The chief findings of this study are that low VD concentrations in the blood were significantly associated with CAD and its severity in Egyptian patients. This prospective study found that low VD levels independently predicted atherosclerotic coronary artery lesions in ischaemic patients. Moreover, it was an independent predictor of multivessel coronary affection diagnosed by cardiac catheterization. The study complemented the laboratory stage of this work that previously revealed the existence of low VD receptor expression in human and animal atherosclerotic arterial strips [7]. Taken together, our findings of low VD levels in the blood of CAD patients and a high prevalence of low VD receptors in atherosclerotic arteries confirmed the significant role of VD in the pathogenesis of atherosclerosis. The most important aspect of the study was its prospective design, which avoided the problems associated with the retrospective collection of data, and the inclusion of a large number of Egyptian patients who underwent cardiac catheterization.

The aetiology of cardiovascular disease is not completely understood. Geographic factors, including place of residence, nutrition, and exposure to sun, may be related to cardiovascular disorders, including atherosclerotic coronary artery disease. These environmental data may influence the exposure of humans to ultraviolet rays and consequently their VD levels. The median level of VD in CAD patients was significantly lower than that in controls [14.65 (9.25–21.45) vs. 42.0 (32–53), p < 0.001]. It acted concurrently with the atherosclerotic lipid profile (high total cholesterol, LDL-cholesterol and triglycerides and low HDL-cholesterol) on the coronary arteries to cause atherosclerosis. Zittermann et al. illustrated how low 25-OH VD levels could result in atherosclerotic changes [18]. Several pathways might be responsible for the pathogenesis of atherosclerosis. 1,25 DihydroxyVD (a precursor of renal conversion of 25-OH VD by 1α-hydroxylase) inhibits the proliferation of vascular smooth muscle cells by acute influx of calcium into the cells. Thus, low VD results in the proliferation of smooth muscle cells, causing thickening of the vascular endothelium. It is also responsible for the low production of matrix G1α protein from vascular smooth muscle cells, triggering massive vascular calcification. Moreover, serum parathyroid hormone might be increased as a compensatory mechanism to raise calcium levels eliciting calcium and phosphate deposition in vessel walls and more insulin resistance. Thickening of the vessel wall due to the proliferation of smooth muscle cells and calcification could encroach into the coronary lumen markedly resulting in myocardial ischaemia and even infarction.

Atherosclerosis is a systemic disease characterized by chronic inflammation with the accumulation of lipids, smooth muscle cell proliferation, cell apoptosis, necrosis, fibrosis, and local inflammation. Endothelial inflammation releases proinflammatory cytokines and adhesive molecules such as interleukin 6 (IL-6), monocyte chemotactic protein 1 (MCP-1), intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1), initiating monocyte adhesion and infiltration of the vessel wall. Consequently, proinflammatory cytokines such as tumour necrosis factor-α (TNF-α) and interferon-γ (IFN-γ) will be released in the vascular environment. Monocyte-derived macrophages engulf oxidized low-density lipoprotein, forming foam cells and fatty streaks and resulting in plaque formation in coronary arteries. The process is tightly regulated by cytokines [19]. Low VD, as noticed in our patients, may be associated with increased levels of proinflammatory cytokines such as TNF-α and IL-6 and decreased levels of anti-inflammatory cytokines such as interleukin-10 (IL-10), enhancing atherosclerotic plaque formation. This is accompanied by endothelial dysfunction [20] and increased renin-angiotensin system expression, aggravating atherosclerotic processes, hypertension and cardiovascular risk [6, 21]. Although the association between low VD and atherosclerosis denotes a causative link, it might be a consequence of atherosclerosis [6]. On the other hand, the high vitamin D group in our population had a significantly higher level of HDL-cholesterol than the low vitamin D group. Recently, HDL-cholesterol has been found to prevent inflammation and atherosclerotic processes [22].

We diagnosed CAD solely by coronary angiography, which is currently considered the gold-standard method. Other modalities used to diagnose CAD and the number of diseased coronary arteries, such as electrocardiographic examination, exercise stress test, stress test and computed tomography yield higher rates of false results. Nevertheless, similar studies that relied on cardiac catheterization to diagnose coronary atherosclerosis and to measure VD in patients with CAD were reported [20, 23]. It is worth mentioning that coronary revascularization depends chiefly on cardiac catheterization [24].

