Dietary Supplement Bioactive Form for Bone and Immune Health Sunlight is the source of life on Earth, but we may not be aware of all the ways it benefits use. Our bodies manufactured vitamin D-3 when the skin is exposed to UV radiation from sunlight, but its ability to do so decreases as we age. Vitamin D-3 maintains health calcium and phosphorous levels in the body for strong bones; it increases muscle strength in older adults; and it also plays an active role in a health immune response. Vitamin D3 (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.