Despite prior epidemiologic and experimental data to the contrary, new research shows that vitamin D supplementation does not reduce blood pressure in individuals with prehypertension or stage I hypertension and vitamin D deficiency.
“There have been a growing number of studies suggesting that people with low vitamin D levels have higher blood pressures. This suggests that there could be a link between vitamin D deficiency and hypertension,” said Thomas Wang, M.D., professor of Medicine, director of the Division of Cardiovascular Medicine, and physician-in-chief of the Vanderbilt Heart and Vascular Institute.
“This evidence raised the possibility that a simple intervention like vitamin D supplementation could be beneficial for hypertension. That’s what led my colleagues and me to start thinking about a randomized trial to look at this question.”
The study was published recently in Circulation. Known as the DAYLIGHT trial, it looked at people who had both vitamin D deficiency (25-hydroxyvitamin D levels < 25 ng/mL) and elevated blood pressure (systolic blood pressure of 120 to 159 mm Hg).
People with very high blood pressure were excluded because they would already be on medication.
Instead, the study targeted people with pre-hypertension or mild hypertension — at the high end of, or just above, the normal range but not on pharmaceutical therapy yet.
The double-blind, randomized, controlled trial was conducted at four sites in the United States.
The participants received Vitamin D drops and were randomized to high-dose (4000 IU per day) or low-dose (400 IU per day, or what one would get in a multivitamin).
Participants wore ambulatory blood pressure cuffs that inflated every 20-30 minutes over a 24-hour period. The cuffs recorded and saved measurements so at the end of 24 hours, the data could be downloaded.
The study found that after six months of intervention there was essentially no difference in blood pressure.
“It was a fairly convincing lack of effect of the vitamin D supplementation, so our conclusion was that vitamin D, given over a 6-month period, had no impact on blood pressure,” Wang said.
“It surprised me a little bit because of the prior epidemiologic and experimental data, but I think it highlights the importance of doing randomized, controlled studies. What you see in observational studies might not pan out in a randomized trial, which is the gold standard.”
Wang added that there remains a great deal of interest in looking at vitamin D deficiency in other outcomes.
There is a large-scale NIH-funded trial to study vitamin D supplementation and heart disease, as well as other trials studying its effect on cancer.
“The DAYLIGHT trial doesn’t necessarily inform expectations of whether these other studies are going to be positive or not,” Wang said.
