Potential Beneficial Effects of Vitamin D in Coronary Artery Disease

Vitamin D plays a pivotal role in bone homeostasis and calcium metabolism. However, recent research has indicated additional beneficial effects of vitamin D on the cardiovascular system. This review aims to elucidate if vitamin D can be used as an add-on treatment in coronary artery disease (CAD). Large-scale epidemiological studies have found a significant inverse association between serum 25(OH)-vitamin D levels and the prevalence of essential hypertension. Likewise, epidemiological data have suggested plasma levels of vitamin D to be inversely correlated to cardiac injury after acute myocardial infarction (MI). Remarkably, in vitro trials have showed that vitamin D can actively suppress the intracellular NF-κB pathway to decrease CAD progression. This is suggested as a mechanistic link to explain how vitamin D may decrease vascular inflammation and atherosclerosis. A review of randomized controlled trials with vitamin D supplementation showed ambiguous results. This may partly be explained by heterogeneous study groups. It is suggested that subgroups of diabetic patients may benefit more from vitamin D supplementation. Moreover, some studies have indicated that calcitriol rather than cholecalciferol exerts more potent beneficial effects on atherosclerosis and CAD. Therefore, further studies are required to clarify these assumptions.


Introduction
Cardiovascular disease (CVD) is a major concern of global health. According to the World Health Organization (WHO), CVD is the most common cause of mortality worldwide. Approximately 17.9 million people died from CVD in 2015, with 7.3 million of these deaths due to coronary artery disease (CAD) [1]. Although CAD is formerly considered a disease mediated by lipid accumulation, its pathophysiology is complex, and the exact underlying mechanisms are still unknown. More recent investigations have suggested an additional excessive inflammatory response in the subintimal arterial space followed by thrombus formation [2,3]. Furthermore, several studies have found that blood microparticle levels are elevated in individuals with CAD [4,5]. Different molecules on the surface of the microparticles mediate procoagulant properties that may lead to an acute coronary event [6].
In 2016, the prevalence of CAD in Denmark was estimated to be approximately 160,000 people [7]. Interestingly, new data suggest that vitamin D is a potentially cost-effective treatment agent for CAD [8]. This review will focus on relevant studies in order to investigate whether vitamin D supplementation may exert beneficial effects on atherosclerosis and CAD.

Literature Search and Investigation
Studies included in this review met the following criteria: participants were adults (>18 years); measured endpoints included risk of myocardial infarction (MI), mortality, plaque burden, CAD events, pulse wave velocity (PWV), adhesion molecules, blood lipids, high-sensitive C-reactive protein (hsCRP) and/or SYNTAX score. Only data from randomized clinical trial (RCT) studies in the publication period from 1 January 2010 to 12 October 2019 were assessed. Only English-language studies that were completed with available results were included. Studies with only stroke or heart failure (HF) as the CVD endpoints were excluded. Moreover, studies with cohorts consisting only of chronic kidney disease (CKD) patients were excluded. The intervention arm was administered calcitriol, cholecalciferol or ergocalciferol.
To prepare the literature review, the following databases were used: PubMed (https://www.ncbi.nlm.nih.gov/pubmed/), Clinicaltrials.gov (https://clinicaltrials.gov/) and Scopus  In order to determine whether vitamin D can reduce vascular inflammation and atherosclerosis by suppression of the NF-κB pathway and be used as a potential treatment agent for CAD in patients with hypovitaminosis D, this review will define CAD and its risk factors, pathophysiology, symptoms and treatment. Next, the physiological role of vitamin D will be described. This review will investigate if vitamin D can be used as a prognostic marker of CAD risk. Finally, the potential role of vitamin D in cardiomyocytes after MI and possible cardioprotective mechanisms will be elucidated. In order to determine whether vitamin D can reduce vascular inflammation and atherosclerosis by suppression of the NF-κB pathway and be used as a potential treatment agent for CAD in patients with hypovitaminosis D, this review will define CAD and its risk factors, pathophysiology, symptoms and treatment. Next, the physiological role of vitamin D will be described. This review will investigate if vitamin D can be used as a prognostic marker of CAD risk. Finally, the potential role of vitamin D in cardiomyocytes after MI and possible cardioprotective mechanisms will be elucidated.

