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Serum vitamin D deficiency (25(OH) D), and therefore reduced 1,25(OH)2 D3 availability, is considered a risk factor for several chronic/inflammatory or autoimmune conditions, including infectious diseases, type 1 diabetes, multiple sclerosis, and especially autoimmune rheumatic diseases (ARD). In ARD in particular, 1,25(OH)2 D3 regulates both innate and adaptive immunity, potentiating the innate response (antimicrobial activity) but reducing adaptive immunity (antigen presentation, T and B cell activities). Regarding a possible synergism between vitamin D and GCs, several studies show that 1,25(OH)2 D3 has significant additive effects on dexamethasone-mediated inhibition of human lymphocyte and monocyte proliferation.
Conversely, vitamin D deficiency seems to play a role in increasing autoantibody production by B cells, and seasonal vitamin D declines may trigger flares in ARD, as recently shown. Finally, 1,25(OH)2 D3 seems to reduce aromatase activity and limit the negative effects related to increased peripheral estrogen metabolism (cell proliferation, B cell overactivity.
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Ysiology is important because about half of the population is being diagnosed with deficiency and treated with supplements.
However, new randomized controlled trials have failed to confirm any cardiovascular benefit from supplementation in the general population. A major concern is that excess vitamin D is known to cause calcific vasculopathy and valvulopathy in animal models. It is not a true vitamin because it can be synthesized endogenously through ultraviolet exposure of the skin.
It is a steroid hormone that comes in 3 forms that are sequential metabolites produced by hydroxylases. As a fat-soluble hormone, the vitamin D-hormone metabolites must have special mechanisms for delivery in the aqueous bloodstream. Because the cardiovascular effects of vitamin D hormones are not straightforward, both toxic and beneficial effects may result from current recommendation.
American Heart Journal
In the cardiovascular system is complex because it regulates expression of genes involved in diverse metabolic processes. It occurs as a series of sequentially activated forms, here referred to as vitamin D-hormones. Since low-density lipoprotein is known to accumulate in the artery wall and atherosclerotic plaque, diet-derived vitamin D-hormones may also collect there, and possibly promote the osteochondrogenic mineralization associated with plaque.
Also, little known is the fact that the body stores vitamin D-hormones in adipose tissue with a half-life on the order of months, raising doubts about whether the use of the term “daily requirement” is appropriate. These considerations suggest that prevalent recommendations for vitamin D-hormone supplementation for the purpose of cardiovascular protection should be carefully reconsidere.
Abstract
New knowledge of the biological and clinical importance of the steroid hormone 1α,25-dihydroxyvitamin D 3 [1α,25(OH) 2 D 3 ] and its receptor, the vitamin D receptor (VDR), has resulted in significant contributions to good bone health.
Over the past several decades, the biological sphere of influence of vitamin D 3 , as defined by the tissue distribution of the VDR, has broadened at least 9-fold from the target organs required for calcium homeostasis (intestine, bone, kidney, and parathyroid). This article identifies the fundamentals of the vitamin D endocrine system, including its potential for contributions to good health in 5 physiologic arenas in which investigators have clearly documented new biological actions of 1α,25(OH) 2 D 3 through the VDR. As a consequence, the nutritional guidelines for vitamin D 3 intake (defined by serum hydroxyvitamin D 3 concentrations) should be reevaluated, taking into account the contributions to good health that all 36 VDR target organs can provide.