When scientists turned up the vitamin D receptor in the leg muscles of rats for 10 days, the muscle fibers got bigger, the muscles made more protein, and key growth signals kicked in—so it looks like boosting this receptor might help muscles grow.

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.

Taking a lot of vitamin D might help your muscles grow bigger by turning down a natural brake that stops them from growing.

When scientists added a form of vitamin D to rat muscle cells in a dish, the cells made more of a protein called VDR — and the more vitamin D they got, the more VDR they made. This suggests vitamin D might help control this protein in muscle cells.

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.

Taking vitamin D pills while doing weight training doesn’t change the levels of a specific protein in your muscles that helps vitamin D work, which means vitamin D probably isn’t helping your muscles grow by using this protein.