Vitamin D helps the intestines absorb calcium. However, the vitamin D must first be activated or "turned on" by parathyroid hormone (PTH). Once activated, vitamin D acts to greatly increase the amount of calcium that the intestines can absorb from food, sometimes by as much as two to four times. The body can either make its own vitamin D using a process that requires sunlight or obtain vitamin D directly from the diet (e.g. in fortified milk or vitamins). Many people across the world have vitamin D deficiency — experts estimate that one-third of Americans are vitamin D deficient.