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Research Article

Meta-Analysis of Long-Term Vitamin D Supplementation on Overall Mortality

  • Yayuan Zheng equal contributor,

    equal contributor Contributed equally to this work with: Yayuan Zheng, Jianhong Zhu

    Affiliation: Department of Pharmacology, Guangdong Medical College, Zhanjiang, China

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  • Jianhong Zhu equal contributor,

    equal contributor Contributed equally to this work with: Yayuan Zheng, Jianhong Zhu

    Affiliation: Department of Pharmacology, Guangdong Medical College, Zhanjiang, China

    X
  • Manru Zhou,

    Affiliation: Department of Pharmacology, Guangdong Medical College, Zhanjiang, China

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  • Liao Cui,

    Affiliation: Department of Pharmacology, Guangdong Medical College, Zhanjiang, China

    X
  • Weimin Yao mail,

    * Email: liuyuyu77@yeah.net (YL); 616286464@qq.com (WY)

    Affiliation: Institute of Respiratory Disease, Guangdong Medical College, Zhanjiang, China

    X
  • Yuyu Liu mail

    * Email: liuyuyu77@yeah.net (YL); 616286464@qq.com (WY)

    Affiliation: Department of Pharmacology, Guangdong Medical College, Zhanjiang, China

    X
  • Published: December 03, 2013
  • DOI: 10.1371/journal.pone.0082109

Abstract

Introduction

It has been suggested that vitamin D is effective to prevent mortality. However, there is no consistent conclusion that the effects of vitamin D supplementation on all-cause mortality are associated with duration of treatment. We conducted a meta-analysis regarding this issue in an effort to provide a more robust answer.

Methods

A comprehensive search in a number of databases, including MEDLINE, Embase and The Cochrane Central Register of Controlled Trials, was conducted for collecting randomized controlled trials (RCTs) on vitamin D supplementation preventing mortality. Two investigators independently screened the literature according to the inclusive and exclusive criteria and the relative data were extracted. Data analysis was performed by using Review Manager 5.0 software.

Results

Data from forty-two RCT s were included. Vitamin D therapy significantly decreased all-cause mortality with a duration of follow-up longer than 3 years with a RR (95% CI) of 0.94 (0.90–0.98). No benefit was seen in a shorter follow-up periods with a RR (95% CI) of 1.04 (0.97–1.12). Results remain robust after sensitivity analysis. The following subgroups of long-term follow-up had significantly fewer deaths: female only, participants with a mean age younger than 80, daily dose of 800 IU or less, participants with vitamin D insufficiency (baseline 25-hydroxyvitamin D level less than 50 nmol/L) and cholecalciferol therapy. In addition, the combination of vitamin D and calcium significantly reduced mortality and vitamin D alone also had a trend to decrease mortality in a longer time follow up.

Conclusions

The data suggest that supplementation of vitamin D is effective in preventing overall mortality in a long-term treatment, whereas it is not significantly effective in a treatment duration shorter than 3 years. Future studies are needed to identify the efficacy of vitamin D on specific mortality, such as cancer and cardiovascular disease mortality in a long-term treatment duration.

Introduction

Vitamin D plays a key role in human health, while the prevalence of vitamin D insufficiency is high, especially among the elderly [1]. It has recently been identified to be associated with skeletal diseases such as osteoporosis [2] and non-skeletal diseases including cancer [3-5], cardiovascular disease [6], and kidney disease [7,8]. Meta-analysis has suggested that low vitamin D baseline levels are associated with increased risks of mortality [9]. This issue is becoming to be paramount importance given the high prevalence of vitamin D deficiency worldwide [10].

It has been documented that vitamin D supplementation prevents fractures and falls [11,12]. In recent years, several studies on meta-analysis of randomized controlled trials with regard to supplementation of vitamin D on total mortality have been published, which found that vitamin D supplementation reduced total mortality when given together with calcium, but not with vitamin D alone [13-16], and. A Cochrane systematic review found that vitamin D significantly decreased mortality in those with vitamin D insufficiency [15]. However, long-term health effects of vitamin D supplementation still remains unclear.

