,25(OH)₂D₃ causes a reduction of proinflammatory molecules and a reciprocal induction of antiinflammatory molecules in MG [127]. MG/MK mediated inflammation is a major obstacle to remyelination [124].

MG have also been identified as actors in depression, and the condition is interpreted as a “MG-associated disorder” (microgliopathy) [128]. Increased calcitriol levels lead to a reduction in MG activation, oxidative stress, and lower BBB permeability [20]. The neuroinflammation in depression results from intrinsic communication between peripheral immune cells, gut microbiota, and immune cells (MG) present in the brain, resulting in glutamatergic dysregulation, reduced synaptic plasticity and monoamine synthesis [123]. The interaction of neuroinflammation and the HPA-axis and their dysregulation could initiate the onset of depression [128, 129].

A disrupted bacterial and viral gut microbiota is thought to be part of the pathogenesis of MS, mediated by an altered gut-microbiota-brain (GMB)-axis. The composition of gut microbiota affects the production of serotonin in the gut, which in turn affects the serotonin-mediated regulation of the systemic immune function [144].

In a systematic review of the gut microbiota composition of 286 PwMS until 2019, most studies found no difference in gut microbiota diversity [145]. However, taxonomic differences from controls were verified and immunomodulatory drugs showed separate taxonomic differences [145].

A recent study demonstrated that inflammatory markers (blood leukocytes, CRP, blood cell gene expression of IL-17A and IL-6) were positively associated with a group of bacteria that are more common in MS [146]. Bacterial species that were more common in disease-active, treatment-naive MS were also positively associated with the plasma cytokines IL-22, IL-17A, IFN-β, IL-33, and TNF-α, some of which are targeted by VitD [146].

Because high dose VitD supplementation with 10,400 IU/day reduced IL-17 producing CD4⁺ T cells and effector memory CD4⁺ T cells and was associated with a concomitant increase in the proportion of central memory CD4⁺ T cells and naive CD4⁺ T cells, a VitD supplementation should be used as a potential therapeutic agent [103]. After 6 months, the 25(OH)D level increased by 34.9 ng/mL [147], which demonstrates that a high dose VitD supplementation is necessary. The active form of VitD3, 1,25(OH)₂D₃, inhibits IL-22 production and can be viewed as an adjuvant therapeutic to regulate IL-22 production [148]. The two proinflammatory cytokines (IL-22, IL-17) correlate with active brain lesions in MS [149]. The bacterial species richness of treatment-naive MS cases was associated with the number of relapses, which is a surprising finding [146].

The gut microbiome regulates the interaction with the brain, and this relationship is understood as the “GMB-axis (GMB-A)”, the connection between the gut microbiome and the CNS [150, 151].

It presents itself as a complex and interactive system that includes a neuronal signaling network, an immune signaling network, and a chemical signaling network [150, 152]. In addition to the involvement of the vagus nerve and the HPA-axis, the pathophysiology involves the MG, Th17/Treg activity, and the gut microbiome [152]. A significant factor, such as psychological stress, can lead to depression via dysfunction of the HPA-axis and dysfunction of the GMB-A [153].

Numerous publications have shown that microbiota promotes both humoral immunity (B cell development and proinflammatory T cell responses) and immune regulation (regulatory B cells and Tregs) [154].

Changes in the gut microbiome (microbial dysbiosis) can affect the severity of autoimmune diseases [154] and patients with depression have a different microbiome than healthy individuals [153]. An altered composition of the fecal microbiota in depression has been pointed out [155, 156] and homeostasis of the gut microbiome should therefore be aimed at. VitD deficiency or VitD supplementation alters the microbiome, and manipulation of bacterial abundance or composition affects disease manifestation [154]. VitD, VDR, or cytochrome P450 family 27 subfamily B member 1 (CYP27B1) deficiency led to an increase in Bacteroidetes and Proteobacteria strains, resulting in dysfunction of the epithelial barrier of the gut and triggering intestinal inflammation [157, 158].

The HPA-axis is the main non-neuronal connection between the gut and brain and primarily regulates stress responses associated with the immune system and vagus nerve [153].

Despite the multifactorial causes of MS and depression, the increased inflammatory activation and dysregulation of the immune system in both diseases is undisputed as a major contributor to the pathophysiology.

