Vitamin D

The term ‘vitamin D’ covers a group of closely-related, lipid-soluble compounds, of which the two most important are cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2). Vitamin D status is assessed by measuring the major storage forms 25-hydroxyvitamin D3 and D2.

Vitamin D is traditionally thought of as a micronutrient but can also be classified as a prohormone. Cholecalciferol is synthesized in the skin by the effect of ultraviolet B radiation. It circulates in the bloodstream and rapidly undergoes 25-hydroxylation in the liver. Subsequent 1α-hydroxylation in the kidney results in the production of the active metabolite, 1,25-dihydroxycholecalciferol (calcitriol). Both cholecalciferol and the plant equivalent ergocalciferol may also be found in the diet. Vitamin D supplements are widely available. In the absence of specific supplementation, the major source of vitamin D is endogenous synthesis.

The best know effects of vitamin D are on calcium metabolism. Calcitriol promotes calcium absorption from the gastrointestinal tract and reabsorption of filtered calcium in the renal tubules, thus increasing the availability of calcium for deposition in bone matrix. Deficiency is therefore associated with reduced bone density. Severe vitamin D deficiency leads to rickets in childhood and osteomalacia in adults.

The actions of vitamin D are not confined to these well known functions: the receptor for vitamin D has been found in many other cells and tissues including macrophages, pancreatic islet cells and skin. There is epidemiological evidence linking vitamin D deficiency to cardiovascular disease, some types of cancer and autoimmune disease.

Vitamin D status can be assessed by measurement of plasma vitamin D which responds to changes in intake. It is, however, subject to the systemic inflammatory response which lowers the concentration by up to 40% following a trauma (see Table).⁸³ This reduction is dependent on the magnitude of the systemic inflammatory response.²⁸

Micronutrient screens in Scotland include 25-hydroxy vitamin D measurement by immunoassay, unless measurement of 25-hydroxy vitamin D by liquid chromatography-tandem mass spectrometry (LC-MS/MS) is specifically requested. Vitamin D measurement by LC-MS/MS occurs in the specialist endocrine laboratory. For further details please visit the Glasgow Royal Infirmary Specialist Endocrine laboratory website.

For questions about vitamin D analysis and interpretation of results please contact the Endocrine Biochemist on 0141 242 9608 or 0141 242 9500 (option 2).

Effect of Systemic Inflammatory Response on Plasma Vitamin D Concentrations

Table: Distribution of median plasma vitamin D concentrations according to increments of CRP concentrations (n = 5154).²⁸

Sample requirements and reference values for Vitamin D

* Absolute minimum volume; this volume is insufficient to carry out repeat analysis if analysis fails.

References