To investigate whether the effects of vitamin D supplementation on all-cause mortality are associated with duration of treatment, we undertook a comprehensive systematic database search and meta-analysis to access the effects of vitamin D supplementation on all-cause mortality.

Materials and Methods

Search strategy

A literature search was conducted on a number of databases, including Medline, Embase and The Cochrane Central Register of Controlled Trials for the period January 1960 to January 2013, to identify RCTs. Our core search terms were “randomized controlled trial”, “vitamin D”, “vitamin D2”, “vitamin D3”, “ergocalciferol”, “cholecalciferol”, “mortality”, “death”. We also searched for any additional studies in the reference lists of recent meta-analysis of vitamin D treatment for mortality. Our searches were limited to human trials, and no language or time restriction was applied.

Eligibility criteria

The preliminary search results were then examed on the basis of the following criteria.

Types of studies.

Randomized controlled trials evaluating an intervention with vitamin D were identified as part of the review, while review articles, commentaries, letters, observational studies were excluded.

Interventions.

The intervention group was restricted to vitamin D alone or in combination with calcium treatment; the control group was placebo, no treatment or calcium only therapy. Studies of patients receiving active vitamin D and intramuscular injection of vitamin D were excluded from the review.

Outcome.

The number of deaths was reported separately for the vitamin D treatment group and the control group. For articles with a large sample size, if the number of deaths was not reported by treatment, we tried to contact the authors to obtain the missing data.

Data Extraction and Quality Assessment

Two statisticians independently extracted information from included trials using a standardized form., and then another statisticians verified them. The following information was subtracted from the study: first author, publishing year, sample size, duration, dwelling, intervention, serum 25 (OH) D levels at baseline, and main results (the number of participants who died). Quality assessment of included trials was conducted using the Cochrane Collaboration’s tool [17]. Methodological features most relevant to the control of bias were examined, including random sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, selective reporting and other bias [17]. Quality assessment was performed by two independent researchers.

Data Synthesis and Analysis

Meta-analysis were undertaken using Review Manager (Version 5.0). The primary outcome was the number of participants who died during follow-up. The pre-planned analysis was vitamin D arm (with or without calcium) versus control arm (placebo, calcium, or no treatment) according to duration of treatment. Mantel-Haenszel method was used to calculate risk ratios (RRs) and their 95% confidence intervals (CI). The I2 statistic was used to assess the presence of heterogeneity, which ranges from 0% to 100% [18]. In case of lack of heterogeneity (I2 < 50%), fixed-effects model was used to assess the overall estimate, or else random-effects model was chosen. The Begg test [19] and Egger test [20] were used to evaluate the presence of publication bias regarding our primary end points (RR of mortality). A 2-tailed P value of less than .05 was considered as statistically significant.

Sensitivity analysis and subgroup analysis

A sensitivity analysis was conducted by excluding studies with high risk of bias. Subgroup analysis was conducted only on trials with the duration of treatment at least 3 years or longer. The effect of vitamin D was assessed according to gender (male or female), age group (< 80 years or ≥ 80 years), dose of oral vitamin D daily (≤800 IU or >800 IU), baseline level of 25- hydroxyvitamin D (< 50 nmol per liter or ≥ 50 nmol per liter), type of vitamin D (ergocalciferol or cholecalciferol) and calcium co-administration status (Vitamin D + calcium vs. Calcium, Vitamin D + calcium vs. Placebo, or Vitamin D vs. Placebo), and specific mortality (cancer mortality, cardiovascular mortality)

Results

Search Results

A total of 4,024 unique titles and abstracts were found from initial searches of the electronic database. With the inclusion/exclusion criteria, 3,861 of which were excluded by scrutinizing the titles and abstracts, and 121 articles were further excluded after full text review. A total of 42 RCTs that met inclusion criteria were included in the final analysis [21-62]. The details of study selection flow were explicitly described in Figure 1.

