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Nutrients 2014, 6, 1501-1518; doi:10.3390/nu6041501
nutrients
ISSN 2072-6643
www.mdpi.com/journal/nutrients
Review
Vitamin D and Depression: A Systematic Review and
Meta-Analysis Comparing Studies with and without
Biological Flaws
Simon Spedding
Nutritional Physiology Research Centre, University of South Australia, City East Campus, North Tce,
Adelaide, SA 5000, Australia; E-Mail: spedding@adam.com.au; Tel.: +61-439-687-866;
Fax: +61-882-900-498
Received: 20 March 2014; in revised form: 4 April 2014 / Accepted: 4 April 2014 /
Published: 11 April 2014
Abstract: Efficacy of Vitamin D supplements in depression is controversial, awaiting
further literature analysis. Biological flaws in primary studies is a possible reason meta-
analyses of Vitamin D have failed to demonstrate efficacy. This systematic review and
meta-analysis of Vitamin D and depression compared studies with and without biological
flaws. The systematic review followed the Preferred Reporting Items for Systematic
Reviews and Meta-Analyses (PRISMA) guidelines. The literature search was undertaken
through four databases for randomized controlled trials (RCTs). Studies were critically
appraised for methodological quality and biological flaws, in relation to the hypothesis and
study design. Meta-analyses were performed for studies according to the presence of
biological flaws. The 15 RCTs identified provide a more comprehensive evidence-base
than previous systematic reviews; methodological quality of studies was generally good
and methodology was diverse. A meta-analysis of all studies without flaws demonstrated a
statistically significant improvement in depression with Vitamin D supplements (+0.78 CI
+0.24, +1.27). Studies with biological flaws were mainly inconclusive, with the meta-
analysis demonstrating a statistically significant worsening in depression by taking Vitamin
D supplements (−1.1 CI −0.7, −1.5). Vitamin D supplementation (≥800 I.U. daily) was
somewhat favorable in the management of depression in studies that demonstrate a change
in vitamin levels, and the effect size was comparable to that of
anti-depressant medication.
Keywords: Vitamin D supplementation; depression; biological plausibility; meta-analysis;
systematic review; 25OHD
OPEN ACCESS
Nutrients 2014, 6 1502
1. Introduction
Depression affects 350 million people worldwide, is the leading cause of disability and the
fourth-leading cause of the global disease burden [1]. However, the effectiveness of conventional
treatments for depression is questioned: meta-analyses of drug treatments demonstrate minimal
difference from placebo, comparisons of real and sham electroconvulsive therapy show little difference
after a month, and the evidence for the use of specific cognitive interventions is weak [2]. Therefore
we examined the evidence for other approaches to the management of depression.
The association between depressive disorders and Vitamin D deficiency from a lack of sun
exposure is well established and was first noted two thousand years ago [3], therefore we considered
the evidence for the effectiveness of Vitamin D supplementation.
Vitamin D is a unique secosteroid hormone formed mainly by photosynthesis, so an indoor lifestyle
and sun-avoidance leads to deficiency (25OHD <50 nmol/L) [4]. Vitamin D deficiency is now a global
public health problem affecting a billion people worldwide [5]. Even in sunny Australia, deficiency
affects one third of the population [6], with much higher rates observed in migrant populations [7,8].
There has been an increase in the prevalence of Vitamin D deficiency [9] and a ten-fold increase in
spending on supplements in the US over the last decade [10].
Knowledge of Vitamin D has grown exponentially [11] and 95% of our current knowledge
was published in the last 15 years [12]. This demonstrates new mechanisms and diseases
associated with deficiency including cancer, cardiovascular disease, diabetes, and premature
mortality [4]. Whilst Vitamin D was believed to follow Funk’s model of vitamins, having a single
mechanism and function limited to calcium and bone metabolism [13], the mechanisms of action of
Vitamin D are now recognized to be endocrine, paracrine and autocrine via Vitamin D receptors
(VDRs) [14] affecting most physiological systems, including the brain [15]. The enzymes necessary
for the hydroxylation of 25hydroxyvitamin D (25OHD) to the active form 1,25dihydroxyvitamin D are
present in the hypothalamus, cerebellum, and substantia nigra [16]. Vitamin D modulates the
hypothalamic-pituitary-adrenal axis, regulating adrenalin, noradrenaline and dopamine production
through VDRs in the adrenal cortex [17]; and protects against the depletion of dopamine and
serotonin centrally [18]. Therefore, biological plausibility for the action of Vitamin D in depression
has been established.
