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.

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.

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 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.

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