Vitamin D is essential for the functioning of many important systems in the body, and recent research has suggested that we may need more vitamin D than previously thought for optimal immune function and long-term health. Vitamin D3 is thought to be more biologically active in the body than D2, the form often used to fortify dairy products and other foods. In some climates, people may be able to produce adequate vitamin D3 from sunlight, but in cooler climates with longer winters, vitamin D deficiency may be more common. Vitamin D is required for optimal absorption of minerals including calcium, making it an important nutrient for supporting healthy bones and structural health. It may also play a role in supporting immune health and cellular integrity.