Introduction
Inflammation is involved in many chronic diseases and concern has been raised about the influence of vitamin D deficiency on inflammatory processes. When studies found an association between inflammatory diseases and low serum 25-hydroxyvitamin D (25(OH)D), further research found evidence of low vitamin D in a large segment of the general population. This led some authorities to declare a world-wide epidemic of vitamin D deficiency and to recommend vitamin D supplementation. Experts are debating the definition of vitamin D deficiency and the appropriate vitamin D doses, while further research is being done to determine if vitamin D supplementation has the intended effect.
The definition of Vitamin D deficiency needs re-evaluation in view of the fact that low 25(OH)D is found in both healthy and sick individuals. Concerns about vitamin D deficiency merit a closer look at the current method of determining vitamin D status because the level of 25(OH)D does not always reflect the level of 1,25-dihydroxyvitamin-D (1,25(OH)2D). Analysis of this active metabolite may reveal elevated 1,25(OH)2D) in the presence of low 25(OH)D and lead to a diagnosis of abnormal vitamin D endocrine system function.
An infectious pathogenesis posits that intracellular bacteria disrupt the vitamin D regulated immune system, resulting in persistent infection and chronic inflammation. In the clinical setting, a novel immunotherapy is demonstrating the ability to resolve vitamin D metabolism dysfunction, restore immune function, and thus, eliminate infection and reduce inflammation.
Vitamin D metabolism
The influence of 1,25(OH)2D on the immune system is one of its most important roles. 1,25(OH)2D regulates the immune system via the VDR which is present in most immune cell types, particularly in antigen-presenting cells (APCs) such as monocytes, macrophages and dendritic cells [7]. 1,25(OH)2D activates the VDR to express antimicrobial peptides (AMPs) such as cathelicidin and beta defensins which attack pathogens [8, 9]. In general, the innate immune system is enhanced and the adaptive immune system is inhibited by 1,25(OH)2D [10, 11]. Thus, an effective immune response is heavily dependent on the vitamin D endocrine system which performs a balancing act of inflammation versus anti-inflammation.
Vitamin D deficiency
Concerns about vitamin D deficiency arose when studies showed patients with autoimmune diseases have lower levels of serum 25(OH)D and study subjects given vitamin D had lower rates of autoimmune diseases and fewer markers of inflammation [12, 13].
Purported reasons for vitamin D deficiency
Is low 25(OH)D among the general population an accurate assessment of vitamin D deficiency? Many reasons are cited for the current ‘epidemic’ of vitamin D ‘deficiency’ but closer examination reveals these beliefs are based on outdated or limited studies and can be challenged with more recent research.
Melanin pigmentation is only one factor that determines the amount of vitamin D3 which is photosynthesized [20, 21]. Bogh et al. [22] measured the baseline serum 25(OH)D and total cholesterol levels of 182 fair-skinned and dark-skinned subjects; and studied the effect of UV radiation on their serum 25(OH)D levels. They found the amount of serum 25(OH)D produced was determined by the amount of cholesterol in the skin, not on skin pigmentation. Skin pigmentation does not appear to negatively affect vitamin D status [25].
Clothing is a barrier to vitamin D photosynthesis but this is an issue only for people who cover themselves from head to toe [26]. Ten to 15 min of sunlight or daylight exposure to a small area of skin (e.g., the forearm or face, etc.) twice a week, without sunscreen, supplies all the vitamin D necessary for health [27].
The belief that sunscreen lotion blocks vitamin D production is based on a 1987 study done by Matsuoka et al. [28] that was funded by the ultraviolet foundation, which is supported by the tanning bed industry.
Although pollution can block some ultraviolet radiation, even in urban areas of high pollution 50 % of UV rays reach the ground [31]. A significant amount of UV radiation exposure can be obtained in dense metropolitan areas; tall buildings provide shade but shade gives up to 50 % of UV rays. Indoor workers receive 10–20 % of outdoor workers’ yearly UV exposure [31]; and for many, this may be adequate, especially if sunlight exposure is higher when they are not working.
