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.

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.

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.

If you're a young man with low vitamin D and you lift weights, your muscles will get stronger and you'll gain more muscle mass—even if you don't take vitamin D pills.

When older men take vitamin D pills while doing weight training, their bodies might slow down the production of a specific enzyme because they have too much vitamin D in their blood — like a safety switch turning itself off.

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.