Can taking too much vitamin D be harmful?

Sun exposure enables us to manufacture Vitamin D, essential to good bone health.  However, because sun exposure can also lead to adverse health consequences like melanoma, many individuals appropriately avoid direct sun exposure and/or use sunscreen or sunblock to protect against harmful UV rays. These measures can lead to lower levels of vitamin D which then often require taking oral supplements.  This raises questions about the optimal dosage to take and whether taking too much vitamin D is not only wasteful but potentially detrimental.

A double-blind, randomized clinical trial conducted over a 3-year period in Canada sought to answer these questions, and findings were recently published in the Journal of the American Medical Association (JAMA).  The 300 subjects between the ages of 55 and 70 enrolled were divided into 3 groups; each received either 400 IU, 4,000 IU or 10,000 IU of vitamin D per day.  Bone mineral density (BMD) and bone strength were measured using a new, high-resolution computed tomography (CT) research scan of bone at the wrist and ankle (XtremeCT) as well as standard dual-X-ray absorptiometry (DXA) at the start of the study and at 6, 12, 24 and 36 months. Vitamin D and calcium blood levels and urinary calcium were also measured.  

The results were surprising- the largest decrease in BMD occurred in the group receiving the highest dose of vitamin D.   Total BMD decreased over the three-year period by 1.4 per cent in the 400 IU group, 2.6 in the 4,000 IU group and 3.6 in the 10,000 IU group.  In addition, those who received higher doses of vitamin D were more likely to develop hypercalciuria, a condition associated with increased risk of kidney stones. Of the 87 subjects with hypercalciuria, incidence increased significantly between the groups receiving 400 IU (17%), 4000 IU (22%), and 10,000 IU (31%) of vitamin D.

Although this study raises questions about the safety of high doses of vitamin D that need to be evaluated in further research, it also confirms that for most healthy adults, 400 IU of supplemental vitamin D daily continues to be an appropriate recommended dose. 

Judith Wolf, MD     Associate Director, WHEP

Lauren A. Burt, Emma O. Billington, Marianne S. Rose, Duncan A. Raymond, David A. Hanley, Steven K. Boyd. Effect of High-Dose Vitamin D Supplementation on Volumetric Bone Density and Bone Strength. JAMA, 2019; 322 (8): 736 DOI: 10.1001/jama.2019.11889

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