THE BODY MAKES VITAMIN D WHEN BARE SKIN IS EXPOSED TO THE SUN. THERE ARE VERY FEW FOOD SOURCES. Almost one out of four people have vitamin D blood levels that are too low or inadequate for bone and overall health. In children, vitamin D deficiency causes rickets, a disease in which the bones become soft, weak, deformed, and painful. In teens and adults, vitamin D deficiency causes osteomalacia, a disorder that causes bone pain and muscle weakness. Vitamin D helps the body absorb calcium, one of the main building blocks for strong bones and preventing osteoporosis. Vitamin D is also needed for muscles to move and for nerves to carry messages between the brain and the body. The immune system needs Vitamin D to fight off invading bacteria and viruses. The recommended amount of Vitamin D for adults 71 and older is 20 mcg (800 IU) daily. For all others, the amount is 15 mcg (600 IU). Vitamin D is available in dietary supplements. The two forms of vitamin D in supplements are D2 and D3. Both forms increase vitamin D in the blood, but D3 might raise it higher and for longer than D2. Because vitamin D is fat-soluble, it is best absorbed when taken with a meal or snack that includes some fat. In the United States, most people have adequate blood levels of vitamin D.