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Figure 1. Study flow diagram.

doi:10.1371/journal.pone.0082109.g001

Study Characteristics

The main characteristics of the included studies are shown in Table 1 (1). Publishing year: The RCTs were published from 1992 to 2012 (2). Treatment duration: 29 RCTs have treatment durations less than 3 years, and the remaining 13 RCTs have treatment durations of 3 years or longer (3). Number of patients: A total of 85,466 patients (42,561 in the vitamin D group and 42,905 in the control group) were included in these 42 RCTs (4). Age of patients: The number of participants in each trial ranged from 46 to 36,282 and mean age of participants ranged from 37 to 89 years, with most participants older than 60 years (5). Vitamin D type and dose: Vitamin D2 was used in 10 studies and vitamin D3 was used in the remaining 32 studies. Vitamin D2 or D3 was given as daily doses ranging from 300 to 3,333 IU. Calcium supplementation was used in 26 trials (6). Baseline vitamin D status: 37 trials (80%) reported the baseline vitamin D status of participants based on serum 25-hydroxyvitamin D levels. Participants in 15 trials had baseline 25-hydroxyvitamin D levels at or above cutoff for vitamin D adequacy (50 nmol/l or 20 ng/ml). Participants in the remaining 22 trials had baseline 25-hydroxyvitamin D levels in a range of vitamin D insufficiency (< 50 nmol/l or 20 ng/ml). The other 5 trials did not report the baseline vitamin D status of participants (7). Bias risk: 26 studies had a low risk of bias, and 16 had a high risk of bias (Table 2).

