The study involved 1 418 patients with stage 3 to 4 CKD, which means moderately to severely reduced kidney function.

All patients also had high parathyroid hormone level (hyperparathyroidism) that may contribute to weakening of the bones in CKD.

The team identified one group of patients who were being treated with calcitriol to lower their parathyroid hormone levels and another group who were not receiving calcitriol. During a two-year follow-up period, mortality rates were compared for patients who were and were not taking calcitriol.

"Although activated vitamin D is known to influence many biological processes, previous clinical knowledge is limited to its effect on parathyroid hormone levels," said Dr. Bryan Kestenbaum of the University of Washington in Seattle, study authors.

The findings revealed that the patients taking calcitriol has lowered their death risk by 26 percent. They were also less likely to develop end-stage renal disease, requiring dialysis to replace lost kidney function.

"Recently, there has been an increased focus on the effects of vitamin D beyond those on bone health," said Kestenbaum

"Vitamin D deficiency has been associated with risk factors for cardiovascular disease, such as high blood pressure, diabetes, and inflammation," he added.

However, the new study showed that treatment with oral activated vitamin D may improve survival in patients with CKD who do not yet require dialysis.

"Randomized clinical trials are needed to test the hypothesis that vitamin D therapy can improve cardiovascular health and survival in CKD," he said.

"Future studies should also examine the role of non-activated vitamin D, which is less expensive and less toxic," he added.

The study appears in August Journal of the American Society of Nephrology.