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.

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.

When scientists added a form of vitamin D to rat muscle cells in a dish, the cells made more of a protein called VDR — and the more vitamin D they got, the more VDR they made. This suggests vitamin D might help control this protein in muscle cells.

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.

When people lift weights, their muscles seem to adjust how they use vitamin D internally, and that change matches up with how much of a certain enzyme they make in the muscle — but it doesn’t match up with how much vitamin D is floating in their blood. So maybe what’s happening inside the muscle matters more than what’s in the bloodstream.