Vitamin D useless for lowering blood pressure, study finds

A new study is casting a shadow on the "sunshine vitamin." Despite widespread use, vitamin D supplements are ineffective when it comes to lowering blood pressure, the study published in JAMA Internal Medicine found.

"It's the second most prescribed or taken vitamin in the country second to a multi-vitamin but there is no benefit that anybody derives. ... It doesn't work in anybody, and yet it may cause significant harm," CBS News medical contributor Dr. David Agus said Tuesday on "CBS This Morning."

Researchers pooled blood pressure data from 46 different clinical trials with over 4,541 participants that used vitamin D supplements for a minimum of four weeks. A random effects model was used for both trial-level and individual patient data, and in both, the study found no effect of vitamin D in the systolic and diastolic blood pressure.

"People with higher blood pressure have lower vitamin D, so it was a logical trial to do, but it failed, and this is one of many trials that have failed with this vitamin," Agus said.

Agus said while there are associations between low levels of vitamin D and heart diseases and cancer, it is not causal. According to the National Institutes of Health (NIH), there is not enough evidence to determine how levels of vitamin D could increase risk for or protect against cancer. NIH also said too much vitamin D in the blood can be harmful, causing nausea, constipation and even damage to the kidneys, among other effects.

"Seventy-five percent of Caucasians and 97 percent of African Americans are low in D, so I think and the community thinks we now have to reassess what is normal," Agus said.

Previous studies have suggested people with low levels of vitamin D may be more likely to develop Alzheimer's disease and die prematurely of cancer or other causes. But the vitamin's role is still unclear.

If a doctor recommends vitamin D, Agus suggested the patient ask for proof of effectiveness.

"These data question what doctors are telling people to do. So I want people, if their doctor says you should be on vitamin D, I want you to say, 'Where is the data it will benefit me?' And challenge for that data," Agus said.

Clarification: The U.S. Preventive Services Task Force says they could not make a recommendation for or against screening for Vitamin D deficiency for the general population. Certain people may have conditions that require supplementation.