Vitamin D3 is a fat-soluble vitamin that is responsible for your body's absorption of calcium and phosphorus. Having optimal Vitamin D3, calcium and phosphorus levels is important for building and maintaining healthy, strong and dense bones in humans. Vitamin D is used to prevent and treat certain bone disorders as well. In the Northern and Western part of the world, Vitamin D3 deficiencies are becoming more and more prevalent, hence why it is a great idea to supplement with it. You can stack your multivitamin supplement with Life Extension's "Vitamin D3" (3,000 IU) to obtain the optimal Vitamin D3 levels in your blood.