StudyPopulation characteristicsTreatment groupsNumber of participantsMean age (years)Baseline Serum 25 (OH) D levels (mmol/l; mean)Follow up
Trials have a follow-up of 3 years or longer
Avenell 2012Community dwelling, with past low-energy ractureVit D3 800 IU daily134377385-8 years
Vit D3 800 IU daily + Ca1000 mg daily13067738
Ca 1000 mg daily13117838
Placebo13327738
Bolland 2011Community-based postmenopausal womenVit D3 400 IU daily + Ca 1000 mg daily1817662467 years
Placebo181066248
Sanders 2010Ambulatory elderly women at risk for fractureVit D3 5000000 IU annually for 3-5 year113177533 years
Placebo11257745
Salovaara 2010Women aged 65–71 yearsVit D3 800 IU daily + Ca 1000 mg daily158667503 years
No treatment16096749
Zhu 2008Community-dwelling women aged 70–80 yearVit D2 1000 IU daily + Ca 1200 mg daily3975.470.25 years
Ca 1200 mg daily4074.166.6
Placebo4174.867.3
Lappe 2007Healthy postmenopausal white womenVit D2 1100 IU + Ca 1400-1500 mg daily44666.771.84 years
Ca 1400-1500 mg daily44566.771.6
Placebo28866.772.1
Lyons 2007Nursing home residentsInitially 100000 IU Vit D2 weekly, then 1000 IU Vit D2 daily + Ca 600 mg daily172584803 years
Ca 600 mg daily17158454
Aloia 2005Healthy black postmenopausal womenVit D3 800 IU daily + Ca 1200 to 1500 mg daily (after 2 years 2000 IU daily)10459.948.253 years
Ca 1200 to 1500 mg daily10461.243
Larsen 2004Community-dwelling residentsVit D3 400 IU daily + Ca 1000 mg daily4957753742 months
No treatment46487533
Trivedi 2003Community dwelling individualsVitD3 100000 IU every 4 months134575745 years
Placebo13417553
Komulainen 1999Nonosteoporotic, early postmenopausal Vit D3 300 IU daily (no intake during June-August) for 4 years and 100 IU daily in the5th year + Ca 500 mg daily11252.9NA5 yeas
Placebo11552.6NA
D-Hughes 1997Healthy, ambulatory eldely older than 65 yearsVit D3 700 IU daily + Ca 500 mg daily1877171.753 years
Placebo2027261.25
Lips 1996Elderly living in apartments or homesVitD3 400 IU daily129180273.5 years
Placebo12878026
Trials have a follow-up of less than 3 years
Alvarez 2012Patients with early chronic kidney diseaseVit D 50000 IU weekly for one year2262.326.71 year
Placebo2462.632.1
Lehouck 2012chronic obstructive pulmonary diseaseVit D3 100000 IU every 4 months for one year9168501 year
Placebo916850
TiIDE trial 2012Patients with type 2 diabetesVit D3 1000 IU daily foe one year60766.7NA6 months
Placebo61466.6NA
Wasse 2012Patients with hemodialysisVit D3 200000 IU weekly for 3 weeks254935.853 days
No treatment275247.5
Witham 2010Older patients with heart failureVit D2 100000 IU at 0 and 10 week5378.820.520 weeks
Placebo5280.623.7
Lips 2010Elderly with vitamin D insufficientVit D3 8400 IU weekly for 3 weeks11478.534.316 weeks
Placebo11277.635.3
Wejse 2009Patients with tuberculosisVit D3 100000 IU at 0, 5, 8 month1873777.51 year
Placebo1803879.1
Chel 2008Nursing home residentsVit D3 600 IU daily, or 4200 IU weekly or 180000 IU monthly for 4.5 month16684.224.94.5 months
Placebo17284.225.0
Bjorkman 2008Aged chronically immobile patientsVit D3 1200 IU daily7383.9246 months
Vit D3 400 IU daily7784.221
Placebo6885.623
Prince 2008Recruited from emergency department or nursing homeVit D2 1000 IU daily + Ca 1000 daily15177451 year
Ca 1000 daily1517744
Burleigh 2007Rehabilitation wards in an acute geriatric unit Vit D3 800 IU daily + Ca 1200 daily10182221 year
Ca 1200 daily1048425
Bolton-Smith 2007Healthy older womenVit D3 400 IU daily + Ca1000 daily6269.462.52 years
Placebo6167.857
Broe 2007Nursing home residentsVit D2 200-800 IU daily998948.75 5 months
Placebo258650
Law 2006Nursing home residentsVit D2 100000 IU/3 months1762855910 months
Placebo19558559
Schleithoff 2006Patients with congestive heart failureVita D3 2000 IU daily + Ca 500 daily for 9 month61573615 months
Ca 500 daily625438
Brazier 2005Ambulatory women aged > 65 years with vitamin D insufficiencyVit D3 800 IU daily + Ca 1000 daily9574.218.251 year
Placebo977517.5
Flicker 2005Institutionalized with vitamin D level between 25 and 90 nmol/lVit D3 initially 100000 IUweekly, then 1000 IU daily + Ca 600 mg daily3138425-902 years
Ca 600 mg daily3128325-90
Porthouse 2005Women aged 70 or over with risk factors for hip fractureVit D3 800 IU daily + Ca1000 daily132177NA25 months
Leaflet199376.7NA
Avenell 2004Participants had had an osteoporotic fracture within the last 10 yearsVit D3 800 IU daily + Ca1000 daily9977NA1 year
No treatment3575.6NA
Harwood 2004Elderly women after hip fractureVit D3 800 IU+ Ca 1000 mg daily2983291 year
No treatment358130
Meier 2004Healthy adultsVitD3 500 IU+ Ca 500 mg daily3055.275.251.5 years
No treatment2557.977
Cooper 2003Women who were ≥ 1 y postmenopausalVit D2 100000 IU weekly + Ca 1000 daily 9356.581.62 years
Ca 1000 daily9456.182.6
Latham 2003Recruited from geriatric rehabilitation centerVit D2 300,000 IU/im/once 10880386 months
Placebo1147948
Meyer 2002Nursing home residentsVit D3 400 IU daily56984472 years
Placebo5758551
Chapuy 2002Elderly ambulatory institutionalized womenVit D3 800 IU daily + Ca 1200 mg daily3938522.52 years
Placebo1908522.7
Krieg 1999Elderly women living in nursing homesVit D3 880 IU daily + Ca 1000 mg daily1248429.82 years
No treatment1248529.3
Bæksgaard 1998Healthy postmenopausal womenVit D3 560 IU daily + Ca 1000 mg daily8062.9NA2 years
No treatment8061.8NA
Ooms 1995Elderly womenVitD3 400 IU daily17780.1272 years
Placebo17180.625
Chapuy 1992Elderly living in apartments or nursing homesVit D3 800 IU daily + Ca 1200 mg daily163484402 years
Placebo16368432.5

