Taking high-dose vitamin D supplements does not prevent cardiovascular disease (CVD), according to the results of a large clinical trial.
In a three-year study of more than 5,000 adults aged 50 to 84 years, with an average age of 66 years, there was no difference in how many people developed CVD between those receiving a high monthly dose of oral vitamin D3 (200,000IU for first dose then 100,000IU thereafter) and those given placebo.
A total of 303 participants (11.8%) in the vitamin D group were later diagnosed with CVD, as were 293 participants (11.5%) in the placebo group.
The analysis, published in JAMA Cardiology
(online, 5 April 2017), also showed no apparent benefit from supplementation among the quarter of patients who were vitamin D deficient before the trial started.
Nor did vitamin D supplementation alter the occurrence of other outcomes, such as heart attack, angina, heart failure, hypertension, and stroke, the team found.
Questionnaires completed by the participants indicated a high degree of compliance, which was confirmed by blood tests in 438 participants showing vitamin D blood levels of more than 20ng/mL higher in the treated group compared with the placebo.
“This study found that monthly high-dose vitamin D supplementation does not prevent CVD,” the researchers conclude, but they add that “the effects of daily or weekly dosing on CVD risk require further study”.
Previous studies have shown that people with low levels of vitamin D have a higher risk of CVD, and other research completed since the trial began suggests that monthly dosing may not be the best way to maintain vitamin D levels, they point out.
Robert Scragg, who led the study, says there are other studies currently looking at the effects of a daily dose of vitamin D.
“We may need to combine the results from the ongoing trials that collected baseline blood samples (which can identify people with low vitamin D levels) to get a final answer on this,” he says.
Other experts echoed the view that vitamin D supplements given in other ways or dosing regimens, or in certain high-risk populations, may show better results.
Commenting on the results, Adrian Martineau, clinical professor of respiratory infection and immunity at Queen Mary University of London, says that while the study “provides definitive evidence that monthly doses of vitamin D did not reduce the risk of CVD in the study population”, it doesn’t rule out the protective effect of daily or weekly dosing.
“As the authors themselves acknowledge — and given the strength of suggestive evidence from observational studies — this trial does not rule out the possibility that vitamin D might have a role in reducing risk of CVD if it were to be given in a different way or to a population with different characteristics,” he says.
Metin Avkiran, associate medical director at UK charity the British Heart Foundation, also comments: “The study revealed that monthly doses provide no protection against CVD, but more research is needed to understand if taking daily or weekly supplements is beneficial.”
He adds: “It’s important that patients with a vitamin D deficiency, who have been prescribed supplements, continue to take them due to their established health benefits, such as improving bone strength.”
Tim Chico, reader in cardiovascular medicine and consultant cardiologist at the University of Sheffield, says the study highlights many of the difficulties faced by researchers trying to work out the best way to prevent heart disease. “This study adds to a large number of others that found disappointing or even harmful effects of vitamin supplementation on a range of diseases, including heart disease and cancer.
“Although low vitamin levels are ‘associated’ with these diseases,” he adds. “It is likely that this is just a pointer to factors that lead to both low vitamin levels and these diseases.”
 Scragg R, Stewart AW, Waayer D et al. Effect of monthly high-dose vitamin D supplementation on cardiovascular disease in the vitamin D assessment study: a randomized clinical trial. JAMA Cardiology 2017. doi: 10.1001/jamacardio.2017.0175