The study, published in the Journal of Endocrinology and Metabolism, reveals a connection between high blood serum levels of vitamin D and an increased risk of cardiovascular deaths such as stroke or heart attack – suggesting that our vitamin D status should be neither too low nor too high.
Study co-author, Professor Peter Schwarz from the University of Copenhagen said results from the study – which retrospectively analysed data from nearly 250,000 people taking part in the CopD Study – suggest that there is an increased risk associated with serum levels of 25-hydroxyvitamin D [25(OH)D] lower than 50 nanomol per litre and also over 100 nanomol per litre.
"We have looked at what caused the death of patients, and when numbers are above 100, it appears that there is an increased risk of dying from a stroke or a coronary. In other words, levels of vitamin D should not be too low, but neither should they be too high,” said Schwarz. “Levels should be somewhere in between 50 and 100 nanomol per litre, and our study indicates that 70 is the most preferable level.”
The new study follows a previous 2012 analysis by the same group, which also suggested that ‘you can have too much vitamin D’.
"These are very important results, because there is such great focus on eating vitamin D,” Schwarz added. “We should use this information to ask ourselves whether or not we should continue to eat vitamins and nutritional supplements as if they were sweets.”
“You shouldn't simply up the dose to feel better. We should only consume such vitamins in close coordination with our GP," he concluded.
High-dose risk?
The team noted that while heart disease is the major cause of death in developed countries around the world, any possible association between 25-hydroxyvitamin D levels and the risk of cardiovascular disease mortality ‘remains unclear.’
As a result, the group used data from the CopD Study to determine whether there is an association between cardiovascular, stroke and acute myocardial infarct mortality and serum levels of 25(OH)D.
"We have studied the level of vitamin D in 247,574 Danes, and so far, it constitutes the world's largest basis for this type of study,” said Schwarz. “We have also analysed their mortality rate over a seven-year period after taking the initial blood sample, and in that time 16,645 patients had died.”
The conclusion from the research is clear, said the team: that there is indeed a correlation between mortality rates and too low levels of vitamin D, but that there is also a link between cardiac deaths and a vitamin D status that is very high.
“Low and high levels of 25-hydroxyvitamin D were associated with cardiovascular disease, stroke and acute myocardial mortality in a non-linear, reverse J-shaped manner, with highest risk at lower levels,” wrote the authors.
“Whether this was a causal or associational finding cannot be determined from our data,” they said – adding that there is a need for randomised clinical trials which include information on the effects of 25-hydroxyvitamin D levels above 100 nmol/L.
According to the study results, a 25-hydroxyvitamin D level of 70 nmol/L was associated with the lowest cardiovascular disease mortality risk.