La vitamine D dans les affections métaboliques et cardiovasculaires. Effet réel ou effet de mode ?

Vitamin D occupies a particular position among vitamins. Firstly, vitamin D is photochemically synthesized in the skin even though small amounts are provided by dietary sources. Secondly, after two consecutive hydroxylations in the liver and kidneys, its effects are due to its final metabolite, i.e. the 1,25 dihydroxyvitamin D [1,25(OH)2D] that interacts with its cognate nuclear receptor: the VDR. As this receptor is widely distributed in numerous target tissues, many properties were attributed to vitamin D. Despite its main action on calcium homeostasis (intestine and bone), many observations indicate that vitamin D exerts a regulatory effect on the insulin sensitivity of peripheral tissues and the ß-cell function, a modulating action on the immune system, and a stimulatory effect on the myocardial contractility. As a consequence, its utilization has been recommended for preventing or treating numerous medical conditions such as diabetes states, metabolic syndromes, obesities, and cardiovascular diseases. Unfortunately, these recommendations are not evidence-based, because randomized interventional trials, which are normally required for establishing such hypotheses are sadly lacking. However, some authors still recommend the use of vitamin D supplementations on the basis of assessment of the vitamin D status from measurements of circulating concentrations of 25 hydroxyvitamin D [25(OH)D]. Definitions of vitamin D deficiencies were usually set at either 20 or 30 ng/mL, i.e. at 25(OH)D thresholds that seem relatively overestimated. Discrepancies between definitions reflect the uncertainties for assessing the vitamin D status of individuals. Today, no one knows exactly whether supplementations in vitamin D should be optimal, recommended, or mandatory, when 25(OH)D levels are found below 20 ng/mL. By contrast, there is no doubt that too aggressive supplementations can exert harmful effects, particularly in those who are hypercalciuric and prone to urolithiasis. Therefore, when it comes to the patient's supplementation with vitamin D, the decision making process should be carefully weighed because, inversely to what is commonly thought, its prescription can display either salutary or deleterious effects.