Our study revealed a significant association between low VD and atherosclerotic CAD. Low VD was an independent predictor of atherosclerotic CAD and was correlated with traditional coronary risk factors, although the correlations were not very strong. A large cross-sectional study showed that patients with a high VD level, ≥ 30 ng/mL, had a more favourable lipid profile than patients with a low VD level, < 20 ng/mL [25], so low VD might be considered a cardiovascular risk factor [26]. More recently, measurements of VD in patients with ST-elevation myocardial infarction were consistent with our study [23]. Of note, most of our patients who presented with acute myocardial infarction (57.4%) had ST-elevation myocardial infarction. Recently, VD receptors were discovered in more than 30 cell types [27]; therefore, they carry out different functions rather than participating in mineral homeostasis alone [28]. VD receptors in the blood vessels might be responsible for the integrity of the vascular wall. Few VD receptors are expressed in the endothelial and smooth muscle cells of atherosclerotic and preatherosclerotic lesions [5, 7, 29, 30].

Most [103 (57.3%)] of our patients low VD (< 30 ng/mL) had ACS, including unstable angina, ST-elevation myocardial infarction and non-ST-elevation myocardial infarction. The odds of low VD in patients with ACS were higher than they were in patients with high VD [OR 2.84 (95% CI 1.99–4.05), p < 0.001]. The rupture of vulnerable plaques was thought to be responsible for most of these events. Vulnerable plaques are characterized by rich lipid content, a thin fibrous cap and high inflammatory activity. Although the molecular function of VD signalling is still not clear [28], VD was shown to reduce the accumulation of cholesterol in macrophages and decrease LDL-cholesterol uptake in atheromas [31]. These favourable effects were associated with inhibition of platelet aggregation and reduction of thrombogenic activity through modulation of thrombomodulin and tissue factor expression in monocytes [32]. Thus, low VD triggers cholesterol accumulation and enhances thrombogenic activity. Additionally, plaque destabilization was boosted through the increase in expression of matrix metalloproteinase (MMP)-2 and MMP-9 which was evident in blood samples derived from male patients with acute myocardial infarction. Therefore, plaques become more vulnerable to rupture, causing ACS [33, 34]. These effects of low VD levels also explain the findings in our study and other prospective studies [2]. Giovannucci et al. demonstrated that the risk of myocardial infarction was significantly higher in men who had VD ≤ 15 ng/mL than those with sufficient VD (≥ 30 ng/mL) at ten-year follow-up [relative risk 2.42 (95% CI 1.53–3.84), p < 0.001] [2]. Of note, our CAD patients had a similar median level of VD, 14.65 ng/mL, explaining why acute myocardial infarction predominated. The study centre was a referral centre for primary coronary intervention, which partly explains why acute myocardial infarction was common in our population. We found a cut-off value of VD at 30 ng/mL, below which atherosclerotic coronary artery disease was more likely to occur. A similar level was reported before [11, 21, 25].

Multivessel disease of coronary arteries would carry an increased risk of future cardiovascular events in CAD patients. Two- and three-vessel disease were prevalent in the majority of patients with low VD, 109 patients (60.6%). The OR of having more than single-vessel disease in the population with low VD versus those with high VD was 4.43 (95% CI 2.87–6.84, p < 0.001). Taking into consideration that other variables, such as diabetes mellitus, left ventricular ejection fraction, LDL-cholesterol, total cholesterol and smoking, were important, a low VD level was an independent predictor of multivessel disease of coronary arteries in our population. However, in peripheral arterial disease, conflicting results of VD levels have been reported [35, 36]. Nevertheless, multivessel disease was seen more frequently in patients with low VD than in those with high VD levels [20]. More confirmatory studies identified a significant association between low VD and the severity of coronary artery disease as assessed by the SYNTAX score [37, 38]. These studies proved that low VD levels were significantly associated with high SYNTAX score (> 22) but not in patients undergoing surgery [39].

To summarize, the study showed that VD was essential for the integrity of coronary arteries, while its reduction was associated with the occurrence of atherosclerotic CAD, especially multivessel disease and possibly ACS. On the other hand, based on the results of the VITAL and WHI studies, which demonstrated that VD supplementation failed to prevent cancer, cardiovascular events [14], mortality [40], and even bone fracture [41, 42], VD supplementation should not be prescribed for primary prevention of CAD [43,44,45], but it would be beneficial in treating CAD patients, especially those with multivessel disease and ACS. Hence, prospective trials would be helpful to demonstrate the effect of VD supplementation in treating CAD patients, especially those with multivessel disease and ACS.