Epidemiological evidence shows that Vitamin D deficiency is associated with an 8%–14%
increase in depression [19–22] and a 50% increase in suicide [23]; however, causality and efficacy of
supplementation remain controversial [10,24] awaiting confirmation by systematic review and meta-
analysis.
Four systematic reviews of Vitamin D efficacy in depression, but no meta-analysis, have been
published [25–28]. These reviews provide conflicting results due to the limited number of studies
found and the inclusion of inappropriate studies. Based on six RCTs deemed relevant, the Institute of
Medicine (IOM) [25] concluded there was “inconclusive evidence of an effect” although four of these
RCTs showed a beneficial effect of Vitamin D supplementation in depression. The inclusion of the
other two studies [29,30] described by the IOM as “RCTs of Vitamin D” was inappropriate as; one
used calcium and not Vitamin D as the intervention, and the other was not an RCT in the opinion of
Nutrients 2014, 6 1503
the study authors as the intervention decreased 25OHD levels. Similarly, consistent conclusions could
not be drawn from the other systematic reviews [26–28], as these found so few of the primary studies.
These reviews mirror the inconsistent results found across Vitamin D research as demonstrated by
the twenty four conflicting meta-analyses for falls, fractures, and all-cause mortality [31]. The reason
Vitamin D meta-analyses fail to produce useful results is thought to be biological flaws in primary
studies. These flaws lead to null results [32] as the intervention does not change the Vitamin D
status however these flaws may be overlooked when evaluating the research for Vitamin D and other
nutrients [33,34].
The concept of “biological flaws” arises from the work of Heaney and others [33,34], and refers to
limitations in the design of primary studies which preclude them from testing the research hypothesis.
The hypothesis being addressed in this review is that rectifying Vitamin D deficiency decreases
depressive symptoms. However some trials have limitations in their study design that prevent this
evaluation. This hypothesis can only be tested if participants are Vitamin D deficient at baseline and
then receive a large enough dose of Vitamin D supplements to achieve Vitamin D sufficiency during
the trial. Vitamin D deficiency cannot be demonstrated if the level of 25OHD is sufficient or higher or
not tested at baseline. An ineffective dose of Vitamin D is one that would not be expected to increase
the level of 25OHD from deficient to sufficient.
Trials with these biological flaws may demonstrate the limitations of the study design rather than
the effectiveness of Vitamin D supplements for changing health outcomes. The parallel in
pharmaceutical research to these nutrient studies with biological flaws would be trialling a drug known
to be ineffective or on patients already taking a full dose of the drug. Thus biological flaws are a
critical element that differentiates nutrient research from pharmaceutical research.
This review was designed to estimate the effect of Vitamin D supplementation in depression and
examine the influence of biological flaws in primary studies on the meta-analyses.
2. Methods
This review followed the PRISMA (Preferred Reporting Items for Systematic Reviews and
Meta-Analyses) guidelines, systematically identifying and appraising peer-reviewed RCTs reporting
on the effect of Vitamin D supplementation for individuals with symptoms of depression with the
objectives of investigating:
• the primary evidence for Vitamin D supplementation and depression from RCTs;
• the types of subjects, the dose of Vitamin D supplementation, the control interventions and the
measures of outcome used;
• methodological quality of the studies;
• biological flaws in the study design, and
• estimates of the size of the effect.
Nutrients 2014, 6 1504
2.1. Search Approach
A systematic search for relevant RCTs was performed evaluating oral Vitamin D supplementation
that included data on depression using four library databases of PsychINFO, MedLine, PubMed and
Cochrane online library. Search approaches for the different databases can be obtained from the
researchers. All databases were searched from inception to October 2012, with eligible papers limited
to English language and human subjects.
