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 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 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.

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.

If you make muscle cells produce more of a protein called the vitamin D receptor, the muscles get bigger because they start making more protein, grow more repair cells, and stop a protein that normally limits muscle growth.