Coronary Artery Disease
CAD is an overall group of clinical conditions including stable angina, unstable angina, myocardial infarction (MI) and sudden death [9]. Important complications are heart failure and arrhythmia. The symptoms of CAD are the result of an inadequate blood supply of the heart caused by the obstruction of a coronary artery. MI is the most common manifestation of CAD and is due to the disruption of a vulnerable atherosclerotic plaque or the erosion of the coronary artery endothelium. Upon rupture, the atherosclerotic plaque releases thrombogenic contents, initiating a coagulation cascade. This hypercoagulable state could especially contribute to the rupture of additional vulnerable fibroatheromas leading to more than one culprit lesion. MI finally ends in an irreversible necrosis of myocardial cells that is detectable by an elevation of cardiac biomarkers [10,11].

Pathophysiology of Atherosclerosis
The underlying mechanisms of atherosclerosis can be divided into two parts: formation of a stable plaque and transition into an unstable plaque. The first process involves endothelial erosion with endothelial activation and denudation [12]. This endothelial dysfunction results in the deposit of low-density lipoprotein (LDL) molecules in the vascular intima [13], which leads to the formation of fatty streaks and eventually stable plaques. Lipoxygenases and myeloperoxidases oxidize the LDL molecules in the vessel wall. This oxidation attracts and stimulates activated macrophages [12]. These macrophages may induce apoptosis of endothelial cells and form a thin fibrous plaque cap that separates the lipid core from the lumen. The plaque consists of degraded smooth muscle cells, endothelial cells, foam cells, cellular debris, lymphocytes and modified lipids [14]. This mechanism is referred to as atherosclerosis and causes a gradual narrowing of the lumen. Atherosclerosis is considered to be a chronic inflammatory process in the vessel wall [15].
Subsequently, atherosclerosis may be followed by rupture of the vulnerable plaque cap. This exposes the lipid core to the vessel lumen. The atheromatous mass is now thrombogenic and causes platelet activation and finally coronary occlusion [16]. Coronary artery narrowing or occlusion may cause the symptoms of angina due to the onset of ischemia.

CAD Symptoms
The CAD symptom spectrum of angina can manifest itself in multiple ways. According to the European Society of Cardiology (ESC) [17], the typical symptoms include discomfort, pain, nausea, fatigue, restlessness, burning, shortness of breath and uncomfortable chest pressure. The sensation of discomfort is most often located at the chest or near the sternum. However, this pain may also be localized between shoulder blades, at the jaw, teeth, in either arm or at the wrist and fingers. In most cases, the pain has a duration of ≤ 10 min and is triggered by physical exercise. Table 1 shows the typical characteristics of pain due to CAD. Table 1. Characteristics of coronary artery disease (CAD) symptoms, modified from [17].

Classification Characteristics
Typical angina Constricting sensation in front of chest or shoulder, neck, jaw or arm. Symptoms relieved by nitrates or rest ≤ 5 min. Triggered by physical exertion.
Atypical angina Meets only two of the characteristics above.
Non-anginal chest pain Meets none or just one of the characteristics above.
CAD may result in stable angina, unstable angina, ST-elevation myocardial infarction (MI) or non ST-elevation myocardial infarction.

CAD Prognosis
A serious complication to acute MI is the progression of heart failure (HF) with reduced function of the left ventricle (LV) [18]. The estimated risk of LV systolic dysfunction after MI is about 40% [19]. Pathogenesis is partly based upon excessive β-adrenergic activation post-MI. This mechanism is complex and promotes cardiomyocyte growth, vasoconstriction and cardiac injury. Subsequently, this might lead to cardiac remodeling and LV dysfunction [20].

Diagnosis of CAD
A thorough prior diagnosis is crucial. This section gives an overview of basic test procedures in patients with suspected diagnosis of CAD. Table 2 depicts some of the useful procedures in diagnosis of CAD. Table 2. Diagnostic tools for the diagnosis of CAD.

