Review: higher vitamin D status and supplementation may be associated with risks.

The prevalence of low vitamin D levels and associated risks has led to an increase in supplementation. However, a "U-shaped" relationship has been suggested between vitamin D status and adverse effects, with risks observed both in low and high levels. While risks associated with low levels of vitamin D have been extensively studied, the risks of higher levels of vitamin D have not been as widely circulated. We sought to describe key observed adverse risks with vitamin D supplementation and higher serum 25(OH)-D levels in healthy adult populations.