The observed association of s25(OH)D deficiency and increased proinflammatory cytokines as well as increased CRP makes correcting the calcidiol levels by VitD supplementation in order to curb chronic inflammation a logic consequence [179].

In addition, activated MG, active T lymphocytes, loss of oligodendrocytes, demyelination, axon damage, and axon loss are potentiating factors [178, 180]. From this perspective, the aim of a VitD supplementation is to slow down the activation of the immune system in PwMS and in the comorbidity of depression.

Calcitriol inhibits the synthesis of IL-6, TNF-α, and nitric oxide (NO) through activated MG [181]. It is undisputed that calcitriol reduces the expression of IL-12 and increases the expression of the antiinflammatory cytokine IL-10, IL-14, and TGF-β and has a dampening effect on various inflammasomes [NOD-like receptor family pyrin domain containing 3 (NLRP3)] [182]. There is evidence that VitD supplementation can increase serum serotonin in people with depression [183]. VitD increases the biosynthesis of monoamines by expressing the enzyme’s tyrosine and tryptophan hydroxylase [123].

VitD as a “pharmacological” treatment to influence the inflammatory activity in the CNS, at least as an add-on therapy, constitutes a biological plausibility.

The biomarker sNfL as a marker of neuroaxonal integrity allows for a better clinical characterization of the depression and is useful for monitoring the therapy. Increased sNfL levels indicate active pathological processes [51]. There is evidence that sNfL levels are a biological marker of inflammatory processes in the CNS and that successful therapeutic agents must be measured by their ability to reduce the sNfL levels [45–51, 54–58]. The outcome of supplementation studies in MS and depression will depend on whether s25(OH)D of more than 50–100 ng/mL can be achieved under daily vitD supplementation [184].

Recognizing that ketamine exposure leads to increased sNfL with persistent neurocognitive impairment and depression, these interactions must be taken into account when interpreting sNfL values [185, 186]. A study with simultaneous high dose vitD supplementation aiming at neuroprotection through 1,25(OH)₂D₃ while controlling sNfL levels would be of great value.

The controversial opinions and results from various studies (study designs) could not be more contradictory. Different initial s25(OH)D values and measuring methods, dose level, daily or bolus doses, type of VitD supplementation (VitD2/VitD3), duration of intervention, age groups (young/older), and missing international levels [definition of deficiency, insufficiency, sufficiency, optimal values in autoimmune diseases of s25(OH)D] stand in the way of PwMS benefiting from VitD supplementation. A frequently cited study by Rolf et al. [187] with a small number of patients with depressive symptoms (n = 20) with a high dose VitD supplementation (14,000 IU/day) over 48 weeks showed a significant decrease in depressive symptoms, but no significant reduction compared to the placebo group.

In order to influence the dysregulation of autoimmunological processes, the duration of VitD supplementation and the s25(OH)D levels reached are crucial [188]. It has been shown that, with a daily intake of 10,000 IU of VitD, a steady state of s25(OH)D of about 80 ng/mL could only be reached after 18 weeks [188]. In the study of Rolf et al. [187], an optimal calcidiol level could therefore only be expected to be achieved at 30 weeks.

The workload of clinicians and general practitioners often only allows the abstract to be read in the secondary literature and less in the original literature. In the case of “supplementation studies with a negative outcome”, the content of the discussion in a publication is often more detailed, in particular, the chapter on “limitations of the study”, in detail: the number of subjects, rare recording of expanded disability status scale (EDSS), selected dosage level/dosage schemes of VitD possibly with a masking effect instead of higher 25(OH)D levels to achieve effectiveness, study duration too short, etc.

A prime example of confusion is a study of the natural variations of VitD and sNfL in RRMS over a one-year period [189].

In conclusion, no significant association between naturally increasing s25(OH)D and neurodegeneration, as measured by sNfL, is supported for RRMS patients. S25(OH)D with more than 20 ng/mL was interpreted as “rather high” [189]. Specifically, s25(OH)D values were measured over a one-year period as an approximative mean value of s25(OH)D of between 18 ng/mL and 44 ng/mL, with 44 ng/mL only being registered over 4 weeks. From these results, it is concluded that “a large effect of VitD supplementation is lacking” [190]. In order to generate a therapeutic potential to influence pathological immunological processes, however, values in the middle or upper range of normal (up to maximum 100 ng/mL or 130 ng/mL) should be aimed at and maintained over a longer period of time in order to achieve long-term effects [75, 147, 191–200].

