Scientists warn popular vitamin D supplement may have a previously unknown downside

A surprising study suggests vitamin D2 supplements may reduce the bodys levels of vitamin D3 - the more effective form of vitamin D. Researchers found D3 not only boosts vitamin D status more efficiently, but may also play a unique role in helping the immune system fight off viruses and bacteria. The discovery is prompting scientists to rethink whether D3 should become the preferred choice for supplementation.