Table 1. Characteristics of the included studies.

Vit D, vitamin D; Ca, calcium
Study Random sequence Generation (selection bias)Allocation Concealment (selection bias)Blinding of participants and personnel (performance bias)Blinding of outcome assessment (detection bias)Incomplete outcome data (attrition bias)Selective reporting Reporting (reporting bias)Other bias
Trials have a follow-up of 3 years or longer
Avenell 2012LLLLLLL
Bolland 2011LLLLLLL
Sanders 2010LLLLLLL
Salovaara 2010LLHHLLL
Zhu 2008LLLLLLL
Lappe 2007LLLLHLL
Lyons 2007LLLLLLL
Aloia 2005LLLLLLL
Larsen 2004UHHHHUH
Trivedi 2003LLLLLLL
Komulainen 1999LLLLLLL
D-Hughes 1997LLLLLLL
Lips 1996LLLLLLL
Trials have a follow-up of less than 3 years
Alvarez 2012LLLLLLL
Lehouck 2012LLLLLLL
TiIDE trial 2012LUUULLH
Wasse 2012LLLLLLL
Witham 2010LLLLLLL
Lips 2010LLLLLLL
Wejse 2009LLLLLLL
Chel 2008UUUULLL
Bjorkman 2008LLLLLLL
Prince 2008LLLLLLL
Burleigh 2007LLLLLLL
Boton-Smith 2007LLLLLLL
Broe 2007LLLLLLL
Law 2006LHHHLLU
Schleithoff 2006LLLLLLL
Brazier 2005LUUULLH
Flicker 2005LLLLLLL
Porthouse 2005LLHHLUH
Avenell 2004LHHHLLH
Harwood 2004LLHHLLH
Meier 2004UUHHLLU
Cooper 2003LLLLLLL
Latham 2003LLLLLLL
Meyer 2002HHLLLLH
Chapuy 2002UUUULLH
Krieg 1999UHHHLLL
Bæksgaard 1998UULLLHL
Ooms 1995LLLLLLL
Chapuy 1992UUUULLL

Table 2. Quality assessment of the included studies.

Primary Analysis

Analysis was performed independently for two categories with follow-up duration either less than 3 years, or 3 years or more. In the category of 29 trials with follow-up less than 3 years, a total of 1,175 (13.3%) participants randomized to the vitamin D group and 1,118 (12.2%) participants randomized to the placebo or no intervention group died. Analysis showed that vitamin D did not significantly decrease all-cause mortality. The risk ratio of mortality for patients treated with vitamin D compared with that of control was 1.04 (95% CI: 0.97–1.12), which was statistically insignificant (P = 0.28), with insignificant heterogeneity (I2 = 12%) (Figure 2A).

