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.

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.

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 people, having more or less vitamin D in the blood doesn’t seem to change how much of the vitamin D receptor gene is active in their muscles — the two just don’t go up or down together.

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.

In older adults who have trouble moving around, higher levels of vitamin D in the blood are linked to more vitamin D receptors in their muscles — meaning the more vitamin D they have, the more of these muscle sensors they seem to have.