Adults who received monthly high doses of vitamin D did not have a decrease in cardiovascular disease, according to a randomized trial.

After a median follow-up period of 3.3 years, 11.8 percent of participants in the vitamin D group and 11.5 percent of participants in the placebo group had cardiovascular disease.

Lead researcher Robert Scragg, PhD, MBBS, of the University of Auckland in New Zealand, and colleagues published their results online in JAMA Cardiology on April 5.

The study enrolled 5,108 participants who were 50 to 84 years old, were residents of Auckland, New Zealand at baseline and expected to live in New Zealand for the four-year study period. The participants did not take vitamin D supplements before enrolling in the study.

The researchers recruited participants mostly from 55 family practices from April 5, 2011, through Nov. 6, 2012, and followed them until July 2015. After the participants completed an interview at baseline, they were randomized to receive a 2.5 mg (100,000 IU) oral capsule of vitamin D or placebo each month.

The mean age was 65.9 years old, while 58.1 percent of participants were males and 83.3 percent were of European or other race/ethnicity. In addition, 6.3 percent were current tobacco smokers and 42.5 percent were former smokers. At baseline, the groups had similar characteristics.

The mean corrected serum calcium levels were similar for the vitamin D and placebo groups throughout the follow-up period. Only one person developed hypercalcemia.

The groups had similar rates of all cardiovascular disease events. The researchers noted they had similar results in vitamin D-deficient participants and when they categorized participants by previous cardiovascular disease.

In addition, a random sample of 438 participants found that the mean follow-up 25 (OH)D level was greater than 20 ng/mL higher in the vitamin D group compared with the placebo group.

“Our results do not support the findings from observational studies that report an inverse association between 25 (OH)D and [cardiovascular disease], which could be explained by residual confounding from other lifestyle risk factors,” the researchers wrote. “It is possible that 25 (OH)D concentrations are a surrogate marker of sun exposure, which may have other health effects entirely separate from vitamin D. Thus, we caution against closure of this topic, given the descriptive epidemiologic findings on inverse associations between solar radiation and [cardiovascular disease] with regard to season, latitude, and altitude.”

The researchers acknowledged the study had a few potential limitations, including a lower than expected event rate and a short follow-up period. They mentioned that they could not rule out the possibility that longer-term vitamin D supplementation could benefit participants.

“This study found that monthly high-dose vitamin D supplementation does not prevent [cardiovascular disease],” the researchers wrote. “The effects of daily or weekly dosing on [cardiovascular disease] risk require further study.”