Curr Rheumatol Rep (2010) 12:259–263 DOI 10.1007/s11926-010-0106-1 Vitamin D and Systemic Lupus Erythematosus: Bones, Muscles, and Joints Nancy E. Lane Published online: 29 April 2010 # The Author(s) 2010. This article is published with open access at Springerlink.com Abstract Vitamin D3, or cholecalciferol, is the naturally occurring form of vitamin D that is converted in the skin and hydroxylated in the liver and kidney to the active form found in humans. The main role for vitamin D is calcium homeostasis, and low levels of vitamin D result in lower gastrointestinal absorption of calcium. Vitamin D is also critical for mineralization of bone tissue, muscle function, and coordination. Recent studies have found prevention of bone mass loss and reduction in falls and fractures in patients supplemented with vitamin D. A high percentage of systemic lupus erythematosus patients are reported to have insufficient or deficient levels of vitamin D. This paper reviews the biology of vitamin D, its role in calcium homeostasis, and how it contributes to the maintenance of bone, muscle, and joint function in older adults and individuals with systemic lupus erythematosus. Keywords Vitamin D . Systemic lupus erythematosus . Bone and muscle function Introduction Vitamin D3, or cholecalciferol, is the naturally occurring form of the vitamin and is also the active form found in humans. In the skin, under the influence of UV light, 7- dehydroxycholesterol is photoconverted to previtamin D3, or cholecalciferol. The previtamin D3, bound to a vitamin D–binding protein, is transported to the liver, where it is N. E. Lane (*) Aging Center, Medicine and Rheumatology, University of California at Davis School of Medicine, 4800 2nd Avenue, Suite 2600, Sacramento, CA 95817, USA e-mail: nelane@ucdavis.edu hydroxylated to 25(OH)D3. In the kidneys, 25(OH)D3 is further metabolized to 1,25 (OH)D3, the biologically active form of vitamin D. The other form of vitamin D, vitamin D2, or ergocalciferol, is found in plants and is much less potent than vitamin D3. It is also biologically inert prior to conversion in the liver and kidney to active 1,25 vitamin D3. 1,25(OH)D3 is nearly 1000 times more potent than the precursor of vitamin D2 or vitamin D3. The major source of vitamin D is sun exposure; thus, active vitamin D in the body can be affected by the season and the duration of direct exposure to sunlight and can be inhibited by sunscreen use and increased skin pigmentation. In contrast to other vitamins, dietary intake is usually a minor source of vitamin D and provides only about 100 IU/d. Vitamin D is rarely found in foods other than eggs, fatty fish, and supplemented dairy products. It can be supplied as a component in combination vitamins or by individual supple- ments that have become available [1]. Mechanism of Action Vitamin D plays a critical role in calcium homeostasis. Maintenance of calcium levels in the body is organized in a balanced system, with vitamin D acting to produce enough gastrointestinal absorption of calcium to satisfy calcium demand and bone turnover. Low levels of vitamin D result in the calcium reserves in bone being depleted in an attempt to correct for the reduced calcium that will be absorbed from the gut. When calcium absorption by bone is reduced and vitamin D status is sufficient, absorption of dietary calcium should be about 30% to 40%. As vitamin D levels fall, absorption of dietary calcium declines to about 10% to 15% [2]. The reduction in intestinal calcium absorption associated with low levels of vitamin D triggers the release