As the skin ages, there is a decline in the cutaneous levels of 7-dehydrocholesterol, resulting in a marked reduction of the skin’s capacity to produce vitamin D3 [32]. However, despite the up to fourfold reduction in vitamin D3 production in a 70-year-old compared to a 20-year-old, the skin has such a high capacity to make vitamin D3 that elders exposed to sunlight will produce an adequate amount of vitamin D3 to satisfy their vitamin D requirement [33, 34].
Low vitamin D is found in healthy subjects
Many studies of healthy subjects have found levels of 25(OH)D that, by some vitamin D definitions, are declared deficient (hypovitaminosis-D) [38, 39]. Vitamin D levels that are considered deficient have even been found in persons who are exposed to abundant sunlight [40]. Binkley et al. [41] showed a mean 25(OH)D level of 31.6 ng/ml among healthy young adult Hawaiian surfers. It is clear that low levels of 25(OH)D are found in both healthy persons and those with autoimmune or chronic inflammatory diseases. Opposing reasoning can be used to explain this contradiction. One explanation reasons that healthy persons with low 25(OH)D will become sick and sick people will develop lower 25(OH)D levels (Fig. 2); however, studies do not support this hypothesis. The correct explanation may be that, in the absence of disease, low 25(OH)D is normal.
Low vitamin D in the presence of diseases
Since low 25(OH)D is found in both healthy persons and those with autoimmune or chronic inflammatory diseases, assessing vitamin D status with the measurement of an additional clinical marker may be helpful. It is asserted that low levels of 25(OH)D accurately reflect vitamin D status; however, measurement of 1,25(OH)2D often demonstrates a positive correlation of elevated 1,25(OH)2D to inflammatory diseases (Fig. 2). This is illustrated by Blaney et al. [42] in a study of 100 patients with autoimmune and chronic disease which found that 85 % of subjects had levels of 1,25(OH)2D higher than 46.2 pg/ml without hypercalcemia.
Vitamin D supplementation
It is reasoned that if low 25(OH)D indicates a current or potential disease state, then increasing 25(OH)D by supplementing with vitamin D should provide some symptom relief and/or protection. So far, there is scant evidence for this hypothesis [56, 57]. According to Ross et al. [16] in the 2010 IOM report, “Outcomes related to autoimmune disorders, cancer, cardiovascular disease and hypertension, diabetes and metabolic syndrome, falls and physical performance, immune functioning, infections, neuropsychological functioning, and preeclampsia could not be linked reliably with calcium or vitamin D intake and were often conflicting.” Despite the recent increase in vitamin D supplementation, chronic diseases have increased and are expected to continue increasing [58, 59].
Consequently, more vitamin D experts are beginning to reconsider vitamin D supplementation among the general population [60]. Recommending higher vitamin D intake to large populations carries the potential risk of overdosing certain individuals [61]. It is difficult to ingest too much vitamin D from food, and natural mechanisms regulate the amount of vitamin D3 photosynthesized from sunlight [62]. However, elevated 25(OH)D and hypervitaminosis-D can occur due to vitamin D supplementation [63].
Decreases in vitamin D levels are a marker of deteriorating health. Ageing and inflammatory processes involved in disease occurrence and clinical course reduce vitamin D concentrations, which would explain why vitamin D deficiency is reported in a wide range of disorders. We postulate that inflammation is the common factor between most non-skeletal health disorders and low 25(OH)D concentrations. Inflammatory processes involved in disease occurrence and clinical course would reduce 25(OH)D, which would explain why low vitamin D status is reported in a wide range of disorders. However, increases in 25(OH)D have no effect on inflammatory processes or on disorders at the origin of these processes.
