If young guys who don’t have enough vitamin D take 4000 IU of vitamin D and 25 grams of whey protein every day while doing strength training for six weeks, their vitamin D levels go up enough to get into a healthy range.

If young guys who don't have enough vitamin D lift weights and also take a combo of whey protein and a high dose of vitamin D every day for six weeks, their body's muscle-building hormones go up and muscle-limiting hormones go down—better than just lifting weights alone.

If young guys who don't get enough vitamin D are working out with weights, taking a protein and vitamin D supplement at night or in the morning works just as well for building muscle and improving body shape — it doesn't matter what time they take it.

If young guys who don't get enough vitamin D lift weights and take a daily combo of whey protein and a high dose of vitamin D, they'll gain more muscle — especially in their legs — than those who just lift weights without the supplements.

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.

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.