2.2. Independence
Two independent researchers investigated the library databases to reduce errors/bias in
accessing evidence. The reference lists of four systematic reviews [25–28] were hand-searched to
identify other RCTs.
2.3. Eligible Studies
RCTs were included where the intervention was Vitamin D supplementation and excluded
where trials were not RCTs or used surrogate interventions. Studies were not excluded on their
methodological quality as the entire evidence base was required to address the aims of this research.
2.4. Decision-Making
Relevant publications were identified from title, abstract and study descriptors by one researcher;
the decision to include was independently validated by a second and disagreements were referred to
third for an independent ruling.
2.5. Critical Appraisal
Methodological quality of articles was critically appraised with PEDro [35]. Trials were rated with
a checklist, the PEDro scale. This considers two aspects of trial quality; internal validity of the trial and
whether the trial contains sufficient statistical information to make it interpretable. It does not rate
external validity or the effect size.
2.6. Data Extraction
Data was extracted for participants, 25OHD levels, study timeframes, interventions, outcome
measures, measures of effect, methodological quality scores, and biological flaws.
2.7. Biological Flaws
Biological flaws in primary studies were identified. These studies included:
• inappropriate interventions (interventions that did not include Vitamin D), or
• interventions producing the opposite effect of that intended (interventions that included Vitamin D,
but reduced the 25OHD level in the intervention group), or
Nutrients 2014, 6 1505
• ineffective interventions that did not improving Vitamin D status (did not significantly change
the 25OHD level), or
• where the baseline 25OHD level was not measured in the majority of participants, or
• where the baseline 25OHD level indicated sufficiency (not deficiency) at baseline.
Studies were grouped according to the presence of biological flaws, and compared by date of
publication, methodological quality, outcome measure, and study outcome.
2.8. Meta-Analysis
Meta-analyses were performed using MedCalc where data was available on diagnosis, dose,
outcome measure, and biological flaws. Estimates of the size of effect using the standardised mean
difference (SMD) were compared according to the presence of biological flaws in primary studies.
For meta-analysis of studies with a continuous measure, MedCalc uses the “Hedges g” statistic as a
formulation for the SMD under the fixed effects model. The SMD is the difference between the two
means divided by the pooled standard deviation, with a correction for small sample bias. Next the
heterogeneity statistic is incorporated to calculate the summary SMD under the random effects model.
The total SMD with 95% CI is given both for the Fixed effects model and the Random effects model.
The SMD has no units or dimensions, however using Cohen's rule of thumb for interpretation of the
SMD statistic: a value of 0.2 indicates a small effect, a value of 0.5 indicates a medium effect, and a
value of 0.8 or larger indicates a large effect.
3. Results
3.1. Systematic Review
From all databases 465 relevant articles were identified with 390 articles remaining after removal of
duplicates. After applying inclusion criteria, 375 were removed and 15 articles remained. These
included 15 RCTs [30,36–49], nine new RCTs and six identified by previous reviews. Seven of the 15
were published in 2011 and 2012 (Table 1).
There was wide variation in study methodology. The study populations were diverse (Table 1).
Smaller studies were performed in patients with specific disorders (depression, seasonal affective
disorder, obesity, post-menstrual tension and hospitalized patients) [30,37–39,41–44,47–49], and
studies in University students [45,46].
Nutrients 2014, 6 1506
Table 1. Study populations, sample sizes (numbers entering intervention and control
groups respectively) and methodological quality score (PEDro Scale).
