Taking a daily vitamin D3 pill (4000 IU) for four months can help older adults with low vitamin D levels build stronger muscle fibers by increasing the number of vitamin D receptors inside their muscle cells.

For older women who have trouble moving and don’t have enough vitamin D, taking a daily vitamin D pill for four months can boost a key protein inside their muscle cells by about 30%, which might mean vitamin D directly helps muscle cells communicate better.

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.

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.

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 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.