Observational studies can assess the exposure (25(OH)D) and outcome (i.e. cardiovascular diseases) at the same point in time (cross-sectional) or they can assess the exposure and then outcome occurring after the exposure is measured (longitudinal). The cross-sectional studies are faster, cheaper and allow us to evaluate the relationship between the variables of interest at a given time point. However, the cross-sectional studies do not provide any information regarding the timing of the events as the longitudinal studies do. All observational studies are subject to confounding, which can be best minimized by randomization to intervention or placebo. In this section we will evaluate the data supporting the effects of vitamin D in cardiovascular diseases by study type.

A growing body of literature suggests that vitamin D insufficiency or deficiency may be associated with an increased risk of cardiovascular disease. However, the evidence thus far is inconclusive and subject to debate. Early studies reported a higher prevalence of hypertension in countries located in northern latitudes where populations are exposed to less sunlight than in countries nearer the equator [23, 24]. Similarly, large population studies such as the Third National Health and Nutrition Examination Survey (NHANES) identified cardiovascular risk factors (elevated blood pressure, plasma glucose and cholesterol) associated with lower 25(OH)D levels [25]. In contrast, an analysis of the Longitudinal Aging Study Amsterdam that included 1,205 participants patients older than 65 year-old did not find any association between 25(OH)D levels and blood pressure [26].

A second analysis of the NHANES database conducted by Melamed et al. included 13,331 free-living adults in the United States. These authors reported an increase in coronary artery disease (CAD) and stroke in patients with total 25(OH)D levels <20 ng/mL vs. those with 25(OH)D of >32 ng/mL [odds ratio (OR) 1.2 (95 % confidence interval (CI): 1.01–1.36)] [27]. Others have described an association between low 25(OH)D (<30 ng/mL) and heart failure (OR 1.7 [95 % 0.87–3.32]) in the same NHANES cohort [28]. In cases of extreme vitamin D deficiency (<15 ng/mL) in children (rickets), severe cardiomegaly and heart failure have been reported [29–33] Interestingly, patients with Vitamin D–resistant rickets as a result of congenital mutations in the VDR mostly die from cardiovascular diseases, but show no abnormalities in angiotensin converting enzyme activity, or in levels of angiotensin II and aldosterone [34].

Ischemic heart disease has been associated with extreme forms of vitamin D deficiency. For example in 1,739 participants of the Framingham Offspring Study, the risk of ischemic heart disease was associated with total 25(OH)D <15 ng/mL (hazard ratio [HR] of 1.62; 95 % CI 1.11–2.36, P = 0.01) compared to individuals with levels ≥15 ng/mL [10]. In a stratified analysis by diagnosis of hypertension at baseline, the association between 25(OH)D levels and ischemic heart disease was evident in patients with hypertension (HR 2.13, 95 % CI 1.30–3.48), but not in those without hypertension (HR 1.04, 95 % CI 0.55–1.96). Similar results were found in a study involving 18 225 men in the Health Professionals Follow-up Study [11] Furthermore, higher vitamin D intake from foods and supplements was associated with a decreased risk of cardiovascular disease including coronary artery disease and stroke in the Nurses Health Study and the Healthcare Professionals Follow- up Study [12]. Large global cohort studies have yielded similar findings. One such study of 10,170 women and men from the Danish general population showed that the risk for myocardial infarction was 64 % higher in participants with extreme 25(OH)D deficiency (<6 ng/mL) than participants with 25(OH)D >24 ng/mL [13]. Similarly, in the Ludwigshafen Risk and Cardiovascular Health (LURIC) study, both low 25(OH)D and 1,25(OH)₂D levels were associated with cardiovascular mortality [14]. Of note, patients with extreme forms of chronic kidney disease who cannot convert 25(OH)D into the active hormonal form 1,25(OH)₂D also have a high risk of cardiovascular mortality [15].