Procedure Explanation
Electrocardiogram (ECG) [21] ECG plays a key role in the initial diagnosis in patients presenting with angina symptoms. A resting 12-lead ECG may reveal abnormalities that support the diagnosis of MI or myocardial ischemia. ST-segment deviations (depression/elevation) may visualize myocardial injury.
Biochemical tests [17,22] This procedure may include blood samples with a lipid profile, fasting glucose and glycated hemoglobin (HbA1c), full blood count and plasma creatinine. Furthermore, it is essential to measure myocardial injury markers such as troponin I, troponin T and creatine kinase myocardial band (CK-MB).
Echocardiography [23] An echocardiography might be performed as a stress test under physical exercise or under concomitant administration of medication such as dipyridamole or dobutamine. This procedure might reveal areas in the left ventricle (LV) with wall abnormalities or hypocontractility.
Cardiovascular magnetic resonance (CMR) [24] This procedure utilizes electromagnetic waves for the imaging of heart and coronary vessels. This technique can be used to assess myocardial viability after MI [25].
Coronary catheterization and angiography [11] As recommended by the 2019 European Society of Cardiology (ESC) guidelines, this technique is now used in cases of inconclusive non-invasive tests and for patients with a high clinical likelihood and severe symptoms refractory to medical therapy or high event risk. A catheter is guided from a peripheral artery to the coronary artery. Subsequently, a contrast medium is injected, and coronary arteries are visualized by X-ray.

Current Treatment Options of CAD
Treatment of CAD must focus on both acute treatment and secondary prophylaxis. Smoking, physical inactivity, high body mass index (BMI), diabetes mellitus, hypertension, excessive dietary fat and genetic dispositions are some of the known risk factors associated with CAD [26]. They all contribute to the lifelong atherosclerotic process and increase the risk of ischemic events. Thus, public health approaches with a focus on smoking cessation, healthy diet, stress reduction, physical exercise and antihypertensive treatment are of great importance [16]. In addition, Ornish et al. [27] showed a 7.9% relative reduction of coronary artery stenosis after five years with the intensive lifestyle interventions mentioned above.
The pharmaceutical treatment of CAD aims to reduce CVD and improve survival. More drug classes can be included in the treatment regimen. Table 3 illustrates an overview of drugs used in the treatment of CAD. Table 3. Overview of medications used for CAD.

Indications Mechanism Drugs
Beta-adrenoceptor antagonists [28] arrhythmia, hypertension, post-MI, angina pectoris, CAD Act through the blockade of beta-adrenoceptors in cardiac muscle cells and vascular SMCs. Lead to decreased heart rate and cardiac output (CO). Secondarily, antagonism of β1-adrenoceptors will cause relaxation of vascular smooth muscle cells, induce vasodilation and lower total periphery resistance (TPR).

Aspirin
Statins [30,31] hyperlipidemia, dyslipidemia, post-MI  [39] were the first to describe surgical techniques with vein grafts in acute MI patients to re-establish blood supply to the myocardium. Since then, great progress has been made in order to improve survival in acute coronary syndrome (ACS) patients.

•
Percutaneous coronary intervention (PCI): The PCI procedure is an effective strategy for revascularization in CAD patients with both acute and stable forms. The intervention is performed by inserting a guidewire catheter into the femoral or radial artery. The guidewire is guided to the coronary artery, where the thrombosis is located. Here, a balloon is inflated, and, for example, a metallic stent might be inserted in order to prevent reinfarction. Stents can either be bare metal or drug-eluting (everolimus, zotarolimus, etc.) to minimize restenosis [17,40]. In principle, PCI is the preferred procedure in patients with ST-segment elevation myocardial infarction (STEMI) within 12 h of symptom onset [41]. In addition, patients with non-ST-segment elevation myocardial infarction (NSTEMI) might be offered PCI within 48 h of symptom onset if no relevant comorbidity is present.

•
Coronary artery bypass grafting (CABG): The CAGB procedure is considered more invasive compared to PCI. The procedure includes bypassing stenosed coronary arteries [42]. Thus, vein or artery grafts are used to anastomose occluded vessels. According to the SYNTAX study, the CABG strategy is preferable in more complex multivessel occlusions [43].

Vitamin D
Vitamin D is mainly synthesized endogenously, when the skin is exposed to ultraviolet radiation from sunlight. Since Askew et al. [44] first isolated vitamin D in 1932, much knowledge has been gained to understand the functions of this vitamin.

