Doctor's Best Vitamin D3 supports healthy bone density and structure as it is required for optimal calcium absorption. More than that, this nutrient enhances and regulates immune function, supports optimal cardiovascular function and enhances cellular function. It is made of D3, the naturally-occurring form of vitamin D. Research shows that supplemental vitamin D3 (cholecalciferol) is significantly more efficient at converting to active vitamin D in the body than vitamin D2 (ergocalciferol).