Comment to: ‘Meta-analysis of long-term vitamin D supplementation on overall mortality’
Vitamin D supplementation to prevent mortality
Goran Bjelakovic,1,2 Dimitrinka Nikolova,1 Jørn Wetterslev,3 Christian Gluud1,3
1
The Cochrane Hepato-Biliary Group, Copenhagen Trial Unit, Centre for Clinical Intervention
Research, Department 7812, Rigshospitalet, Copenhagen University Hospital, Copenhagen,
Denmark
2
Department of Internal Medicine - Gastroenterology and Hepatology, Medical Faculty, University
of Nis, Nis, Serbia
3
The Copenhagen Trial Unit, Centre for Clinical Intervention Research, Department 7812,
Rigshospitalet, Copenhagen University Hospital, Copenhagen, Denmark
Words in text: 1811.
Corresponding author:
Professor of Medicine Goran Bjelakovic, MD, Dr. Med. Sc.
The Cochrane Hepato-Biliary Group, The Copenhagen Trial Unit,
Centre for Clinical Intervention Research, Department 7812,
Rigshospitalet, Copenhagen University Hospital
Blegdamsvej 9, DK-2100, Copenhagen, DENMARK,
And Department of Internal Medicine - Gastroenterology and Hepatology,
Medical Faculty, University of Nis
Boulevard Dr Zorana Djindjica 81, 18000 Nis, SERBIA
Telephone: +381 18 532381 Facsimile: +381 18 4238770
E-mail: goranb@junis.ni.ac.rs
Short running head: Vitamin D and mortality
1
Zheng et al. published a meta-analysis of long-term vitamin D supplementation effect on overall
mortality. [1] The conclusions they reached were that vitamin D (i.e., D3 and D2) is effective in
preventing all-cause mortality in trials with duration of follow-up longer than 3 years but not in trials
with shorter duration.
Generally, Zheng et al.[1] followed methodology similar to The Cochrane Collaboration
methodology that we used to conduct our Cochrane systematic review, published in 2011.[2]
Zheng et al. included and analyzed almost the same set of trials that were included in our
Cochrane review[2], but they applied two restrictions, namely they included only trials of oral
administration and they included only trials assessing vitamin D3 or vitamin D2.[1] At the time when
Zheng et al. review was published, we have been updating our Cochrane review.[3] Following our
protocol inclusion criteria, we could include 6 newly published trials, with another 1,138
participants.[3] Our updated review is now based on 56 trials with 95,286 participants in total.[3]
Our present conclusions on the evidence on vitamin D3 and D2 seem less optimistic than those
reached by Zhen et al.[1] and originally ourselves.[2] Therefore, we would like to point out several
important caveats in the meta-analysis of Zheng et al.[1] that should have influenced their results
and conclusions.
One of the issues is related to the different effects of vitamin D3 and vitamin D2 on all-cause
mortality. Zheng et al. meta-analyzed together the trials on vitamin D3 and vitamin D2. Our
Cochrane review suggested a difference between the two forms of supplemental vitamin D.
Vitamin D3 seemed to significantly decrease mortality, while the effects of vitamin D2 seemed to be
neutral or even detrimental.[3] Therefore, we decided here to reanalyze the set of 49 trials (37
vitamin D3 trials; 11 vitamin D2 trials; and 1 vitamin D3 and vitamin D2 trial) out of the 56 trials
included in our updated Cochrane review.[3] We excluded 7 trials that tested active forms of
vitamin D (4 alfacalcidol trials and 3 calcitriol trials). We performed random-effects model metaregression analyses in order to identify potential covariates that could predict intertrial
2
heterogeneity, that is, the covariates that were statistically associated with the estimated
intervention effects. The included covariates were: percentage of women; mean age; duration of
treatment; and dose of vitamin D as continuous co-variates; the form of vitamin D (D3 or D2);
vitamin D status at trial entry (adequate or deficient); and co-administered calcium (vitamin D singly
or vitamin D combined with calcium) as binary co-variates. In addition, we performed subgroup
analyses comparing trials of vitamin D3 (cholecalciferol) to trials of vitamin D2 (ergocalciferol). We
compared the intervention effects in subgroups of trials using the method described by Bornstein et
al.[4] which is implemented in RevMan 5.2 for all types of meta-analyses.