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Figure 2. Primary analysis (fixed effect model): A, studies under 3 years; B, studies over 3 years.

doi:10.1371/journal.pone.0082109.g002

In the second category of 13 trials with follow-up of 3 years or longer, a total of 3,693 (10.9%) participants randomized to the vitamin D group and 3,880 (11.5%) participants randomized to the placebo or no intervention group died. Data analysis showed that vitamin D significantly decreased all-cause mortality with a risk ratio of mortality 0.94 (95% CI: 0.90–0.98), which was statistically significant (P = 0.001), with insignificant heterogeneity (I2 = 0%) (Figure 2B).

Sensitivity analysis

Sensitivity analysis was conducted by excluding the trials that had a high risk of bias, the results remain robust. For studies under 3 years (16 RCTs), the risk ratio of mortality for patients treated with vitamin D compared with control was 1.03 (95% CI: 0.86–1.24), which was not statistically significant (P = 0.72), with insignificant heterogeneity (I2 =0%) (Figure 3A). For studies over 3 years (10 RCTs), the risk ratio was 0.94 (95% CI: 0.90–0.98), which was statistically significant (P = 0.008), with insignificant heterogeneity (I2 =0%) (Figure 3B).

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Figure 3. Sensitivity analysis (fixed effect model): A, studies under 3 years; B, studies over 3 years.

doi:10.1371/journal.pone.0082109.g003

Subgroup analysis of long-term follow-up studies

In subgroup analysis (Table 3), significantly decreased mortality was seen in women (RR= 0.91; 95% CI: 0.83–1.00). Data on men were limited with only one related trial. Fewer death were found in patients younger than 80 years (RR= 0.93; 95% CI: 0.88–0.97), but not statistically significant in patients aged 80 years or older (RR= 0.97; 95% CI: 0.90–1.04). A dose of 800 IU or less (RR= 0.93; 95% CI: 0.89–0.98) was found to be more favorable than a dose greater than 800 IU (RR= 0.95; 95% CI: 0.89–1.03). Patients with baseline of 25-hydroxyvitamin D level less than 50 nmol/l treated with vitamin D resulted in significant reduction of mortality (RR= 0.93; 95% CI: 0.89–0.98), whereas no effect was seen in patients with baseline of 25-hydroxyvitamin D leve higher than 50 nmol/l (RR= 0.96; 95% CI: 0.89–1.03). Treatment with cholecalciferol (RR= 0.93; 95% CI: 0.89–0.97) was more favorable than with ergocalciferol (RR= 0.98; 95% CI: 0.90–1.06). Vitamin D combined with calcium was effective to reduce mortality when compared to placebo (RR= 0.94; 95% CI: 0.88–0.99), but not significantly effective when compared to calcium (risk ratio 0.97, 95% CI 0.91 to 1.03). The effect of vitamin D alone treatment was statistically insignificant compared to placebo (RR= 0.93; 95% CI: 0.86–1.00). Vitamin D treatment significantly reduced the cancer mortality (RR= 0.88; 95% CI: 0.79–0.98), but did not decrease cardiovascular mortality (RR= 0.91; 95% CI: 0.81–1.02).

SubgroupNo. of participants No. of deathRisk ratio (95% CI)P ValueI2, %
Vitamin D groupControl groupVitamin D groupControl group
Gender
Male only101910181992200.88 (0.73–1.07)0.19 0
Female only22111224018319190.91 (0.83–1.00)0.04 0
Age
<80 yr3073630710269828590.93 (0.88–0.97) 0.002 0
≥80 yr3016300299510210.97 (0.90–1.04) 0.39 0
Dose of oral vitamin D, IUP=0.61
≤8002906628715271228510.93 (0.89–0.98) 0.003 0
>8004686499798110290.95 (0.89–1.03) 0.20 19
Baseline 25-hydroxyvitamin D*
<502717726788269528350.93 (0.89–0.98) 0.003 0
≥506459680899810440.96 (0.89–1.03) 0.23 0
Type of vitamin D
Ergocalciferol221025297177350.98 (0.90–1.06) 0.59 45
Cholecalciferol3154231183297631450.93 (0.89–0.97) 0.001 0
Calcium coadministration status
Vitamin D + calcium vs. calcium31743170112911640.97 (0.91–1.03) 0.32 0
Vitamin D + calcium vs. placebo2636726054200820980.94 (0.88–0.99) 0.02 0
Vitamin D vs. placebo5110508796710340.93 (0.86–1.00) 0.06 0
Specific mortality
Cancer mortality22170220905586320.88 (0.79–0.98) 0.03 0
Cardiovascular mortality22170220904895370.91 (0.81–1.02) 0.11 0

