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 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 scientists added a form of vitamin D along with leucine and insulin to muscle cells from mice, the cells made more protein than usual—about 14 to 16% more—suggesting these three things work better together than alone.

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 mouse muscle cells in a dish, the cells start making more of two important proteins: one that helps them respond to insulin, and one that helps them respond to vitamin D itself.

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.