The univariate meta-regression analyses including 49 trials with 94,276 participants revealed that
the duration of treatment was significantly associated with a significantly lower estimated
intervention effect on mortality (RR 0.98, 95% CI 0.95 to 0.999, P = 0.04) and vitamin D2 was
associated with a significantly higher estimated intervention effect on mortality (RR 1.13, 95% CI
1.03 to 1.25, P = 0.01). None of the other covariates, i.e., sex of participants, dose of supplement,
vitamin D status, and co-administered calcium were significantly associated with the estimated
intervention effect on mortality. In a multivariate meta-regression analysis including all covariates,
the mean age of participants was significantly associated with a significantly lower estimated
intervention effect on mortality (RR 0.98, 95% CI 0.96 to 0.99, P = 0.02), the duration of treatment
was significantly associated with a significantly lower estimated intervention effect on mortality
(RR 0.92, 95% CI 0.86 to 0.98, P = 0.01), and vitamin D2 was associated with a significantly higher
estimated intervention effect on mortality (RR 1.13, 95% CI 1.00 to 1.28, P = 0.048).
In addition, we also performed random-effects model meta-regression analyses using the same
covariates but now only including the 38 vitamin D3 trials with 75,927 participants. Univariate and
multivariate meta-regression analyses revealed that none of the covariates were significantly
associated with the estimated intervention effect on mortality.
3
We also performed meta-regression analyses using the same covariates but only including the 12
vitamin D2 trials. Univariate meta-regression analyses (RR 0.90, 95% CI 0.83 to 0.99, P = 0.02)
and multivariate meta-regression analysis (RR 0.87, 95% CI 0.77 to 0.98, P = 0.02) revealed that
duration of follow-up was associated with a significantly lower estimated intervention effect on
mortality.
Our subgroup analysis in which we compared the effect of vitamin D3 to vitamin D2 on mortality,
using random-effects model meta-analysis, showed that vitamin D3 significantly decreased
mortality (RR 0.94, 95% CI 0.91 to 0.98, P = 0.002, I2 = 0%) while vitamin D2 had no statistically
significant effect on mortality (RR 1.02, 95% CI 0.96 to 1.08; P = 0.54; I2 = 4%). The difference
between the estimate of the effect of vitamin D on mortality in the trials using vitamin D3 and the
trials using vitamin D2 was statistically significant by the test of interaction (P = 0.02) (Figure 1).
These results indicate that the observed beneficial effect of vitamin D in trials with duration of
follow-up longer than 3 years in Zheng et al. meta-analysis may be related to the different forms of
vitamin D used. Most trials with duration of follow-up longer that 3 years used vitamin D3 for
supplementation. A number of recently published clinical trials,[4-7] and a systematic review[8]
found evidence that vitamin D3 increases serum 25-hydroxyvitamin D more efficiently than vitamin
D2. However, we do not know yet if this may influence clinical outcomes.
Another issue is related to the different kind of biases that may influence the Zheng et al. metaanalysis results. Zheng et al. meta-analysis is potentially severely misleading due to the risk of
attrition bias (originating from substantial dropout of participants), outcome reporting bias due to a
number of trials not reporting on mortality, and other weaknesses, like variable inclusion criteria.
Most of the included trials in the Zheng et al. meta-analysis as well as in our updated Cochrane
review[3] had a substantial number of missing outcome data. Our analyses of the 56 trials (37
vitamin D3, 11 vitamin D2, 1 vitamin D3 and vitamin D2, 4 alfacalcidol, and 3 calcitriol trials) with
95,286 participants pointed to the fact that outcome data reporting was lacking for more than 8% of
4
the randomized participants.[3] This proportion is too high when mortality in our Cochrane review
amounts to only 12% to 13% in the placebo or no intervention groups. Accordingly, our intentionto-treat 'best-worst’ case and 'worst-best’ case scenario analyses revealed that our results were
compatible with both a very large beneficial effect and a very large detrimental effect of vitamin D 3
on mortality.[3] Although the results of these extreme sensitivity analyses are unlikely, they are still
indicative how few unaccounted for patients should have died to substantially shift the findings
from a modest benefit to nil effect, or maybe even harm.[3]
We abstained from conducting 'uncertainty' analysis.[9] The latter analysis accepts the point
estimate from the complete-participant analysis, assuming that the distribution of deaths among
the participants lost to follow-up is equal to the distribution of deaths among all participants. But the
distribution of dead participants among the lost to follow-up participants may indeed be different
from the distribution of dead participants among participants actually followed up through the whole
observation period, making the 'uncertainty' analyses themselves uncertain.
