Vitamin D supplements may reduce mortality in chronic kidney disease (CKD) patients who are substantially deficient in vitamin D — including those on dialysis, but clinical trials are required to assess how effective such treatments are, a Chinese study concludes.

The study “Effects of vitamin D or its analogues on the mortality of patients with chronic kidney disease: an updated systematic review and meta-analysis,” appeared in the European Journal of Clinical Nutrition.

Patients with CKD are often lacking in vitamin D due to impaired kidney function. If left untreated, this can cause mineral and bone disorders and boost the risk for cardiovascular-related mortality. In fact, vitamin D deficiency correlates with an increased prevalence of hypertension, heart failure and cardiovascular events.

Researchers at the Third People’s Hospital of Changzhou in China set out to investigate “whether vitamin D treatment alters the overall all-cause and cardiovascular mortalities in a CKD population.” They searched several databases including PubMed, EMBASE, Web of Science and the Cochrane Central Register of Controlled Trials for published studies comparing vitamin D treatment with no treatment and how it affected the  mortality of CKD patients.

In total, researchers included 38 studies involving 223,429 patients in their analysis. The pool of selected studies included 17 randomized controlled trials and 21 observational studies, which they analyzed separately from the other study types.

While randomized controlled trials are typically small- to medium-sized, observational studies are usually large samples, but there is no randomization, and the non-randomized selection of vitamin D treatment over non-vitamin D treatment are compared in routine clinical practice. These differences may affect the results.

“Randomized controlled trials, which minimize the effects of both measured and unmeasured confounders, represent the purest comparison between two treatment strategies and minimize the bias of treatment selection,” authors wrote. “Nevertheless, observational studies may be more generalizable and less subjective to enrollment bias.”

This systematic review and meta-analysis showed that in the randomized controlled trials, the all-cause mortality rates were unaffected by whether patients received vitamin D. On the contrary, in the observational studies vitamin D supplements led to a 38 percent drop in all-cause mortality and a 45 percent reduction in cardiovascular mortality.

“The different findings between the randomized controlled trials and observational studies demonstrate that reliance on neither should be absolute,” said researchers.

Until now, randomized controlled trials using the intention-to-treat principle showed no signs that vitamin D supplements affect the mortality of CKD patients. However, “subsets of patients that could potentially benefit from vitamin D treatment can be identified by using the existing data from the randomized controlled trials,” researchers wrote, adding that in the future, performing these type of studies with larger samples is key to assessing any potential differences in survival.