Bacterial pathogenesis of low vitamin D hypothesis
If evidence indicates that most people get adequate vitamin D from sunlight exposure but healthy persons are found to be ‘deficient’ by recent standards, what is the explanation for this phenomenon? Vitamin D proponents use a disease deficiency model to explain low levels of 25(OH)D. Their hypothesis states low 25(OH)D causes chronic diseases; however, a pathogenesis has not been elucidated [68]. Low serum 25(OH)D in the presence of disease can also be explained with a dysregulated vitamin D metabolism model [69]. This hypothesis proposes that low vitamin D is the consequence of a chronic inflammatory process caused by persistent infection. The bacterial pathogenesis theorizes that intracellular (cell wall deficient) bacteria invade nucleated cells, use strategies to avoid destruction and cause abnormal vitamin D endocrine function, resulting in low vitamin D. Excess 1,25(OH)2D is produced in an effort to up-regulate the VDR to transcribe AMPs; and 25(OH)D is rapidly metabolized in the process, resulting in a low serum level. The resulting elevated 1,25(OH)2D causes chronic, systemic inflammation and its accompanying symptoms (Fig. 3).
Effects of intracellular pathogens on the immune system
Pathogens gain many advantages by parasitizing immune cells and altering nuclear receptor activity. Tissue invasion provides a privileged niche with access to host protein and iron, sequestration from immune response, and a means for persistence [90]. In the arms race of host–microbe co-evolution, successful microbial pathogens have evolved innovative strategies to evade host immune responses. For example, ‘crosstalk manipulation’ undermines host defenses and contributes to microbial adaptive fitness [91, 92].
It is theorized that bacteria have developed some of these strategies in order to invade host cells and remain undetected within cellular cytoplasm. Many bacterial pathogens form antibiotic-tolerant persister cells which can replicate within macrophages. In this form they can cause subclinical infection and have been associated with chronic diseases [95, 97, 98].
Macrophage microbicidal mechanisms are responsible for the control and elimination of pathogens. 1,25(OH)2D production and action in macrophages activates toll-like receptors to increase expression of the AMP cathelicidin which kills infectious invaders [101, 102]. When the immune system is fighting a persistent microbe, inflammatory molecules are continuously released in an effort to kill the pathogen [103]. Immune defenses stimulate Th17 cells and contribute to the development of chronic inflammatory conditions [104, 105]. An ineffective immunological response causes low-grade inflammation and phagocyte-inflicted tissue damage plays an important role in many chronic diseases [106]; autoimmune patients acquire a distinct pathogenic microbiota and multi-morbidity often results [107, 108]. Therefore, it is reasonable to infer that bacteria have evolved strategies which allow them to persist within host cells. The exact mechanisms are unknown and warrant further study.
The compromised immune system, infection and vitamin D
Regulation of the VDR is a common mechanism used in the host defense against pathogens but certain microbes have been shown to slow innate immune defenses by down-regulating the VDR:
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Mycobacterium tuberculosis down-regulates VDR activity [112].
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Mycobacterium leprae inhibits VDR activity through down-regulation of CYP27B1 in monocytes [113].
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Aspergillus fumigatus secretes a toxin capable of down-regulating the VDR in macrophages [114].
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Epstein–Barr virus lowers VDR activity [115].
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HIV completely shuts down VDR activity [116].
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In VDR knockout mice, a circumstance that closely mimics extreme VDR dysregulation, 1,25(OH)2D levels increase by a factor of ten [117].
Studies also point to immune system depression and elevated 1,25(OH)2D in chronic diseases [118]:
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Sarcoidosis patients are deficient in cathelicidin despite healthy vitamin D3 levels [119].
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1,25(OH)2D is high (>60 pg/ml) in 42 % of Crohn’s patients and the source of the active vitamin D may be the inflamed intestine [52].
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1,25(OH)2D is elevated in the synovial fluid of patients with RA (rheumatoid arthritis) [120].
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Crohn’s disease decreases expression of cathelicidin [121].
