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 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 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.

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.

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.