Author Year
Reference
Citation # Population Sample Size
Quality
Score
Arvold et al. 2009 [36]
Individuals with Vit D deficiency
(10–25 ng/mL) seen for medical
care at a primary healthcare clinic
100 (I 50, C 50) 10
Belcaro
et al. 2010 [42]
Menopausal women with signs of
depression and mood disorder 65 (I 33, C 32) 8
Bertone-Johnson
et al. 2012 [38]
Postmenopausal
Women with depressive symptoms 36,282 (I 18176, C 18106) 11
Dean et al. 2011 [45] Young healthy adults (University
students) 128 (I 63, C 65) 11
Dumville
et al. 2006 [43]
Older women with seasonal
affective disorder 2117 (I 912,C 1205) 11
Gloth et al. 1999 [44] Adults with Season Affective
Disorder 15 (I 8,C 7) 6.5
Harris &
Dawson-Hughes 1993 [30]
Women with seasonal affective
disorder 250 (I 125, C 125) 5
Jorde et al. 2008 [37] Overweight and obese adults 441 (IH 150, ILl 142, C 149) 8
Khajehei
et al. 2009 [46]
University female students with
premenstrual syndrome 180 (IOes 60, I 60, C 60) 9
Khoraminya
et al. 2013 [49]
Adults with major depressive
disorder based on DSM-IV criteria,
without psychosis
40 (I 20, C 20) 10
Landsdowne &
Provost 1998 [39]
Adults with seasonal affective
disorder 44 (I 22, C 22) 8
Sanders
et al. 2011 [47]
Community dwelling older women
with seasonal mood disorders 2012 (I 1001, C 1011) 11
Veith et al. 2004 [40]
Adults with serum 25(OH)D
<61 nmol/L in summer, expected to
develop 25(OH)D concentrations
<40 nmol/L by winter
64 ( I 32, C 32) 10
Yalamanchilli &
Gallagher 2012 [48]
Older post-menopausal women with
depression
488 (Ioes+Calcitrol 122, Ioes 122,
Calcitrol 123, placebo 123 ) 11
Zhang et al. 2011 [41] Hospitalized patients 32 (I 17, C 15) 9
C = control group and I = intervention group. Where there are two intervention groups; IH is used to indicate where a
high dose and IL for where a low dose of Vitamin D supplements were given. Where one intervention group took a
hormone, this was designated IOes.
Baseline 25OHD levels were not reported in six papers [36–41] but were performed in eight
studies [42–49] (Table 2). For one study [30], Vitamin D data was sought from an earlier paper [50]
showing 25OHD levels were not measured at baseline. However 25OHD levels were measured twice
during the study. This demonstrated that the 25OHD levels decreased 5% in the intervention group
during this part of the study due to the decreased availability of sunlight with the change in season,
overwhelming the effect of the low dose of Vitamin D supplements provided.
Nutrients 2014, 6 1507
Daily doses varied from 400 I.U. to 18,400 I.U. across the 15 trials (Figure 1). Three studies [30,38,43]
used doses lower that 800 I.U./day. In the Women’s Health Initiative [38], the Vitamin D dose would
be inadequate to change vitamin levels; the actual dose ingested was ≈200 I.U., as the stipulated dose
was 400 I.U. but compliance was 46%. The doses shown in two papers were misprints; reported
as 200 mg Vitamin D [42] and 0.25 g of calcitriol [48], equating to millions of international units.
However, attempts to clarify this with authors and editors were unsuccessful. The intervention in
another study [47] was high dose Vitamin D (500,000 I.U.) probably inducing side effects; a 15%
increase in falls and 26% increase in fractures.
Figure 1. Daily dose of Vitamin D per study. This shows the range of equivalent daily
doses. (These were calculated after estimating the actual dose rather than using the dose
shown in their published papers).
Low doses of 400 I.U. in Harris & Dawson-Hughes [30] and Bertone-Johnson et al. [38]; High doses were
over 15,000 I.U. per day in Belcaro et al. [42] and Khoraminya et al. [49]; Jorde et al. [37] and Landsdowne
& Provost [39] both tested three groups; two differing dosages and one placebo.
Validated outcome measures of depression (Table 2) included Beck Depression Index in three
studies [37,45,49] the Profile of Mood States in two studies [30,41] and the mental component score of
the SF12 in two studies [43,47]. Questionnaires about pre-menstrual syndrome [46], fibromyalgia [36],
and menopause [42] included depression as a domain. One early study used an unvalidated
questionnaire [39]. There was no significant differences at baseline measures and methodological
quality of studies was generally high (9 out of 11) (Table 1).