Vitamin D Metabolism
The active form of vitamin D (named 1,25(OH) 2 D 3 ) is based upon a modified steroid scaffold with lipophilic properties [45]. Its chemical structure contains a secosteroid with an open B-ring. Figure 2 shows a schematic overview of vitamin D metabolism. principle, PCI is the preferred procedure in patients with ST-segment elevation myocardial infarction (STEMI) within 12 h of symptom onset [41]. In addition, patients with non-ST-segment elevation myocardial infarction (NSTEMI) might be offered PCI within 48 h of symptom onset if no relevant comorbidity is present. • Coronary artery bypass grafting (CABG): The CAGB procedure is considered more invasive compared to PCI. The procedure includes bypassing stenosed coronary arteries [42]. Thus, vein or artery grafts are used to anastomose occluded vessels. According to the SYNTAX study, the CABG strategy is preferable in more complex multivessel occlusions [43].

Vitamin D
Vitamin D is mainly synthesized endogenously, when the skin is exposed to ultraviolet radiation from sunlight. Since Askew et al. [44] first isolated vitamin D in 1932, much knowledge has been gained to understand the functions of this vitamin.

Vitamin D Metabolism
The active form of vitamin D (named 1,25(OH)2D3) is based upon a modified steroid scaffold with lipophilic properties [45]. Its chemical structure contains a secosteroid with an open B-ring. Figure 2 shows a schematic overview of vitamin D metabolism. Typically, the marker 25-hydroxyvitamin D (25(OH)D) is used as a surrogate endpoint of vitamin D status in plasma [47]. Historically it has been difficult to establish evidence-based recommendations for optimal plasma levels of vitamin D. Several health regulatory agencies have published slightly different definitions of vitamin deficiency based on the serum levels of 25hydroxyvitamin D. An international consensus on the definition of vitamin D deficiency and sufficiency is lacking. Table 4 summarizes the definitions of selected health organizations and the Mayo Clinic [48][49][50][51][52][53]. Typically, the marker 25-hydroxyvitamin D (25(OH)D) is used as a surrogate endpoint of vitamin D status in plasma [47]. Historically it has been difficult to establish evidence-based recommendations for optimal plasma levels of vitamin D. Several health regulatory agencies have published slightly different definitions of vitamin deficiency based on the serum levels of 25-hydroxyvitamin D. An international consensus on the definition of vitamin D deficiency and sufficiency is lacking. Table 4 summarizes the definitions of selected health organizations and the Mayo Clinic [48][49][50][51][52][53].  [54], elucidating the local synthesis of active 1,25(OH) 2 D 3 . These findings suggest that vitamin D has autocrine and paracrine functions [54]. This probably exerts a positive impact on cardiovascular health and the immune system and prevents the development of diabetes mellitus [55].

Cardiovascular Effects of Vitamin D
In a recent review summarizing the current knowledge of the effects of vitamin D on cardiovascular disease, Saponaro et al. [56] demonstrated that this scientific field has drawn considerable attention in recent years. As detailed below, vitamin D deficiency is associated with hypertension [57], which is a risk factor in the atherosclerotic process. Moreover, in vitro models have been used to understand the possible mechanistic effects of vitamin D in CAD progression [58] and the suppression of renin synthesis [59]. Al-Ishaq et al. [60] have stated that vitamin D deficiency activates the renin-angiotensin-aldosterone system, which might lead to cardiac hypertrophy and increased CVD risk. However, RCTs and Mendelian studies have been inconclusive regarding the causality of vitamin D supplementation and improved cardiovascular outcomes [61].

Vitamin D and Essential Hypertension
In a review on vitamin D and essential hypertension [62], it was pointed out that, based on data from the third National Health and Nutrition Examination Survey (NHANES III), vitamin D deficiency is associated with essential hypertension [63,64]. In addition to these epidemiological findings, Yuan et al. demonstrated that vitamin D can suppress renin synthesis in vitro [59]. Nevertheless, RCTs performed to assess the impact of vitamin D supplementation on hypertension showed equivocal results [62]. This can, in part, be attributed to suboptimal study designs.

