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.

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.

Doing strength training for 12 weeks can make your thigh muscles bigger and stronger, whether you're young or old—but taking vitamin D pills won’t make that improvement any better.

When older men lift weights for 12 weeks and take a daily vitamin D3 pill, their muscles get stronger relative to their size compared to men who only take calcium — even though their muscles don’t get bigger or stronger in absolute terms.

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.