When scientists add a form of vitamin D to human muscle cells in a lab, the cells start making a lot more of a specific molecule—like turning up a volume knob—showing that these muscle cells can respond to vitamin D.

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.

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.

A form of vitamin D that your body uses helps muscle cells grow better, improves how muscles use insulin to build protein, and makes the energy factories inside muscle cells work harder.

When you give a special form of vitamin D to muscle cells in a lab dish for a day or three, it slows down how fast they multiply and helps them turn into mature muscle cells by changing which genes they use.