A meta-analysis of randomized clinical trials increases the power and precision of the estimated
intervention effect, but this effect may be influenced by systematic errors or random errors and can
lead to a report of false significant results.[10,11] It is probable that the results of Zheng et al.
meta-analysis[1] are influenced by both random errors and systematic errors. By dividing the metaanalysis of 42 trials into two meta-analyses, Zheng et al. significantly increased the risk of random
error (‘play of chance’). A cumulative meta-analysis runs the risk of random errors due to analysis
of sparse data and repetitive testing of data.[11-13] In our Cochrane review[3] we conducted trial
sequential analyses to control the risk of random errors and to prevent premature statements of
superiority of the experimental or control intervention or probably falsely declarations of absence of
effect in cases for which we have too few data.[11-13] We performed trial sequential analyses with
a type I error of 5%, a type II error of 20% (80% power), and a diversity-adjusted required
information size.[11-14] We assumed an event proportion of 10% of deaths in the control group
5
and an anticipated intervention effect of 5% relative risk reduction. The diversity was zero. Trial
sequential analysis of 20 vitamin D3 trials with low risk of bias showed that the required information
size had not yet been reached and that the cumulative Z-curve did not cross the trial sequential
monitoring boundaries for benefit, harm, or futility. Consequently, the trial sequential analysis does
not exclude risk of random errors (Figure 2). This finding is in agreement with the findings of
Bolland et al. trial sequential meta-analysis.[15]
Another issue is related to the trial inclusion criteria. Zheng et al. included 2 trials in patients with
chronic kidney disease[16,17] but have left out, without any explanation, many other randomized
trials in patients with chronic kidney disease supplemented with vitamin D.[18,19] This way of
selection increases the selection bias (i.e., bias from unrepresentative sample). In our opinion
these two trials[16,17] should be excluded from the meta-analysis by Zheng et al.[1] due to the fact
that patients with chronic kidney diseases have deranged vitamin D metabolism. In a similar vein,
Zheng et al. included one randomized trial in patients with tuberculosis,[20] while few other
randomized trials in these patients have again been omitted.[21] In addition, Zheng et al. included
one quasi-randomized clinical study[22] and omitted some randomized trials that fulfill their
inclusion criteria.[23-25] These facts additionally threat the reliability of their meta-analysis.
In conclusion, the meta-regression analyses and subgroup analyses of vitamin D trials included in
our updated Cochrane review revealed that the effect of vitamin D3 and vitamin D2 on all-cause
mortality seems to be opposite although we lack direct head-versus-head comparisons. Our results
indicate that the observed beneficial effect of vitamin D in trials with duration of follow-up longer
than 3 years in Zheng et al. meta-analysis may actually not be related to the longer duration of
follow-up but rather to the different forms of vitamin D used, simply because most of the trials with
follow-up longer than 3 years were vitamin D3 trials.
Furthermore, the results of Zheng et al. meta-analysis are influenced by different kind of biases
and did not thoroughly consider the icreased risks of random errors. We therefore draw the
6
attention to our findings of high risks of attrition bias in vitamin D trials, which may invalidate results
of meta-analyses.[2,3] We do not know yet whether vitamin D affects mortality.[15,26-30]
7
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Figure 1. Intervention effect of vitamin D3 and vitamin D2 vs placebo or no intervention on mortality
11
Figure 2. Trial sequential analysis of 20 vitamin D3 trials with low risk of bias. The diversityadjusted required information size (DARIS) was calculated based on mortality in the control group
of 10%; relative risk reduction of 5% in the experimental group; type I error of 5%; and type II error
of 20% (80% power). No diversity was noted. The required information size was 110,505
participants. The cumulative Z-curve (blue line) did not cross the trial sequential monitoring
boundaries for benefit (red line) at any time. Accordingly, the crossing of the conventional statistical
5% boundary (the horizontal brown line) may be due to random errors.
12