So physiologist Kyoji Ikeda and colleagues at the National Center for Geriatrics and Gerontology in Obu, Japan, and Chugai Pharmaceutical Co., in Gotemba, Japan, tried to tease apart the effects. By adding vitamin D to a petri dish full of osteoclast precursors, they discovered that the hormone blocked a signal called RANK ligand that tells these cells to become osteoclasts. That's how vitamin D cuts back on osteoclasts.