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 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.

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.

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.

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.