When people do supervised weight training for 20 weeks, their muscles make more of a protein called VDR—and that’s linked to gaining more muscle mass, but not to getting stronger or having more vitamin D in their blood. So VDR might be a sign that muscle growth is happening, even if vitamin D levels don’t change.

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.

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.

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.

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 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.