For older women who have trouble moving and don’t have enough vitamin D, taking a daily vitamin D pill for four months can boost a key protein inside their muscle cells by about 30%, which might mean vitamin D directly helps muscle cells communicate better.

Taking a daily vitamin D3 pill (4000 IU) for four months can help older adults with low vitamin D levels build stronger muscle fibers by increasing the number of vitamin D receptors inside their muscle cells.

When older women who have trouble moving around get more vitamin D in their blood, their muscle cells seem to respond by making more vitamin D receptors — and this happens no matter how they got the vitamin D, like from pills or sun exposure.

For older women who have trouble moving around and don’t have enough vitamin D, taking a daily vitamin D pill for four months might make their muscle fibers about 10% bigger than if they took a sugar pill — which could mean it helps build muscle.

In older women who have trouble moving and low vitamin D, taking a daily vitamin D supplement for 4 months might help their muscles make more of a key protein that helps vitamin D work, while those who take a fake pill might see that protein drop instead.

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.