Skeletal and extra-skeletal consequences of vitamin D deficiency

Vitamin D is obtained from cutaneous production when 7-dehydrocholesterol is converted to vitamin D(3) (cholecalciferol) by ultraviolet B radiation or by oral intake of vitamin D. Rickets appeared to have been conquered with vitamin D intake, and many health care professionals thought the major health problems resulting from vitamin D deficiency had been resolved. However, rickets can be considered the tip of the vitamin D deficiency iceberg. In fact, vitamin D deficiency remains common in children and adults. An individual's vitamin D status is best evaluated by measuring the circulating 25-hydroxyvitamin D (25(OH)D3) concentration. There is increasing agreement that the optimal circulating 25(OH)D3 level should be approximately 30 ng/mL or above. Using this definition, it has been estimated that approximately three-quarters of all adults have low levels. In utero and during childhood, vitamin D deficiency can cause growth retardation and skeletal deformities and may increase the risk of hip fracture later in life. Vitamin D deficiency in adults can exacerbate osteopenia and osteoporosis, cause osteomalacia and muscle weakness, and increase the risk of fracture. More recently, associations between low vitamin D status and increased risk for various non-skeletal morbidities have been recognized; whether all of these associations are causally related to low vitamin D status remains to be determined. The discovery that most tissues and cells in the body have vitamin D receptors and that several possess the enzymatic machinery to convert the 25-hydroxyvitamin D3, to the active form, 1,25-dihydroxyvitamin D3, has provided new insights into the function of this vitamin. Of great interest is its role in decreasing the risk of many chronic illnesses, including common cancers, autoimmune diseases, infectious diseases, and cardiovascular disease. In this review I consider the nature of vitamin D deficiency, discuss its role in skeletal and non-skeletal health, and suggest strategies for prevention and treatment.