A new meta-analysis of randomized controlled trials published in PLOS ONE has found that vitamin D supplementation reduces mortality, specifically over the long run, as opposed to the short run.
There have been previous meta-analyses of randomized controlled trials looking at vitamin D supplementation and mortality. The most notable of which was a Cochrane Review in 2011. In their meta-analysis, researchers included 50 randomized controlled trials and found that vitamin D3 supplementation reduced risk of death by 6% compared to placebo.
In this present, newer analysis, researchers from Guangdong Medical College in Zhanjiang, China, wanted to examine the issue a little differently. They wanted to know, what effect does duration of vitamin D treatment have on mortality? If we systematically review all the trials on vitamin D supplementation, might some studies show greater or lesser efficacy than others due to duration of treatment?
The researchers searched PubMed, Embase and Cochrane Central Register of Controlled Trials for randomized controlled trials where vitamin D supplementation was the intervention, there was a control or placebo group to compare to, and number of deaths were reported.
In all, they found 42 studies that met their criteria. They divided these studies into two categories:
- Studies with a follow-up of less than three years. Twenty-nine studies fell into this category.
- Studies with a follow-up of three years or longer. Thirteen studies fell into this category.
When they looked at risk of death in these categories, here’s what they found:
- In the trials with less than three years follow up, there was no difference between vitamin D and placebo. The risk ratio of mortality for patients treated with vitamin D compared to control was 1.04 (95% Cl: 0.97-1.12). However, this was not statistically significant (p=0.28).
- In the trials with three years or longer follow-up, vitamin D protected against death compared to placebo. They found vitamin D reduced risk by 6% in these types of studies. The risk ratio of mortality for patients treated with vitamin D compared to control was .94 (95% Cl: 0.90-0.98). This was statistically significant (p=0.001).
In addition to looking at duration of trial, they also found that vitamin D supplementation has a greater effect in decreasing mortality in those younger than 80 years, as opposed to those over 80 years.
The researchers concluded,
“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.”
They further recommended that future studies look at more specific mortality outcomes, like cancer mortality and cardiovascular disease mortality. Thankfully, almost all large-scale vitamin D trials underway right now are roughly five-year interventions looking at specific outcomes.
This study also demonstrates, as studies often do, that vitamin D sufficiency over a long period of time is important, as opposed to short durations of sufficiency and trying to find outcomes in shorter durations.
Bjelakovic G, Gluud LL, Nikolova D, Whitfield K, Wetterslev J et al. (2011) Vitamin D supplementation for prevention of mortality in adults. Cochrane Database Syst Rev, 2011.
Zheng Y, Zhu J et al. Meta-Analysis of Long-Term Vitamin D Supplementation on Overall Mortality. PLOS ONE, 2013.