The study, published in Nutrition, Metabolism and Cardiovascular Diseases, reported no significant benefits in several markers of cardiovascular health for stroke patients with low serum levels of vitamin D when a single large dose was administered. The authors suggested that given the current uncertainty around the benefits of vitamin D for stroke patients, high dose supplementation should not be recommended.
“Our results fail to show a significant effect of vitamin D supplementation on a range of markers of vascular health ... This is despite a modest but significant increase in 25OHD levels, which persisted 16 weeks after a single large oral dose of vitamin D2,” said the researchers, led by Dr Miles Witham from the University of Dundee, Scotland.
“Our results lead us to question whether a blanket approach to vitamin D supplementation is the most appropriate method of dealing with low vitamin D levels in the population,” they said.
Vitamin D
The authors noted that low circulating vitamin D (25 hydroxyvitamin D [25OHD]) levels are common in temperate latitudes, “where limited ultraviolet intensity and short winter days conspire to limit production of vitamin D in the skin.”
They said that low 25OHD levels are common in many patients with cardiovascular disease, including stroke, myocardial infarction and hypertension – and are associated with higher rates of future cardiovascular events.
“In patient groups other than those with stroke, several previous studies have suggested that vitamin D supplementation may be able to reduce blood pressure and improve markers of vascular health including endothelial function,” said Witham and colleagues
However they added that is not known whether supplementation with vitamin D in stoke patients with low baseline 25OHD levels could leads to a reduction in further cardiovascular events, including stroke and myocardial infarction.
“We therefore carried out this study to ascertain whether a single large dose of vitamin D could reduce blood pressure and improve markers of vascular health in a cohort of stable patients who had previously suffered a stroke,” they said.
Study details
In the randomised trial of patients with a history of stroke and low baseline 25-hydroxyvitamin D levels received 100,000 units of oral vitamin D2 or placebo at baseline.
The researchers then measured blood pressure, blood vessel function, cholesterol, oxidised low density lipoprotein, and heart rate turbulence at the beginning of the trail, and follow up after eight and 16 weeks.
Witham and co workers reported higher levels of serum 25OHD in the intervention group at 8 weeks compared to placebo, and noted that these levels remained higher at 16 weeks.
They also observed significantly higher blood flow and dilatation in the 25OHD intervention group at 8 weeks, but said that such benefits were not sustained up to the 16 week follow up. They also found no significant changes in blood pressure between the groups at 8 or 16 week follow ups.
“Our results suggest that vitamin D supplementation cannot be recommended at present for use in normotensive stroke patients, even when 25 hydroxyvitamin D levels are low,” said the researchers.
Given that there are known side effects (such as renal stones) associated with vitamin D supplementation, Witham and colleagues said that clear evidence of a beneficial effect on cardiovascular health is needed before advocating vitamin D replacement in different at-risk groups.
“It is possible however that a larger dose, and use of repeated dosing (with the more bioavailable vitamin D3) would show greater effects. These hypotheses require testing in future trials,” they added.
Source: Nutrition, Metabolism and Cardiovascular Diseases
Published online ahead of print, doi: 10.1016/j.numecd.2010.11.001
“The effect of vitamin D replacement on markers of vascular health in stroke patients – A randomised controlled trial”
Authors: M.D. Witham, F.J. Dove, J.A. Sugden, A.S. Doney, A.D. Struthers