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
Daily dose of Vitamin D (I.U./day)
Nutrients 2014, 6 1508
Table 2. Key depression outcome measures, within and between group findings.
Author Year Outcome
Measures
Follow-up
Time Period Within Group Findings Between Group Findings
Arvold et al. 2009
Fibromyalgia
Impact
Questionnaire
8 weeks FIQ score Mean pre-post difference total (95%CI) intervention −3.71
(−7.5 to 0.1) (p < 0.03), control 1.91 (−2.9 to 6.7) (p > 0.05) p < 0.05 favoring intervention
Belcaro et al. 2010
Menopause
Symptoms
Questionnaire
8 weeks Total average symptom score reduced by 48% for intervention group
(p < 0.05), control group increased by 10% (p > 0.05). p < 0.05 favoring intervention
Bertone-
Johnson et al. 2012 Burnam
Depression Scale
At 2 weeks,
then twice
yearly for
2 years
Mean overall change (SD) 0.004 (0.143) intervention,
−0.002 (0.113) (control) p > 0.05
Dean et al. 2011
Beck Depression
Index 6 weeks
Baseline: follow up mean (95%CI): Intervention 7.24
(5.58–8.90); 6.40 (4.73–8.07) (p > 0.05); control 5.72
(4.09–7.36); 5.38 (3.74–7.02) (p > 0.05)
p > 0.05
Dumville et al. 2006 SF12 mental
component 6 months
Mean difference (95%CI) between intervention and control at baseline
−0.6 (−1.5 to 0.3) (p > 0.05);
at follow up 1.8 (−0.8 to 1.2) (p > 0.05)
Mean adjusted (age- and baseline
score) between group difference
(95%CI) −0.49 (−1.34 to 0.81)
p > 0.05
Gloth et al. 1999 SAD-8 1 month Significant improvement in SAD-8 scores for intervention group, not control
(explanatory data not provided)
Significant association between
improvement in Vit D levels and
SAD-8 scores in overall cohort
(r2 = 0.26)
Harris &
Dawson-
Hughes
1993 Profile of Mood
States
3 monthly for
12 months
No difference in pre-post scores for any domain of PoMS for either
intervention or control (p > 0.05)
No difference between intervention
or control change over time in any
domain (p > 0.05)
Jorde et al. 2008 Beck Depression
Index (total score) 12 months
Baseline: DD group 4.5 (0.0-24.0); DP group 5.0 (0.0–28.0); PP group 4.0
(0.0–24.0). Follow-up: DD group 3.0 (0.0–23.0) (p < 0.05); DP group 4.0
(0.0–26.0) (p < 0.05); PP group 3.8 (0.0–18.0)
DD and DP groups change was
similar (p > 0.05) but significantly
greater from PP (p < 0.05)
Nutrients 2014, 6 1509
Table 2. Cont.
Author Year Outcome Measures Follow-up
Time Period Within Group Findings Between Group Findings
Khajehei et al. 2009
PMS symptom rating form
which captured psychological
and physical symptoms
including depression
Pre-mens for
2 cycles
Mean % total symptoms
Pre: Dydrogesteron group 52.1%, Calcium plus Vitamin
D group 50.7%, Placebo 53.7%.
