Post hoc analysis
The current study analyzed new data from the Calgary Vitamin D study.
That study found no benefit in BMD or bone strength (JAMA. 2019;322[8]:736-45), contrary to the researchers’ hypothesis that high-dose vitamin D supplements would be associated with greater calcium absorption and parathyroid hormone suppression and, thus, reduced age-related bone loss (improved bone density and strength).
Instead, they found a negative dose-response relationship, which “should be regarded as hypothesis generating, requiring confirmation with further research,” they wrote.
The current study sought to determine if there were sex differences in the effect of vitamin D supplements on bone health in this population.
From October 2013 to December 2017, the Canada Vitamin D study enrolled 311 participants (53% male). To be eligible for the study, participants had to have serum 25-hydroxyvitamin D levels greater than 30 nmol/L and less than 125 nmol/L. They also needed to have adequate calcium intake (1,200 mg/day, as defined by the U.S. Institute of Medicine), or if not, they were instructed to take an appropriate calcium supplement dose.
Patients were randomized to receive 400, 4,000, or 10,000 IU/day of vitamin D3 cholecalciferol, given as 5 drops/day of liquid (Ddrops), with roughly 50 men and 50 women in each dose group.
Researchers selected the 400 IU/day dose as the comparator because the Institute of Medicine recommends a vitamin D intake of 600 IU/day for adults under age 70 years to provide the vitamin D needed for bone health. The typical Canadian diet includes 200-300 IU/day of vitamin D, so individuals would need a supplement of 400 IU/day to reach the recommended intake. The 4,000 IU/day dose is the recommended tolerable upper intake level, according to the Institute of Medicine. And the 10,000 IU/day dose is the tolerable upper intake level of vitamin D as identified in a review by Hathcock and colleagues (Am J Clin Nutr. 2007;85:6-18).
Participants underwent scans with high-resolution peripheral quantitative computed tomography (HR-pQCT) to measure total volumetric BMD at the radius and tibia at baseline, 6, 12, 24, and 36 months. Finite element analysis was used to estimate bone strength.
After 3 years, women had lost significantly more BMD at the radius after taking high-dose versus 400 IU/day of vitamin D. Losses in BMD at the tibia followed a similar trend but were smaller (Figure 1). There were no significant changes in this measure among men (Figure 2).
There were also no significant changes in bone strength among men or women.
Biological mechanism remains to be determined
Dr. LeBoff said a “possible biological explanation” for the findings is that “women, particularly when they are younger, lose more bone than men.”
“Postmenopausal females do lose bone at an accelerated rate compared with males,” Dr. Burt agreed, “but at the time the study was designed, there was no reason to believe that high-dose vitamin D supplementation would accelerate the problem.”
“The biological mechanism of the vitamin D–related bone loss needs further investigation,” Dr. Burt added, “but there are laboratory data suggesting that supraphysiologic doses of active metabolites of vitamin D may stimulate bone resorption.”
The study was funded by the Pure North S’Energy Foundation. Dr. Burt has reported no relevant financial relationships. Disclosures for the other authors are listed with the article. Dr. LeBoff has reported receiving grants from the National Institutes of Health for the VITAL analysis.
A version of this article originally appeared on Medscape.com.