When scientists remove the vitamin D receptor in mice, the genes that help muscles grow and repair don't work right anymore, which might mess up how their muscles heal or develop.

When mice can't use vitamin D properly, their muscles get weaker, their muscle fibers shrink, and a protein that stops muscle growth increases—so vitamin D seems to help keep muscles strong and healthy.

Even if the levels of calcium, magnesium, and phosphate in the mice’s bodies were kept the same, removing the vitamin D receptor or not giving them enough vitamin D still changed their muscle strength and which genes were active.

When mice don’t have enough vitamin D or can’t use it properly, their muscles weaken because the genes that help move calcium around in muscle cells don’t work as well.

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.

When mice don’t get enough vitamin D from their diet, their muscles start breaking down more because certain proteins that signal muscle loss become more active.