Post (respectively): 47.9%, 46.1%, 53.7%
Both active treatment groups had significant decreases
The dydrogesterone
and calcium plus Vitamin D
treatments
were significantly more effective than
placebo in lessening the severity of
PMS symptoms
(p < 0.05)
Khora-minya et al. 2013
24-item Hamilton Depression
Rating Scale (HDRS) (1°),
21-item Beck
Depression Inventory (BDI) (2°)
Every 2 weeks
for 8 weeks
BDI
Intervention
Wk0 32.45 ± 7.35; Wk2 27.73 ± 7.50; Wk4 20.44 ±
6.56; Wk6 16.73 ± 8.11; Wk8 13.2 ± 8.64 (p < 0.05)
Control. Wk0 31.65 ± 7.33; Wk2 29.17 ± 6.78; Wk4
25.18 ± 6.93; Wk6 21.00 ± 6.81; Wk8 17.95 ± 6.31
(p < 0.05)
p < 0.05 for both outcomes, favoring
intervention
Lands-downe &
Provost 1998 PANAS 5 days Sig within-group improvements for both active
interventions (p < 0.05)
Sig improvements for both active
interventions cf control for positive
and negative affects (p < 0.05)
Sanders et al. 2011
General Health Questionnaire
SF12 (PCS, MCS), WHO
Wellbeing Index
3–5 years
Intervention: no intervention
SF12 PCS effect size (95%CI)
0.27 (−2.40 to 2.94)
0.23 (−0.88 to 1.34)
Treatment effects SF12 effect size
(95%CI) PCS 0.22 (−70.75 to 1.19);
MCS 70.14 (−71.00 to 0.72)
Veith et al. 2004 Self-developed Wellbeing Scale 2–6 months
Pre-post mean (SD): 600 I.U. 2.2 (2.0); 2.3 (2.3)
(p > 0.05)
4000 I.U. 2.0 (2.3); 1.1 (1.8)
(p < 0.05)
Significant improvement in wellbeing,
favoring higher Vit D dose
Nutrients 2014, 6 1510
Table 2. Cont.
Author Year Outcome Measures Follow-up Time
Period Within Group Findings Between Group Findings
Yalamanchilli &
Gallagher 2012 Geriatric Depression Scale
1. HT alone
2. calcitriol alone
3. HT & calcitrol
4. placebo
% with depression (pre/post)
13.8%; 8.9%; 9.7%; 7.3%; 8.2%; 6.6%
13.8%; 8.9%
All groups p > 0.05
No effect on depression in any
treatment group compared with
placebo (p > 0.05)
Zhang et al. 2011
Profile of Mood States
questionnaire Average 8 days Vit D group pre-post 23.1 ± 27.2; 22.4 ± 22.4 p > 0.05
Vit C group pre-post 28.6 ± 21.8; 18.8 ± 19.4) p < 0.05 p < 0.05 favouring Vit D
Nutrients 2014, 6 1511
3.2. Biological Flaws
Biological flaws were found in eight of the 15 studies (Table 3). These flaws limit the ability of
these studies to demonstrate a change in vitamin status in the intervention group. The most common
flaw, occurring in five studies, was not measuring 25OHD. Two studies [30,38] utilized doses below
the minimum effective dose of 600-800 I.U. [51] and one study [45] had such high baseline 25OHD
levels that supplements could not improve the Vitamin D status of participants.
One intervention was associated with a decrease in 25OHD level [30], and another caused falls and
fractures minimising the potential to see any health benefits [47]. Biological flaws were more
prevalent (70%) in recent studies (since 2010) than in earlier studies (50%), and in larger studies than
in smaller studies (Table 3).
Table 3. Comparison of studies by presence of biological flaws to the study findings and
methodological quality.
Study
Biological
Flaws NOT
Present
Biological
Flaw(s)
Present
Type of Flaw Quality Score
(Max 11)
Date of
Publication
25OHD not
Assessed
Dose not
Appropriate
Belcaro et al. X X 8 2010
Bertone-Johnson et al. X X X (L) 11 2012
Dumville et al. X X 11 2006
Harris & Dawson-Hughes X X X (L) 5 1993
Dean et al. X X X (H) 11 2011
Khajehei et al. X X (I) 9 2009
Sanders et al. X X (SE) 11 2011
Yalamanchilli & Gallagher X X (I) 11 2012
Total-8 Studies with
Biological Flaws 0 8 5 6 3 5
Arvold et al. X 10 2009
Gloth et al. X 6.5 1999
Jorde et al. X 8 2008
Khoraminya et al. X 10 2013
Landsdowne & Provost X 8 1998
Veith et al. X 10 2004
Zhang et al. X 9 2011
Total—7 studies
without flaws 7 0 0 0 5 2
↑ = significant improvement favouring Vitamin D; Dose incorrect (I), low (L), high (H) or produces side effects (SE).
