If young guys who don’t have enough vitamin D do strength training without taking extra protein or vitamin D, they can get stronger legs—but their muscles won’t grow much, and their muscle hormones won’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.

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 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.

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.

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.