Sunlight is the source of life on Earth, but we may not be aware of all the ways it benefits us. Our bodies manufacture vitamin D-3 when the skin is exposed to UV radiation from sunlight, but the ability to do so decreases as we age. Vitamin D-3 maintains healthy calcium and phosphorus levels in the body for strong bones: it increases muscle strength in older adults; and it also plays an active role a healthy immune response. Vitamin D-3 (cholecalciferol) is the optimal form of vitamin D. It is the form of vitamin D that the body manufactures in sunlight, and the form most efficient for the body's needs.