Vitamin D: Ten Beliefs
We would like to begin by clarifying a few issues
regarding our paper that seem to have been misunder-
stood. First, our paper is not a systematic review. It is a
thorough narrative review that generally follows the prin-
ciples of a systematic review but, as explained in the title,
is a narrative review. Second, as outlined on the first page
of the paper, observational studies were discussed to clar-
ify the origin and support for a belief in the benefits of
vitamin D supplementation for a specific health outcome.
We used only the highest level evidence, randomized
meta-
analysis of RCTs, to determine whether vitamin D sup-
plementation truly influences clinical outcomes. Third, in
addition to fracture prevention, we suggested that vitamin
D supplementation may reduce falls and have a small
impact on mortality.
Mokry and Richards suggest that vitamin D is off-patent so
RCTs are unlikely, but as our review shows, some meta-
analyses included
G. Michael Allan 1 2
James McCormack 0
Christina Korownyk 1 2
0 Faculty of Pharmaceutical Sciences, University of British Columbia , Vancouver, British Columbia , Canada
1 6-10 University Terrace, Evidence- Based Medicine, Department of Family Medicine-Research Program, University of Alberta , Edmonton, AB , Canada
2 Evidence-Based Medicine, Department of Family Medicine, University of Alberta , Edmonton, Alberta , Canada
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examine potential cause-and-effect relations in a
none x p e r i m e n t a l w a y. H o w e v e r , d e s p i t e t h e t e r m
Brandomization,^ this methodology is still observational with
some of the typical risks of bias seen in observational studies,
and they have been shown to provide findings discordant with
RCT evidence.2
The Mendelian randomization paper of Mokry and
colleague does support a potential causal role of vitamin D in
the etiology of MS. While intriguing, we strongly agree with
the authors’ own conclusions: BWhether vitamin D sufficiency
can delay, or prevent, MS onset merits further investigation in
long-term randomized controlled trials.^3 We believe before
clinicians recommend wholesale vitamin D screening and
treatment for any condition, the evidence for benefit should
be based on findings from RCTs.
1. Allan GM , Cranston L , Lindblad A , McCormack J , Kolber MR , Garrison S , Korownyk C . Vitamin D: a narrative review examining the evidence for ten beliefs . J Gen Intern Med . 2016 ; 31 ( 7 ): 780 - 91 .
2. Sheehan NA , Didelez V , Burton PR , Tobin MD . Mendelian randomisation and causal inference in observational epidemiology . PLoS Med . 2008 ; 5 ( 8 ): e177 .
3. Mokry LE , Ross S , Ahmad OS , Forgetta V , Smith GD , Leong A , Greenwood CM , Thanassoulis G , Richards JB . Vitamin D and risk of multiple sclerosis: a Mendelian randomization study . PLoS Med . 2015 ; 12 ( 8 ): e1001866 . (...truncated)