VitD could support the immune defense at s25(OH)D levels above 50 ng/mL in the fight against infection [severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection] and thus prevent potentially stressful situations [201–203] and should be incorporated into the medical treatment plan. In “lifelong” autoimmune processes in progressive MS, the immunomodulatory activity of vitD can only reduce excessive inflammatory damage if optimal levels of s25(OH)D are sustainably achieved. Taking into account the dose-response relationship, this ongoing adjuvant therapy is therefore a rational approach.

VitD is a secosteroid hormone and acts through transcription in the cell nucleus and therefore takes a while to act [204]. Both for prevention and for the treatment of depression with VitD supplementation, women had the highest benefit when the VitD dose was over 2,800 IU/day and the duration of the intervention was longer than 8 weeks. The results also showed that high dose VitD supplementation was associated with a reduction in the incidence of depression, while low dose VitD supplementation (below 2,800 IU/day) was futile [204]. This is also confirmed by a study with 1,600 IU/day over 6 months, which showed no effect on the symptoms of depression but did show an effect on the symptoms of anxiety [205].

In a systematic review of a VitD supplementation at 1,500 IU/day to 2,800 IU/day or 50,000 IU weekly/biweekly over 8–12 weeks without checking the s25(OH)D values, the wide range of results did not provide any conclusions for real-life application [206]. Negative correlation between VitD supplementation and depressive symptoms have been recorded when s25(OH)D levels are above 32 ng/mL [91, 207].

If only 50% of patients respond to first-line antidepressant therapy and patients also are afraid of the side effects of antidepressants, a “permanent” VitD supplementation can be offered as an add-on therapy or as an alternative for milder forms [204].

To achieve a therapeutic effect of VitD on the (pathological) immunological mechanisms, a constant, continuous s25(OH)D measurement is required. Long-term supplementation is also necessary because depressive symptoms in PwMS can extend over 10 years [208]. A vitD supplementation with 10,000 IU/day over 12 months showed a significant improvement in symptoms [209].

A significant problem in assessing the results of such supplementation studies consists in avoiding “non-daily VitD doses” but bolus doses. After an oral dose of VitD, blood levels peak after 12 h and the half-life is around 12–24 h. Because of this short half-life, large bolus doses of VitD of 50,000 IU to 100,000 IU are cleared from the circulation within a week and are no longer detectable.

Continued daily supplementation of VitD results in a slow, sustained increase in s25(OH)D, reaching a steady state after 3–4 months. Bolus doses of VitD led to fluctuating levels in the blood, affecting the immune effects and, thus, the outcome of VitD supplementation [210]. These strong fluctuations have an adverse effect on the activity of the enzymes responsible for the synthesis and depletion of the active VitD metabolite, 1,25(OH)₂D₃. This dysregulation results in decreased levels of this metabolite in the extrarenal tissue [211–213]. Randomized controlled trials (RCTs) of VitD supplementation in respiratory disease have shown that intermittent high dose bolus doses are ineffective, although sufficient 25(OH)D levels have been achieved [214, 215]. One possible explanation is that the bolus doses induce 24-hydroxylase activity and result in the inactivation of VitD3 through the formation of 24,25-dihydroxyvitamin D [24,25(OH)₂D]. This effect persisted for at least 28 days after the bolus [216].

In 18 studies on the effect of VitD supplementation on the occurrence and prognosis of depression, patients in 10 studies received bolus doses of 50,000 IU to 300,000 IU weekly, every 14 days or 4 weeks, and only in 8 studies daily [208]. In adults with a VitD supplementation the greatest benefit on depressive symptoms was registered with a daily dose of more than 4,000 IU [217].

With intermittent bolus doses of VitD and short-term administration, a study over 8 weeks with 50,000 IU VitD every 2 weeks showed significant improvement in the severity of depression without any effect on the measured neurotransmitters, IL-1β, IL-6, and hsCRP [96, 218].

