Natural Factors Vitamin D3 is essential for strong bones and teeth as it increases the body’s absorption of calcium from food. Its role in regulating calcium levels also helps regulate the immune system and maintains healthy nerves and muscles. Natural Factors Vitamin D3 is an excellent way to support the body’s normal functioning. You may not always be able to get your Vitamin D from the sunlight or from a proper diet; many people take a Vitamin D supplement. It’s best to take Vitamin D in combination with calcium to maintain a proper balance. Natural Factors Vitamin D3 is a great source to ensure adequate levels, stays in your system longer and is a more effective source of Vitamin D.