Table 3. Subgroup benefits at the longer duration of Vitamin D, as compared with control group (Trial level data).

*Based on the reported mean baseline level, irrespective of type of vitamin D assay, in a sample of study participants

Publication bias

No evidence of publication bias was detected for the risk ratio of mortality in this study by either Begg or Egger’s test. For studies under 3 years, Begg’s test P= 0.837, Egger’s test P= 0.623; For studies over 3 years, Begg’s test P= 0.059, Egger’s test P= 0.055.

Discussion

We conducted a systematic review and meta-analysis to evaluate the best available research evidence regarding vitamin D supplementation on overall mortality. A total of 42 RCTs were included in the present study, quality assessment suggested that the overall study quality was fair and no significant publication bias was detected. Our results demonstrates that vitamin D supplementation longer than 3 years leads to a significant reduction on overall mortality. When trials with a high risk of bias excluded in the sensitive analysis, the results remain robust. The effect of vitamin D on mortality reduction was significant in several subgroups of individuals: female patients, participants with a mean age younger than 80, dose of 800 IU or less, participants with vitamin D insufficiency (baseline 25-hydroxyvitamin D level less than 50 nmol/L) and cholecalciferol therapy. In addition, compared with placebo, vitamin D in combination with calcium significantly reduced mortality.

Our findings confirmed those in an earlier Cochrane systematic review [15] on the effect of vitamin D treatment on overall mortality, which showed that participants with vitamin D insufficiency (25-hydroxyvitamin D level less than 20 ng/ml) decreased the overall mortality significantly, and indicated that cholecalciferol therapy was more favorable than ergocalciferol and that vitamin D as daily doses of 800 IU or less was more favorable than daily doses more than 800 IU. In contrast with two meta-analysis [13,16], which compared daily dose of 800 IU or greater with that less than 800 IU and suggested that daily dose of vitamin D did not differ in the effect on the outcome, our analysis indicated that the beneficial effect of vitamin D is clearly observed in the low daily dose. One explanation may be that several included trials [23,27,30] used intermittent and high dose of vitamin D, which has been suggested less likely to have a benefit, or to even have a negative effect among the elderly [23]. Consumption of intermittent and high dose of vitamin D leads to high concentrations of plasma 25-hydroxyvitamin D. Michaëlsson et al [63] concluded that both high and low concentrations of plasma 25-hydroxyvitamin D were associated with elevated risks of overall and cancer mortality.

A previous meta-analysis conducted by Autier et al [13] suggested that no relationship was found with duration of vitamin D supplement. In contrast, our results indicated that vitamin D supplementation significantly reduced the overall mortality when duration was longer than 3 years compared with that of control. However, no benefit was seen in those with durations less than 3 years. Additionally, two meta-analyses of RCTs with vitamin D treatment on falls also reported that patients benefit from vitamin D supplementation in a longer time duration [64,65].

Our results indicated that vitamin D was effective in reducing mortality among female patients. There was a lack of evidence to draw a conclusion of vitamin D’s influence on male patients with only one identified trial collected death data by subgroup of gender. We concluded that vitamin D may decrease mortality in patients younger than 80 years old, but not in patients aged 80 years or older. However, no statistically significant difference was found for risk ratio of overall mortality between the two age groups (P= 0.59). This results support an early meta-analysis of vitamin D treatment on falls, which indicated that participants with a mean age younger than 80 benefited from vitamin D supplementation [64].

