The human body requires vitamin D in many of its daily functions. Receptors for this vitamin are present on cells in the brain, heart, colon, breast and immune system, reports M. F. Holick, MD in ”Vitamin D Deficiency.” Vitamin D is also important in calcium absorption and plays a key role in maintaining bone density. A individual acquires vitamin D from dietary intake, supplements or from adequate sun exposure. A deficiency of this essential vitamin may lead to serious disease.
Because Vitamin D is essential to the formation of bone, inadequate amounts of this vitamin result in weakening of the bones. In children this disease is called rickets. Common signs of rickets are an outward bowing of the legs and unusually widened wrists in a growing child. Bone pain has also been reported with rickets, especially when pressure is applied directly to the bone. In adults, osteomalacia is the term used to describe softening of the bones due to insufficient vitamin D levels. Fractures are also common in people with osteomalacia and healing time is sometimes slowed.
An inadequate supply of vitamin D makes it difficult for the body to effectively use calcium to continue to maintain bone density. Osteoporosis is a disease in which the bones become less dense and as a result are more prone to fracture. The spine, hip and wrist are frequent sites of fractures due to osteoporosis.
People deficient in vitamin D report muscle weakness and difficulty in performing normal tasks as a result. The cells of skeletal muscles have Vitamin D receptors, and it is believed that they are unable to work to maximum efficiency without adequate levels of this vitamin.
According to D.T. Felson, MD, in “Osteoarthritis: New Insights. Part 1: The Disease and Its Risk Factors,” which appeared in "Annals of Internal Medicine" in October 2000, progression of osteoarthritis is thought to be related to very low levels of Vitamin D. While it has not been connected to the development of arthritis, a deficiency of Vitamin D may play a part in the advance of the disease because the body is unable to repair damage to the affected joints.