**4. Biologic plausibility**

Although biologically plausible, the characterization of vitamin D deficiency as a primary risk factor for CVD is challenging because of the complexity and number of interplaying pathways vitamin D is involved with.

The vitamin D receptor is nearly ubiquitous. It has been found in many cells including vascular smooth muscle cells (VSMC), endothelial cells, cardiac myocytes, and juxtaglomerular and most immune cells, all of which have been implicated in the pathogenesis and progression of CVD [3, 6, 10, 15–17, 32, 37–41].

Activated CD4+ and CD8+ T cells, B cells, neutrophils, macrophages, and dendritic cells all possess the capacity to convert 25(OH)D3 into its active form 1,25(OH)D3. Moreover, it is known that the rate‐limiting enzyme in this pathway, 1,25 hydroxylase, is also present in activated macrophages [41, 42]. VSMC [37] and endothelial cells also express their own 1,25 hydroxylase, suggesting that these cells contain an autocrine mechanism to modulate the effects of vitamin D on the vasculature [43].

Vitamin D has direct and indirect cardiovascular effects. In a direct manner, 1,25(OH)2 enhances proliferation of vascular smooth muscle cells and expression of vascular endothelial growth factor via the VDR and CYP27B1 expression in VSMCs and endothelial cells [37]. It also plays an important role in inflammation and thrombosis. Inverse associations between vitamin D deficiency and thrombogenicity, vascular inflammation, and vascular calcification have been demonstrated [7, 38]. Cardiac and smooth muscle contractility is controlled partly by intracellular handling of calcium that depends on extracellular calcium which is regulated by vitamin D. 1,25(OH)D3 has an inhibitory effect on hypertrophy and proliferation of VSMCs *in vitro* and in cultured cardiac myocytes, ultimately inducing apoptosis [37]. (**Figure 2**) The lack of VDR signaling results in chronically low nitric oxide production, caused by defective NO synthase.

Indirectly, the expression of renin *in vivo* is strongly regulated by vitamin D, and an inverse relationship between vitamin D levels and renin expression has been demonstrated experi‐ mentally [6, 27, 39, 40, 44, 45]. 1,25(OH)2 binds to the renin promoter region and inhibits renin transcription, thus reducing plasma renin activity [27, 40]. VDR knockout mice were proven to have increased levels of renin and angiotensin II and therefore higher prevalence of hypertension [27, 28, 40, 44, 45]. Thus, vitamin D may indirectly regulate blood pressure and affect cardiac hypertrophy through this mechanism.

Another indirect effect of vitamin D on CVD involves the production of matrix metallopro‐ teinase 2 and 9, which promote insulin uptake beta‐cell function and suppress pro‐inflamma‐ tory cytokine release while increasing anti‐inflammatory cytokine levels (IL‐10) [34, 46]. These mechanisms help delay the inflammatory pathways implicated in coronary artery disease, by maintaining glycemic control and hindering secondary hyperparathyroidism and the forma‐ tion of vascular calcification [33].

Vitamin D deficiency may also indirectly act deleteriously by inducing hyperparathyroidism. Parathyroid hormone (PTH) controls calcium homeostasis through specific receptors that are also present within vessel walls and the myocardium. PTH may promote the release of inflammatory cytokines, modulate vascular remodeling and lead to impaired glucose metab‐ olism [47]. Several studies have demonstrated an association between high PTH levels and hypertension, myocardial dysfunction and vascular disease. In addition, hyperparathyroidism is also associated with increased mortality [6, 47, 48].

Lastly, increased biosynthesis and hyperlipidemia have also been associated to vitamin D deficiency. This is thought to result from decreased transcriptional activity of the VDR leading to the downregulation of insulin‐induced gene‐2 (Insig‐2) expression. This ultimately results in increased 3‐hydroxy‐3‐methylglutaryl‐coenzyme reductase expression [49].
