Vitamin D (also known as calcitriol or 1,25-dihydroxyvitamin D in its hormonal form) acts on the intestines, bones, and kidneys. It also maintains calcium and phosphate levels in the plasma that are required for bone mineralization.1,2 Normal calcium levels are also important for the proper functioning of the neuromuscular junction, nerve transmission, vasodilation, and hormonal secretion. Parafollicular, or “C” cells, found in the thyroid gland, secrete calcitonin if the level of calcium in the plasma is too high. This prevents calcium release from bones, called resorption, and regulates plasma calcium levels.3
Vitamin D receptors are present in many tissues in the body that are not involved in maintaining the plasma levels of calcium and phosphate such as multiple types of immune cells and epidermal keratinocytes. The physiological consequences of the presence of vitamin D receptors in these tissues are not clearly known.3
Vitamin D deficiency in adults can lead to osteomalacia, in which bone does not mineralize properly and results in wide unmineralized bone matrix seams. In children, deficiency can lead to rickets (widening of end of long bones, rachitic rosary, and skeletal deformations).3
References
- Deluca HF. The transformation of a vitamin into a hormone: the vitamin D story. Harvey lectures. 1979;75:333–79
- Holick MF. Vitamin D: photobiology, metabolism, mechanism of action, and clinical application, Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism. Favus MJ, editor. Philadelphia, PA: Lippincott-Raven; 1996. pp. 74–81
- Ross AC, Taylor CL, Yaktine AL, et al., editors. Institute of Medicine (US) Committee to Review Dietary Reference Intakes for Vitamin D and Calcium; Dietary Reference Intakes for Calcium and Vitamin D. Washington (DC): National Academies Press (US); 2011. 3, Overview of Vitamin D.Available from: https://www.ncbi.nlm.nih.gov/books/NBK56061/