Although vitamin D is usually categorized as a fat-soluble vitamin, it actually functions as a hormone in the body. Vitamin D helps to activate calcium and phosphorus (another key mineral for keeping bones strong) into the bloodstream. When the body is depleted of vitamin D or has an insufficient supply, the blood levels of calcium and phosphorus plummet as well. Your body turns to the bones for replenishing this mineral.Loss of the minerals calcium and phosphorus is directly correlated to osteoporosis, a bone-thinning disease that results in painful fractures, and a host of other bone-weakening problems. There are also some new findings from Boston University showing that vitamin D is an important nutrient for multiple facets of health - including insulin function, cancer prevention (especially of the breast/colon/prostate), and cardiovascular health.