In lab-grown rat muscle cells, a form of vitamin D helps change how the cells develop into muscle fibers, but only partly — another protein called the vitamin D receptor is involved, and it’s probably not the only one doing the job.

When scientists added a form of vitamin D to rat muscle cells in a dish, the cells made more of a protein called VDR — and the more vitamin D they got, the more VDR they made. This suggests vitamin D might help control this protein in muscle cells.

When scientists gave a specific form of vitamin D to muscle cells from young, healthy people for 18 hours, they noticed the cells made 36% more of a protein that helps vitamin D work — suggesting vitamin D might tell these cells to make more of this protein themselves.

When scientists lower a specific vitamin D sensor in rat muscle cells, the cells make less of a protein linked to fast-twitch muscles—but giving them a form of vitamin D brings that protein back up.

When scientists reduced a specific vitamin D sensor in rat muscle cells, the cells made less of a protein linked to slow-twitch muscles—unless they added a lot of vitamin D, then the protein came back.

When scientists reduce a specific vitamin D sensor in rat muscle cells, the cells make less of two important muscle-building proteins—but giving them a tiny amount of active vitamin D brings those proteins back up.