Vitamin D comes in two forms:D2 and D3. Of the two, D3 is more bioactive. The body synthesizes vitamin D3 from cholesterol and sunlight; which makes it difficult to obtain adequate amounts of vitamin D during cloudy or winter months. In the body, vitamin D is responsible for maintaining normal blood levels of calcium and phosphorus, and helping to build strong bones. It works with other vitamins, minerals and hormones to promote bone mineralization. Research shows that vitamin D may help support a healthy immune system by regulating lymphocyte function, cytokine production and macrophage activity. Studies also show that Vitamin D may improve mood.