Studies evaluating the role of vitamin D in patients with left ventricular hypertrophy (LVH) show mixed results. An analysis of 256 participants from the Hoorn study, which is a population-based cohort in the Netherlands, demonstrated an association between low 25(OH)D and LVH only in individuals with history of cardiovascular disease and in those with low renal function (median estimated glomerular filtration rate ≤77.5 mL/min/1.73 m²) [16]. The effect appeared to be attenuated when adjusted for parathyroid hormone (PTH) levels. The Prospective Investigation of the Vasculature in Uppsala Seniors (PIVUS) study suggested an association between low 25(OH)D levels and altered left ventricular (LV) geometry and function at baseline (left ventricular end-systolic diameter and left ventricular ejection fraction), but failed to show the same association after 5 years of follow up [17]. In contrast, an analysis of 711 participants from the Baltimore Longitudinal Study of Aging revealed a relationship between 25(OH)D levels and left ventricular mass index (by echocardiogram), but did not find an association with left ventricular ejection fraction or geometry [35]. Similarly, a study that utilized cardiac magnetic resonance in 992 Icelandic community dwelling individuals found an association between 25(OH)D and left ventricular mass [36]. Populations with extreme forms of 25(OH)D deficiency such as the chronic kidney disease have a higher prevalence of LVH than patients with similar risk factors with normal renal function [37]. Heart failure patients with low 25(OH)D levels have a poor prognosis and higher inflammatory markers than those with higher 25(OH)D levels [18]. Furthermore, 25(OH)D levels are negatively correlated with natriuretic peptides and New York Heart Association class [19]. However, the relationship between vitamin D and heart failure is still controversial [38].

Blood pressure and vitamin D have been inconsistently linked in longitudinal studies. A combined analysis of the Health Professionals’ Follow-Up Study and the Nurses’ Health Study with 1,811 patients encompassing up to 8 years of follow-up reported a 2.67 fold increase in the relative risk of hypertension when comparing individuals with 25(OH)D <15 vs. >30 ng/mL [20] In contrast, the Women’s Health Initiative study reported no association between 25(OH)D and hypertension [21]. In the Norwegian Tromso study (n = 2,385), no difference in the incidence of hypertension was found in participants with lower vs. higher quartiles of 25(OH)D after 14 years, except for a small increase (4 mmHg) in systolic blood pressure when the lowest (<16 ng/mL) vs. the highest 25(OH)D quartiles (>25 ng/mL) were compared [39].

It is important to note that none of these studies evaluated the role of vitamin D repletion in the aforementioned outcomes. Observational studies cannot rule out other characteristics of these populations that may explain the associated cardiovascular effects. Well-designed randomized trials are needed to confirm or rule out the suspected associations [40].

Cardiac tissue expresses the VDR gene, suggesting that vitamin D may have a direct effect on the heart [6, 7]. Furthermore, VDR expression is increased in hypertrophied hearts [58]. Cardiac tissue also expresses the 1-alpha hydroxylase and 24-hydroxylase genes, so in theory has the potential to convert 25(OH)D into active 1,25(OH)₂D locally as well as convert 1,25(OH)₂D into inactive 1,24,25-tetrahydroxy vitamin D [59].

Animal models that are vitamin D deficient develop cardiac hypertrophy, fibrosis and hypertension [60, 61]. Other animal models such as Dahl salt-sensitive rats, which develop hypertension and proteinuria, also become profoundly vitamin D deficient [52, 62]. These rats develop LVH, high left ventricular end diastolic pressures and elevated levels of natriuretic peptides that can be reversed with administration of 1,25(OH)₂D analogs (such as paricalcitol) [52, 63] or doxercalciferol (a pro-hormone vitamin D2 analog) [63]. Similarly, vitamin D analogs prevent progression of LVH and heart failure [64] in animal models. One possible mechanism whereby hypertrophic changes occur is through a decrease in protein synthesis in cardiomyocytes, such as actin [54]. Treatment with paricalcitol has also been associated with higher VDR expression, reduction in Proliferating cell nuclear antigen (PCNA) [65], and inhibition of Protein kinase C alpha (PKCα) in the heart [51] Several other animal models including spontaneously hypertensive rats [66, 67], uremic rats [65], and even in swine [68] have shown similar associations. Among this abundance of positive data, one study utilizing a murine aortic constriction model failed to show that administration of a vitamin D analog could reverse cardiac hypertrophy, but reduced extracellular matrix proteins and natriuretic peptides [69]. Similarly, calcitriol reduced fibrosis and collagen synthesis in the hearts of rat models after partial nephrectomy, with no differences in heart weights [70].