High levels of 1,25(OH)2D may result when down-regulation of the VDR by bacterial ligands prevents the receptor from expressing enzymes necessary to keep 1,25(OH)2D in a normal range [42]. Elevated 1,25(OH)2D further reduces VDR competence, suppresses macrophage function, and blocks the nuclear factor kappa-B pathway; thus inhibiting immune system function [116, 122, 123]. Reducing the ability of the VDR to express elements of innate immune function allows intracellular bacteria to persist in the cytoplasm of nucleated cells and may account for the increased susceptibility to non-bacterial co-infections that are commonly found in patients with chronic illnesses [124, 125].
Elevated 1,25(OH)2D appears to be evidence of a disabled immune system’s attempt to activate the VDR to combat infection.
Autoimmune disease
Vitamin D appears to have a positive effect on autoimmune disease due to immune system suppression [122, 134, 135] and immune suppression is considered therapeutically beneficial for autoimmune diseases [136, 137]. However, vitamin D proponents have failed to recognize that positive effects are due to the immunosuppressive effect of elevated 25(OH)D or to understand that immunosuppression is contraindicated because of the probable presence of intracellular infection. When the immune system is suppressed clinical disease markers and symptoms are reduced but immunosuppression does not address an underlying cause of persistent bacteria, thus relapse is common [138]. Verway et al. [79] wonder, “Is a specific pathogen responsible for disease or rather is a dysregulated immune response generated against a complex microbial population? Why would immune-suppressive drugs be efficacious if the primary defect is an immune deficiency?” Much of current research focuses on finding drugs to suppress inflammation but, according to Collins [139], 95 % of these studies have failed It seems clear a better direction is needed. Immunotherapy which restores VDR competence corrects dysregulated vitamin D metabolism and eliminates intracellular bacteria could be the answer (as discussed in the section titled Restoring VDR Competence).
Dysregulated vitamin D metabolism
In a healthy individual, the complex interplay between innate and adaptive immunity cooperates to mount an appropriate response to infection through regulation of the vitamin D endocrine system [140]. The immune system detects and responds to the presence of intracellular bacteria by producing more 1,25(OH)2D to activate the VDR and express the crucial endogenous AMPs which enable the innate immune system to target intracellular pathogens [141]. Renal production of 1,25(OH)2D is tightly self-regulated, with the end product down-regulating its own further production. In contrast, extra-renal tissues (e.g., uterine decidua and placenta, colon, breast, prostate, spleen, bone, keratinocytes, melanoma and synovial cells, pulmonary monocytes and macrophages, etc.) which produce 1,25(OH)2D are regulated by cytokines (e.g., interferon-gamma), lipopolysaccharide, nitric oxide and intracellular VDBP, which activate the enzyme CYP27B1 to stimulate conversion of 25(OH)D to 1,25(OH)2D [142]. This extra-renal production of 1,25(OH)2D in tissues infected with intracellular bacteria can result in an excess in production of 1,25(OH)2D which may contribute to depletion and low levels of 25(OH)D [143] (Fig. 4).
Because extra-renal production of 1,25(OH)2D is primarily dependent on the availability of 25(OH)D [144], supplementation with vitamin D to increase 25(OH)D may promote the production of 1,25(OH)2D in non-renal tissues that are sites of intracellular infection and result in hypervitaminosis-D.
We hypothesize that when nucleated cells are parasitized by intracellular bacteria, extra-renal production of 1,25(OH)2D increases, the kidneys lose control of 1,25(OH)2D production, and pro-hormone 25(OH)D decreases due to rapid conversion to 1,25(OH)2D. The following mechanisms are thought to be responsible (Fig. 5):
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Inflammatory cytokines activate CYP27B1, an enzyme that causes more 25(OH)D to be converted to 1,25(OH)2D [146].
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The microbial-repressed VDR cannot transcribe CYP24A1 (formerly 24-hydroxylase), an enzyme that breaks down excess 1,25(OH)2D [147].
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Excess 1,25(OH)2D binds the PXR (pregnane X receptor), to inhibit conversion of vitamin D3 to 25(OH)D so 25(OH)D is down-regulated [148].