Of the seven studies without flaws, six [36,37,39,40,44,49] showed improvement in depression with
supplementation, whereas six of the nine flawed studies [30,38,42,45–48] had a null result (Table 3).
The positive results in two flawed studies maybe due to the unknown contents [46] or the effects of the
herbs [42] used in these studies.
Nutrients 2014, 6 1512
3.3. Meta-Analysis
3.3.1. Meta-Analysis of Studies without Biological Flaws (Right Panel of Figure 2)
Two studies (Jorde et al. [37] and Khoraminya et al. [49]) were included as they used the same
outcome measure; the Beck Depression Inventory.
The standardized mean difference for these studies without flaws is shown in the Right Panel of
Figure 2. It shows a statistically significant positive effect of Vitamin D in depression of 0.78 (CI 0.24,
1.27). The random effects model was used due to the diverse populations studied.
Figure 2. The figures show the meta-analysis of studies from the systematic review.
Left Panel—Two studies with biological flaws were combined, Dumville et al. [43] and Sanders et al. [47];
Right Panel—Two studies without biological flaws were combined, Jorde et al. [37] and Khoraminya et al. [49],
showing two intervention groups for Jorde et al. [37] (high and low dose Vitamin D) and the data from the
Khoraminya et al. [49] at 2, 4, 6, and 8 weeks.
The Jorde et al. [37] trial (n = 387) had three study groups; two interventions with different doses of
Vitamin D and a control. The Khoraminya et al. [49] trial (n = 40) compared Vitamin D plus
fluoxetine to fluoxetine alone. The studies had similar baseline level of 25OHD (Jorde et al. [37]
55 nmol/L) (Khoraminya et al. [49] 57 nmol/L), and the doses of Vitamin D over 800 nmol/L in both
studies. The participants in both studies were patients; Khoraminya et al. [49] depressed patients and
Jorde et al. [37] obese patients. Depression and obesity overlap, as there is a reciprocal relationship
between obesity and depression indicated by the 50% increase in one condition when the other is
present [52].
3.3.2. Meta-Analysis of Studies with Biological Flaws (Left Panel of Figure 2)
Options for meta-analysis were examined and performed combining the Dumville et al. [43] and
Sanders et al. [47] studies, due to the diverse outcome variables used in other studies. There was a
statistically significant negative effect of Vitamin D administration evident from the forest plot in the
Nutrients 2014, 6 1513
standardized mean differences as shown in the Left Panel of Figure 2. The effect size was −1.1 (CI
−0.7, −1.5) (random effects). These studies were of high methodological quality, had similar subjects
(community dwelling women aged >70 years) and baseline 25OHD, and used the same outcome
measure. The studies differed in the dosing schedule, daily and annually.
4. Discussion
This is the most comprehensive systematic review of randomized controlled trials investigating the
effectiveness of Vitamin D in the management of depression. Fifteen RCTs were found, whilst
previous reviews captured few of the available RCTs. Although the methodological quality was good,
biological flaws were common and more prevalent in recent studies.
For the meta-analysis of studies without biological flaws, the size of the effect was statistically
significant being +0.78 (CI 0.24, 1.27). As the measure of effect size was the standardized mean
difference (SMD), this was 0.78, using Cohen’s Rule-of-Thumb, a SMD of 0.8 is considered to
indicate a large effect.
As less than half the study population were deficient the effect of the intervention was diluted such
that if all subjects had been deficient the size of the effect would have been higher, perhaps double,
1.5 points on the BDI scale. This is similar to the size of effect seen in a large RCT of antidepressant
medication, which was 0.8 point on the BDI scale for the blinded parts of the study and 1.7 points
overall [53]. A review of antidepressant efficacy published in the NEJM [54] shows that the effect size
of antidepressant medication was increased by selective publication of trials and altering the effect
size. However the overall mean weighted effect size value for antidepressants was only 0.15 (CI 0.08,
0.22) for unpublished studies and 0.37 (CI 0.33, 0.41) for published studies. Thus, the effect size of
Vitamin D demonstrated in our meta-analysis may be comparable with that of anti-depressant
medication. For the meta-analysis of studies with biological flaws, the size of the effect was
statistically significant and negative being −1.1 (CI −0.7, −1.5), indicating that Vitamin D
supplementation in flawed studies may lead to deterioration in depression.