Association between Serum Vitamin D and Myocardial Injury
Using NHANES III data, Ahmad et al. [65] examined a possible association between serum vitamin D concentration and subclinical myocardial injury. In this cross-sectional study, recruited individuals were sought to be representative of the background population [66]. Hence, 8561 participants underwent a 12-lead ECG to visualize the electrical conduction of the heart. Participants with earlier diagnosed CVD were excluded, and thus 6079 participants were included for this analysis in the period between 1988 and 1994. To evaluate subclinical myocardial injury (SC-MI) in ECG measurements, the objective multivariate tool named the Cardiac Infarction Injury Score (CIIS) [67] was chosen. Participants were divided into three tertiles based on their serum levels of 25(OH)D (<20, 20-30 and >30 ng/mL).
The first group (serum-25(OH)D < 20 ng/mL) showed a prevalence of SC-MI = 23.0%, while the second group (serum-25(OH)D 20-30 ng/mL) had a prevalence of SC-MI = 21.1%. The third group (reference) with serum-25(OH)D > 30 ng/mL had a prevalence of SC-MI = 19.5%. A comparison of groups one and three revealed that SC-MI was inversely associated with 25(OH)D levels with an odds ratio (OR) of 1.27 (95% CI: 1.04-1.55) after adjustments for potential confounders [65]. Hence, the study found a significant incremental increase in SC-MI prevalence associated with vitamin D deficiency (p = 0.04).
Verdoia et al. [68] conducted another cross-sectional study to investigate the relationship between serum 25(OH)D-levels and CAD. The examined cohort comprised 1484 patients, all of whom underwent elective coronary angiography. The results showed that vitamin D deficiency is significantly associated with the severity of CAD. Comparing the odds of CAD in patients with severe hypovitaminosis D (<10 ng/mL) and patients with normal vitamin D status yielded an adjusted OR of 1.73 (95% CI: 1.18-2.52).
The strengths of these studies can be attributed to the large sample size. Moreover, data included in the NHANES III study were derived from a sample group without prior CVD history. So far, the NHANES III survey is the most comprehensive study, where both information on serum 25(OH)D-levels and markers of myocardial injury can be extracted. However, the methodological limitations include potential confounding factors, as the evaluations of exposure and outcomes were not temporally separated. Likewise, seasonal variation in serum 25(OH)D-levels might be a concern. This is due to fact that information is lacking about the time of year at which the participants had blood samples collected [65]. Even though both studies found strong associations between vitamin D status and CAD, this does not necessarily substantiate causality.

Impact of Vitamin D on Cardiac Function after MI
Le et al. [69] conducted an in vivo study to explore the effects of vitamin D on cardiac function in post-MI mice. One group of 1,25(OH) 2 D 3 supplemented mice (n = 5) was compared to non-vitamin D supplemented controls (n = 5). The experimental mice were offered an optimal diet and husbandry conditions. At the start of the study, MI was induced in all mice by ligating the left anterior descending (LAD) artery. Subsequently, the intervention group was administered calcitriol 0.6 µg/day/kg for 14 days and examined by echocardiography and histological analysis. The results showed a significant reduction in the fibrotic scar area in the LV in the intervention arm compared with controls (p < 0.05). Likewise, LV wall thinning after MI was attenuated in calcitriol-supplemented mice versus controls (p < 0.05). Le et al. [69] performed an in vitro experiment to provide a mechanistic explanation for these findings (i.e., that vitamin D suppresses cell cycle progression in cCFU progenitor cells). Thus, cardiac myofibroblast differentiation might be decreased after calcitriol supplementation.

