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.

In muscle cells from mice, a form of vitamin D makes the cells respond stronger to leucine and insulin, which are signals that tell the body to build muscle — so it might help muscles grow better.

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.

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.

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.