Vitamin D and Cardiovascular Disease: Where We Currently Are

Over the last few decades, there has been a recurring trend in cardiac nutritional science. Observational data shows a strong association between a nutritional deficiency and worse cardiovascular (CVD) outcomes, generating a hype for supplementation. This is followed by small intervention trials of nutritional supplementation that yield mixed results, with meta-analyses unable to provide clarity due to heterogeneity in study designs. Meanwhile, the buzz catches on with health care practitioners and the lay public alike because who doesn't like the simple idea of taking a natural supplement to improve heart health? Finally, large-scale randomized controlled trials (RCTs) are undertaken and fail to show any benefit (vitamin C and E, beta-carotene, folic acid to reduce homocysteine, and, most recently, omega-3 fatty acids), and in some cases even show harm (vitamin E).

The evidence for an association between vitamin D and CVD is robust. The prevalence of vitamin D deficiency increases with distance from the equator, and Northern European countries have the highest deficiency rates. In a similar fashion, rates of diabetes, hypertension, and CVD increase with the distance from the equator. Epidemiologic studies show a strong association between vitamin D deficiency and not only CVD risk factors (hypertension, diabetes, metabolic syndrome, left ventricular hypertrophy), but also myocardial infarction, and cardiovascular and all-cause mortality.1-5 However, it is well known that association does not equal causation. True biological causality requires consistency of association, strength of association, dose response, temporality, and biological plausibility. So does the availabe data imply causality of vitamin D deficiency for CVD?

Vitamin D is obtained from dietary sources, like fatty fish or fortified foods, or through the conversion of 7-dehydro cholesterol in the skin from exposure to UVB radiation. It is then hydroxylated by the liver to 25-hydroxy vitamin D (25-OH D). This is still inactive, but its level correlates well with the overall vitamin D status. It is then further hydroxylated in the kidneys to its active form, 1,25-dihydroxyvitamin D (or calcitriol). Calcitriol regulates calcium metabolism by enhancing intestinal calcium absorption and mobilizing calcium from the skeleton.

Although there is no consensus on the optimal blood level of 25-OH D, most experts believe that a level below 20-30 ng/ml signifies deficiency. Using this cutoff, the prevalence of deficiency is estimated at 50-60% among U.S. adults; worldwide, it affects up to 1 billion people.1 Darker-skinned ethnic groups require considerably more sun exposure to synthesize adequate amounts of vitamin D compared with Caucasians. Other risk factors for vitamin D deficiency include sun avoidance strategies like sunscreen, sedentary indoor lifestyle, obesity, renal and hepatic dysfunction, older age, and institutionalization. A report from the Institute of Medicine (IOM) recommends a daily intake of 600 IU for adults, with older adults (>70 years) needing 800 IU, and a safe upper intake level of 4000 IU.6 Each additional 100 IU of vitamin D taken daily increases 25(OH) D levels by about 1 ng/ml.

Many cell types, including vascular smooth muscle cells, endothelial cells, and cardiomyocytes, produce 1α-hydroxylase, which converts 25-OH D to calcitriol. Vitamin D deficiency leads to upregulation of the renin-angiotensin-aldosterone system. Small intervention trials of vitamin D supplementation have shown significant reduction in blood pressure,7 though meta-analyses have reported conflicting results.8 Vitamin D deficiency leads to pancreatic beta-cell dysfunction and insulin resistance. Although there is strong observational data connecting vitamin D deficiency with incident diabetes, trials of supplementation have yielded mixed results.8 Vitamin D deficiency also leads to increased parathyroid hormone (PTH) levels, which in turn can lead to left ventricular and vascular smooth muscle hypertrophy.

Data from the Offspring Cohort of the Framingham Heart Study show an inverse relationship between vitamin D levels and incident CVD.9 A report from the National Health and Nutrition Examination Survey III (NHANES III) showed an inverse relation with all-cause mortality but not cardiovascular mortality.10 Both of these studies also point to a U-shaped relationship between vitamin D levels and cardiovascular mortality, with high levels (>50 ng/ml) associated with a slightly increased mortality. A novel method to overcome the limitations inherent in observational data is the Mendelian randomization approach, in which causality is inferred from associations between genetic variants that mimic the influence of a modifiable risk factor and the clinical outcome of interest. This approach largely circumvents confounding and reverse causation, but is still limited by pleiotropic effects of genetic variants. Mendelian randomization studies of large cohorts showed no causal link between genetic variants causing low 25-OH D and obesity,11 and possible causal links with hypertension12 and diabetes.13 Mimicking the observational data, vitamin D deficiency was linked with all-cause mortality, but not cardiovascular mortality.14

In a meta-analysis of 18 RCTs of over 57,000 participants, the summary relative risk for all-cause mortality was reduced by 7% with vitamin D therapy.15 Another meta-analysis of eight RCTs showed a statistically non-significant reduction in CVD risk.16 The included trials had considerable heterogeneity in study design and vitamin D dose used for supplementation. None had CVD as a pre-specified primary outcome.17 A British RCT of 2,700 elderly participants who received either vitamin D 100,000 IU or placebo every four months for five years found no statistically significant difference in various CVD outcomes.18 A systematic review done by the Agency for Healthcare Research and Quality in 2009 found the overall evidence for vitamin D supplementation and improved CVD outcomes as inconclusive.19

There are two National Institutes of Health (NIH)-funded RCTs with CVD as a primary outcome that will hopefully provide some clarity. The VITamin D and OmegA-3 TriaL (VITAL) is an ongoing double-blind, placebo-controlled RCT to assess the impact of 2,000 IU vitamin D and/or omega-3 fatty acids on heart disease, stroke, and cancer in 20,000 healthy men and women in the U.S.17 Enrollment for VITAL is complete, and results are expected in 2017. Another RCT (Vitamin D, Insulin Resistance, and Cardiovascular Disease, NCT00736632) is currently enrolling patients with coexistent diabetes, hypertension, and vitamin D deficiency.

Despite the widespread prevalence of vitamin D deficiency, the strong and consistent association with CVD risk factors and mortality, as well as biological plausibility from mechanistic laboratory studies, it is still unclear whether vitamin D is just a marker for increased CVD risk or actually plays a role in the pathogenesis of CVD. Potential confounders to the observational data include obesity, physical activity, sun exposure, and nutritional status. Small interventional trials done so far have not given a convincing answer, so we are eagerly awaiting the results of ongoing RCTs. For now, as the IOM report recommends, treat vitamin D deficiency for bone health, but not heart health.


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