In this clip, Bruce Ames explains how vitamin D functions as both a vitamin and a hormone. As a hormone, it controls thousands of genes in the human body. Bruce says that while darker skin color protects you from the sun, it also decreases the production of vitamin D. Rhonda further elaborates that the daily RDA of vitamin D is 600 IUs but that may not be enough for many in the US. Bruce argues that while diseases of severe vitamin D deficiency like rickets are rare, the criteria for vitamin D deficiency should be based on long-term health biomarkers and not just short-term severe diseases. Lastly, Bruce explains that the reason many nutritional research studies do not show positive effects is because they are under-powered and poorly designed.