Possible Mechanisms behind Vitamin D Effects on CAD
To clarify possible underlying mechanisms of vitamin D effects on CAD, Chen et al. [58] performed a study in swine. Here, epicardial adipose tissue (EAT) cells were extracted and cultured as preadipocytes in vitro. Interestingly, this study indicated that vitamin D suppresses the nuclear factor 'kappa-light-chain-enhancer' of the activated B-cells (NF-κB) pathway and thereby attenuates the progression of CAD. Figure 3 depicts how vitamin D interferes with the NF-κB pathway. EAT cells are deeply involved in the progression of coronary atherogenesis mediated through the synthesis of local inflammatory cytokines [70]. First, KPNA4 mRNA is transcribed in the nucleus and transported to the endoplasmic reticulum in the cytosol, where translation takes place. Subsequently, the nascent KPNA4 protein is released and incorporated in the nuclear membrane. KPNA4 is a membrane transporter that is responsible for shuttling NF-κB from the cytosol to the nucleus [71]. NF-κB acts as a transcription factor in the nucleus via binding to different κB elements [72], which promote the transcription of proinflammatory cytokines such as IL-6, IL-8 and TNF-α. These cytokines are involved in the progression of atherogenesis in coronary arteries [73,74].
Interestingly, it appears that liganded 1,25(OH)2-vitamin D3-VDR actively suppresses the transcription and translation of KPNA4 in EAT cells. The reduced expression of KPNA4 leads to compromised shuttling of NF-κB into the nucleus. Hence, sufficient levels of intracellular 1,25(OH)2vitamin D3 are capable of reducing the inflammatory response in the atherosclerotic process. This might to some extent be a mechanistic explanation as to how vitamin D deficiency is linked to CAD. However, it still remains to be elucidated how vitamin D3 indirectly or directly suppresses KPNA4 transcription [58].

Vitamin D Supplementation and CAD
A comprehensive review of relevant studies has been prepared in order to investigate whether vitamin D supplementation may exert beneficial effects on atherosclerosis and CAD. All eligible RCTs for this review are listed in Table 5. EAT cells are deeply involved in the progression of coronary atherogenesis mediated through the synthesis of local inflammatory cytokines [70]. First, KPNA4 mRNA is transcribed in the nucleus and transported to the endoplasmic reticulum in the cytosol, where translation takes place. Subsequently, the nascent KPNA4 protein is released and incorporated in the nuclear membrane. KPNA4 is a membrane transporter that is responsible for shuttling NF-κB from the cytosol to the nucleus [71]. NF-κB acts as a transcription factor in the nucleus via binding to different κB elements [72], which promote the transcription of proinflammatory cytokines such as IL-6, IL-8 and TNF-α. These cytokines are involved in the progression of atherogenesis in coronary arteries [73,74].
Interestingly, it appears that liganded 1,25(OH) 2 -vitamin D 3 -VDR actively suppresses the transcription and translation of KPNA4 in EAT cells. The reduced expression of KPNA4 leads to compromised shuttling of NF-κB into the nucleus. Hence, sufficient levels of intracellular 1,25(OH) 2 -vitamin D 3 are capable of reducing the inflammatory response in the atherosclerotic process. This might to some extent be a mechanistic explanation as to how vitamin D deficiency is linked to CAD. However, it still remains to be elucidated how vitamin D 3 indirectly or directly suppresses KPNA4 transcription [58].

Vitamin D Supplementation and CAD
A comprehensive review of relevant studies has been prepared in order to investigate whether vitamin D supplementation may exert beneficial effects on atherosclerosis and CAD. All eligible RCTs for this review are listed in Table 5.