However, after a period of 12 weeks, a decrease in the hsCRP values was registered [219]. VitD supplementation for the treatment of depressive symptoms in women led to a significant improvement in depressive symptoms, both at a dose level of VitD of 5,000 IU/week and at 50,000 IU/week, with no difference for either group [220].

An 11-year study showed that higher levels of VitD predicted better cognitive performance in patients with clinically isolated syndrome (CIS) [221]. A 20 ng/mL higher mean 25(OH)D level in the first 2 years was associated with a 20% lower sNfL level. VitD can be a prognostic marker for long-term cognition and neuroaxonal integrity [221].

Depression may be associated with reduced cognitive performance in PwMS [222, 223]. For about 10 years there has been evidence that the molecular biomarkers sNfL, VitD, and VitB12 play an important diagnostic role in cognitive impairment in PwMS, namely to predict long-term cognitive performance [221, 224]. A slower information processing speed (IPS) was confirmed in PwMS [225].

However, IPS was able to use a VitD supplementation and VitB12 as long-term neuroprotection [221, 224, 226, 227]. A recent study showed that sNfL and s25(OH)D correlate with measurements of the IPS [225]. This knowledge opens up the opportunity of an extended indication for a permanent sufficient vitD supplementation of PwMS and depression.

A high-quality diet in PwMS was already positively associated with lower severity of depressive symptoms and disability [198]. A systematic literature review on the influence of a VitD supplementation confirmed the positive effect on mental health in PwMS [228].

Not only because of the fear of side effects in the treatment of depression with antidepressants (e.g., restrictions on the ability to drive, etc.), PwMS with comorbidity depression want a “non-pharmacological treatment”. Over 80% of PwMS used dietary supplements and 65% used vitD [229].

PwMS require medical advice on this concomitant therapy. If you leave this to your own initiative, it could confuse or overwhelm the patient [230].

In the future, probiotic treatment affecting the gut microbiota of PwMS will be increasingly in demand. The improvement in mental health parameters through probiotic supplementation can be demonstrated by reductions in the hsCRP and the proinflammatory IL-6 and increased levels of IL-10 [231–233]. Hypotheses on the mechanism of the multifactorial interaction between the intestinal flora, the nervous, immune, and endocrine systems (GMB-A), and therapy in PwMS are presented [234]. In the context of personalized medicine, supporting PwMS through a multitherapeutic approach could help in managing the disease.

Confusion concerning the value of a vitD supplementation has existed for decades due to the claim that only RCTs can be used as a benchmark for assessment, especially since there are considerable discussions concerning the evaluation of such studies [235–237]. However, there are potential limitations so that RCTs alone are not able to provide a definitive answer [238–241]. There is a demand for the improvement of statistical significance in randomized VitD control studies using Tregs as a benchmark [242].

It is undisputed that the Tregs respond to VitD supplementation. The proportion of Tregs (Treg percentage) should be considered as a biomarker in the results of vitD studies. After a 12-week vitD supplementation with 4,700 IU/day, s25(OH)D levels of 55 ng/mL were achieved and the Treg percentage had increased significantly compared to placebo [242].

The entire process, from the beginning of the study to its evaluation, could span an entire generation of PwMS, which is unacceptable for ethical reasons. RCTs are designed for therapeutic (pharmacological) drugs and not for a nutrient, such as VitD [177, 188, 243, 244].

The evaluation of individual studies with specific, high-aiming, and hardly achievable primary and secondary objectives [245] does not do justice to the complexity and range of effectiveness of a VitD supplementation at PwMS.

Study data suggest that an optimal s25(OH)D level could reduce the risk of depression and MS [4]. Therefore, the cost of a daily VitD supplementation is negligible in the treatment of autoimmune diseases. For the management of VitD supplementation is highly promising that in Canada, for example, calculations have shown that a reduced annual economic burden of disease in the general population of $12.5 billion ± $6 billion could be achieved. This would require that the s25(OH)D values be increased to 40 ng/mL. In the case of PwMS, the overall economic burden of the VitD supplementation estimated to be reduced by $1.5 billion [280, 281]. The estimated cost-saving effect of an improved VitD status in the population in Germany could add up to €37.5 billion per year [282]. The established biomarkers make it possible to record at least a subgroup of PwMS where a VitD supplementation promises optimal success and is economically justifiable.

“The practices of medicine remain more in art than a science” [283].