Taking a daily vitamin D3 supplement while doing strength training might help young men build more of a certain type of muscle fiber and reduce a protein that limits muscle growth, compared to just taking calcium.

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.

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.

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.

Taking a high-dose vitamin D pill every day for three months won't help young men who are low on vitamin D build more muscle or gain more lean body mass from weight training, even though their vitamin D levels go up to normal.

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.