(HealthDay News) — Monthly high-dose vitamin D supplementation may not prevent the risk of cancer among adults aged 50 to 84 years, according to a study published online in JAMA Oncology.

Robert Scragg, MBBS, PhD, from the University of Auckland in New Zealand, and colleagues conducted a post-hoc analysis of data from the Vitamin D Assessment study, which recruited participants from family practices and community groups. A total of 5110 participants aged 50 to 84 years were randomized to receive vitamin D3 (2558 adults; initial bolus dose of 200,000 IU followed by monthly doses of 100,000 IU) or placebo (2552 adults) for a median of 3.3 years; 5108 were included in the primary analysis.

The researchers found that the mean follow-up 25-hydroxyvitamin D concentration was consistently more than 20 ng/mL higher in the vitamin D vs the placebo group in a random sample of 438 participants. The primary outcome of cancer (328 total cases of cancer) occurred in 6.5 and 6.4% of participants in the vitamin D and placebo groups, respectively, for an adjusted hazard ratio of 1.01 (95% confidence interval, 0.81 to 1.25; P=0.95).

“High-dose vitamin D supplementation prescribed monthly for up to 4 years without calcium may not prevent cancer,” the authors write. “This study suggests that daily or weekly dosing for a longer period may require further study.”

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Scragg R, Khaw KT, Toop L, et al. Monthly High-Dose Vitamin D Supplementation and Cancer Risk: A Post Hoc Analysis of the Vitamin D Assessment Randomized Clinical Trial. JAMA Oncol. DOI:10.1001/jamaoncol.2018.2178. [Published online July 19, 2018]