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1,25(OH)2D inhibits the hepatic synthesis of 25(OH)D [149].
Thus, low 25(OH)D may be a consequence of the inflammatory process. More studies are concluding that suboptimal circulating levels of vitamin D appear to be caused by the disease process. Waldronn et al. [150] found serum 25(OH)D was decreased following an acute inflammatory insult (i.e., orthopedic surgery) and concluded that hypovitaminosis-D may be the consequence rather than cause of chronic inflammatory diseases.
Diagnosis of dysregulated vitamin D metabolism
Vitamin D status is currently determined by measuring the level of serum 25(OH)D which, presumably, reflects the serum levels of other vitamin D metabolites (e.g., vitamin D3, vitamin D2 and 1,25(OH)2D, etc.). This measurement may not, however, provide enough information to assess vitamin D endocrine function. The clinical utility of measuring 1,25(OH)D is not fully understood, but it is clear that associations are being made between this active metabolite of vitamin D and disease states [154].
In the context of solving the puzzle of low 25(OH)D, the reasons cited for this lapse fail to consider the possibility of abnormal levels in the presence of chronic inflammation:
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1,25(OH)2D has a short half-life (hours) and fluctuates rapidly.
However, a high result may be discovered even at trough level.
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1,25(OH)2D levels are regulated by PTH, calcium, phosphate.
This is not true if extra-renal production is prevalent [143].
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1,25(OH)2D does not decrease until 25(OH)D is very low.
A low 25(OH)D may be a sign that 1,25(OH)2D is abnormally high [55].
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1,25(OH)2D is only over-produced in hypercalcemic disease states such as sarcoidosis.
Studies show this is not true [42].
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1,25(OH)2D may be elevated as a result of up-regulation of the CYP27B1 enzyme.
This begs the question, Why is this enzyme elevated [146]?
Measuring both 25(OH)D and 1,25(OH)2D (and PTH, calcium, phosphate when indicated) as clinical markers in chronic disease is more likely to provide a true picture of vitamin D status, than measuring 25(OH)D alone [155, 156] (Table 1). Measuring 1,25(OH)2D should be considered in patients with low 25(OH)D, abnormal laboratory results (especially inflammatory markers), a diagnosis of autoimmune disease or other chronic inflammatory illness, or signs of chronic systemic inflammation. For example, elevated 1,25(OH)2D may serve as a marker of Crohn’s disease [52].
Restoration of VDR competence
The ability to mount an appropriate immune system response to intracellular infection is highly dependent on a competent VDR [159]. When it appears that 1,25(OH)2D is unable to up-regulate the VDR due to microbial activity, VDR competence may be restored with another VDR ligand which acts as an agonist; an agonist increases the signal transduction activity of a cell when bound to a receptor on that cell. Over 3000 synthetic VDR ligands have been identified, but most of these 1,25(OH)2D analogues have no clinical use because of their undue disruption to calcium regulation [160]. A number of non-vitamin D VDR ligands have also been identified: curcumin, omega-6 fatty acids (e.g., arachidonic acid, linoleic acid), and lithocolic acid (LCA) but are not being used for this purpose [161, 162].
Angiotensin receptor blockers (ARBs) have been shown, via in silico molecular modeling, to modulate VDR activation [163]. The most promising ARB, olmesartan medoxomil (brand name Benicar®) was estimated to have a Ki value in the low nanomolar range, similar to the Ki values of the natural VDR ligands [163].
Olmesartan is believed to decrease elevated 1,25(OH)2D by several VDR-mediated effects (Fig. 6). The up-regulated VDR:
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transcribes CYP24A1 and CYP3A4 (enzymes which reduce 1,25(OH)2D production) [147].
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represses CYP27B1 (the enzyme that hydroxylates 25(OH)D to 1,25(OH)2D) so less 1,25(OH)2D is produced [146].
Referência :
Inflamm Res. 2014 Oct;63(10):803-19.