Several previous meta-analyses suggested that vitamin D supplementation reduced all-cause mortality when given together with calcium, but did not support an effect of vitamin D alone treatment [14-16]. In contrast, our results suggested that vitamin D combined with calcium reduced all-cause mortality significantly when compared with placebo (RR= 0.94; 95% CI: 0.88–0.99), but the effect was insignificantly when compared with calcium therapy (RR= 0.97; 95% CI: 0.91–1.03). Vitamin D alone had a trend to decrease mortality (RR= 0.93; 95% CI: 0.86–1.00) when administrated in a long time. It may indicate that calcium therapy does not increase risk of death [66]. Whether vitamin D given together with calcium is more beneficial than calcium alone treatment needs more RCTs to be clarified.

There is not sufficient evidence to draw conclusions of the effect of vitamin D on specific mortality with only 3 trials collected mortality data in a rigorous fashion. Vitamin D may have a beneficial effect on cancer related mortality. But it needs more RCTs to better understand the effect of vitamin D on cancer. Meta-analyses of cohort studies have suggested that vitamin D intake was associated with reduced risk of colorectal cancer [67], breast cancer [68] but not prostate cancer [69]. Vitamin D had no significant effect on cardiovascular disease mortality. A growing number of literature suggest that low levels of vitamin D are associated with cardiovascular disease risk [70-74]. A limited number of interventional studies that investigated the effects of vitamin D supplementation on cardiovascular disease risk showed mixed results [75-78]. The effect of vitamin D on cardiovascular diseases remains to be identified.

The mechanism of vitamin D benefit on overall mortality is not clear. Both forms of vitamin D (D2 and D3) are converted to 25-hydroxyvitamin [25(OH)D] in the liver, and then hydroxylated to 1,25-dihydroxyvitamin D in the kidney [79]. 1,25(OH)2D is the only biologically active form of vitamin D, which increases calcium absorption and bone formation to maintain bone health, regulate blood pressure and insulin production, prevent heart disease, regulate immune function to prevent diabetes and autoimmune disease, regulate cell growth to prevent cancer [80,81].

Vitamin D insufficiency (< 50 nmol/L) has now been linked to a broad spectrum of human diseases from cancer to cardiovascular to autoimmune conditions [82]. Though vitamin D can acquire through cutaneous synthesis after sunlight exposure and nutrition [83], it is often not sufficient to reach the required levels of vitamin D, especially in patients with osteoporosis and fracture risk [84]. In that case, supplementation of vitamin D is required, in order to prevent vitamin D insufficiency and associated adverse outcomes.

Similar to other meta-analyses, our review has several limitations. First, though extensive searches were made, there were no data of Hispanic or Orientals. Second, most of the participants in the present study were older women, the effects of vitamin D on mortality in younger, healthy persons and in males are still inconclusive. Third, the overall RR (95% CI) effect was modest and could be the result of chance alone. Fourth, there were only 13 trials that have durations of follow up longer than 3 years.

In conclusion, our results implicated that long-term supplementation of vitamin D may have a beneficial effect on overall mortality, especially in patients with vitamin D insufficiency and younger than 80 years. Vitamin D in a dose of 800 IU daily or less was found to be more favorable than a dose greater than 800 IU and treatment with cholecalciferol was more favorable than ergocalciferol. Future studies are needed to test the efficacy of vitamin D on specific mortality, such as cancer and cardiovascular disease mortality in a long-term treatment duration.

Supporting Information

Checklist S1.

PRISMA checklist.

doi:10.1371/journal.pone.0082109.s001

(DOC)

Author Contributions

Conceived and designed the experiments: YL WY. Performed the experiments: JZ YZ MZ. Analyzed the data: JZ YZ WY. Contributed reagents/materials/analysis tools: YL. Wrote the manuscript: YZ JZ. Revised the manuscript: LC YL.

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