Discussion
Overall, this review attempted to elucidate whether vitamin D supplementation could be beneficial as a treatment agent in CAD patients. A possible mechanistic link was provided by Chen et al. [58], who explained how vitamin D alters the inflammatory response of CAD through suppression of the NF-κB pathway. Likewise, Le et al. [69] suggested that calcitriol might decrease fibroblast differentiation in progenitor cCFU cells after MI. However, this study was conducted in mice, which might make it problematic to directly transfer these findings to the human organism. Overall, these results are consistent with epidemiological studies reporting serum 25(OH)D to be inversely correlated with CAD and myocardial injury [65,68]. However, the possibility of unknown confounding factors in these cross-sectional studies cannot be excluded. Hence, randomized prospective studies are in high demand. Table 5 shows recent the RCTs that have attempted to address this issue. Nevertheless, the results are ambiguous. The two large scale studies [75,78] failed to demonstrate a beneficial effect of vitamin D on MI risk and CVD events. These studies must be given greater weight due to the large sample size. Interestingly, Scragg et al. [75] used monthly bolus doses of 100,000 IU in their study. One consideration worth following with this study design is the bioavailability of vitamin D. A high-dose intervention with a long dosage interval might be a suboptimal study design [90]. All RCTs that used this study design failed to demonstrate major benefits of vitamin D supplementation, even though plasma levels were restored [75,79,80,82,86].
Hin et al. [76] found no cardiovascular benefits of daily cholecalciferol therapy. Remarkably, plasma levels of 25(OH)D were above 50 nmol/L at baseline and 12 months after the intervention in both the intervention arm and controls. According to the Danish Health Authority [52], the threshold of vitamin D sufficiency is achieved at plasma concentrations between 50-160 nmol/L. Thus, possible additional cardiovascular benefits might be difficult to detect in this sample group.
The two studies by Seibert et al. [77] and Sokol et al. [84] have more similarities. Both studies had a study period of 12 weeks and did not show significant changes in endothelial markers, BP, inflammation or blood lipids. Perhaps a longer duration of follow-up in these studies would have clarified the effect of this intervention.
Only four small RCTs succeeded in showing major cardiovascular improvements following vitamin D supplementation. Wu et al. [81] examined whether daily supplementation of 0.5 µg calcitriol for six months could improve CAD. The results revealed a significantly decreased SYNTAX score (−3.9; p < 0.001) and reduced vascular inflammation. This was the only RCT to employ the administration of calcitriol (1α, 25-(OH) 2 D 3 ), which is the active form of vitamin D. This vitamin D analogue is more potent compared to cholecalciferol and might be more suitable treating vitamin D insufficiency [91]. Therefore, it could be speculated as to whether this analogue is more effective in exerting positive effects on atherosclerosis and CAD. Nevertheless, further studies are needed to investigate if calcitriol is a better treatment agent in cardiovascular disease.
In a sub-study of the Women's Health Initiative, Manson et al. [87] did not find evidence for reduced coronary calcification after seven years of cholecalciferol treatment. It is important to state that this RCT did not obtain information about vitamin D status in participants.
In a small study by Arnson et al. [85], five days of cholecalciferol treatment attenuated some inflammatory and endothelial markers (CRP, VCAM-1 and IL-6). Likewise, Raygan et al. found reduced vascular inflammation (hsCRP) and metabolic improvements in diabetic patients supplemented with 12 weeks of vitamin D [88,89]. Low calcium intake and low vitamin D status have been associated with obesity and diabetes mellitus [92,93]. This hints towards a tendency for better cardiovascular outcomes with vitamin D supplementation in certain patient subgroups, such as type 2 diabetics. Nevertheless, other studies in diabetic type 2 patients failed to demonstrate any effects on vascular inflammation [80,82,83]. In addition, the role of vitamin D in Ca 2+ -mediated apoptosis in obesity indirectly supports the recommendation to reach an optimal vitamin D status [94].
In general, the results in this field are conflicting. A comprehensive 2019 meta-analysis by Barbarawi et al. [95] including 83,000 participants did not find beneficial cardiovascular outcomes following vitamin D supplementation. However, further studies are needed to clarify if special subgroups could benefit from this intervention.

Conclusions
CAD is one of the most prevalent cardiovascular diseases. This disease is mainly caused by the progression of atherosclerosis. Predisposing factors represent a complex interaction between lifestyle, environmental and genetic contributors. Recent large-scale observational studies have demonstrated a strong inverse correlation between plasma levels of 25(OH)D and coronary atherosclerosis. Interestingly, in vitro studies suggest that vitamin D may attenuate CAD through the downregulation of the NF-κB pathway. However, the results obtained from a review of relevant RCTs presented here did not clearly show cardiovascular improvements following cholecalciferol supplementation.
Only a few RCTs have supported the hypothesis of the benefits of vitamin D in the treatment of CAD. In one study that employed calcitriol as the intervention, the results indicated a significant reduction in CAD and vascular inflammation. Hence, future studies could focus on the effects of more potent vitamin D analogues, such as calcitriol. Likewise, considerations of sufficient doses are important to conducting optimally designed studies. Finally, future studies may consider if certain subgroups such as type 2 diabetics with vitamin D insufficiency are more suitable for vitamin D supplementation.