The main finding is that all studies without flaws and the meta-analysis of studies without
biological flaws support the efficacy of Vitamin D supplementation for depression, as compared with
the negative results of meta-analysis for studies with biological flaws. The Womens Health
Initiative [38] (WHI), with more participants that all the other studies combined, had the highest
methodological quality and the most biological flaws leading to non-significant outcomes for both
bone strength and mood. Due to its sheer size, the WHI has dominated previous meta-analysis leading
to null results.
The main limitation of this review was the diversity of study methodology precluding more
extensive meta-analyses, and leaving only two studies in each meta-analysis. The variability in
outcome measures and reporting suggest agreement should be sought within the research community
to underpin standard conduct and reporting of future studies to support meta-analysis.
5. Conclusion
Traditional evidence, biological plausibility and epidemiological studies indicate Vitamin D has
therapeutic effects in depression. There are no previous meta-analyses of Vitamin D and depression as
Nutrients 2014, 6 1514
the evidence was deemed to be insubstantial [25]. This may be due to previous systematic reviews
identifying few of the available studies and including RCTs with inappropriate methodology and
biological flaws.
Meta-analysis of studies without biological flaws demonstrates that improving Vitamin D levels
improves depression, whereas the meta-analysis of flawed studies had a negative result. Heaney [34]
identified the most common flaw “baseline status” and the most pernicious flaw “(in)effective dosing”.
However we found other flaws: not measuring 25OHD levels throughout the study limits the ability to
know if the 25OHD level actually changed. In this case, there would be no reason to believe that the
intervention caused a biological difference in Vitamin D levels between intervention and control
groups. We also found more fundamental biological flaws where the intervention was not Vitamin D
but calcium, and caused a decreased in the 25OHD level. These two studies were included in previous
systematic reviews but rejected by this review.
The finding that meta-analyses for studies with biological flaws had the statistically significant
effect of increasing depression, may lead to a conclusion that some of these trials led to levels for
Vitamin D above the therapeutic range. This would be supported by a recent paper indicating that the
therapeutic range for 25OHD in depression is 50 and 85 nmol/L [55].
It may be argued that meta-analysis including flawed RCTs reflect the trial methodology more
than the efficacy of the intervention, leaving reviewers unable to make valid conclusions about
efficacy [34], resulting in uncertainty amongst researchers and clinicians. This has led to calls for more
RCTs and less “torturing of the data” by meta-analysis [56]. However, as this review demonstrates,
it is excluding biological flaws that will lead to greater understanding of Vitamin D, not simply
increasing the quantity of studies.
We note that biological flaws are more frequent in recent studies; this may be due to the belief that
vitamins exert a function beyond deficiency. Hence RCTs should test whether using supplementation
to correct deficiency is beneficial, rather than testing whether additional supplementation on top of the
recommended doses is beneficial in reducing disease [57]. Thus, it is unremarkable that Vitamin D
supplementation would not benefit a population that are not deficient or where the dose was
ineffective. To test the hypothesis that correcting Vitamin D deficiency leads to an improvement in
depression, it is critical to exclude biological flaws from future studies.
The effect size for Vitamin D in depression demonstrated in this meta-analysis is comparable with
the effect of anti-depressant medication, an accepted treatment for depression. Should these results be
verified by future research, these findings may have important clinical and public health implications.
Acknowledgments
The authors would like to acknowledge Karen Grimmer, Kate Beaton, Khushnum Pastakia and
Ellie King for their invaluable technical support, Howard Morris for his advice, and Jan
Drewery-Clark, the journal editors and staff for their assistance preparing the manuscript for
publication.
Nutrients 2014, 6 1515
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