Fourteen studies (194,932 patients) were included in the review: seven studies were prospective and seven were retrospective. None of the studies were randomised or blinded. Where reported, loss to follow-up was less than 12%. Study quality was low to satisfactory.
Receiving vitamin D therapy significantly reduced the risk of all-cause mortality (RR 0.73, 95% CI 0.65 to 0.82; Ι²=97%; 13 studies), cardiovascular mortality (RR 0.63, 95% CI 0.44 to 0.92; Ι²=92%; six studies), mortality at three years (RR 0.72, 95% CI 0.65 to 0.80; Ι²=23%; six studies) and five years of therapy (RR 0.67, 95% CI 0.45 to 0.98; Ι²= 69%; three studies) compared to control. The risk reduction was shown to be greater in patients with higher parathyroid hormone serum levels.
According to the authors, Egger’s test, funnel plot and trim and fill test showed no evidence of publication bias of studies on all-cause mortality. According to fail safe N calculation, 26 null studies were required to negate the observed findings. Sensitivity analysis by removing one study at a time did not significantly alter the results.
Further subgroup analyses were reported in a supplementary paper.