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.

When you give human muscle cells a specific form of vitamin D, they start using more oxygen — but only after 24 hours if they’re young cells, and after 5 days if they’ve turned into mature muscle fibers.

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 muscle cells from humans are treated with a specific form of vitamin D for five days, they become more active in building proteins — especially when insulin is also around.

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 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.