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However, absence of significant association between serum leptin and CRP levels as well as between serum leptin concentrations and ESR values in HD patients requires further investigation and clarification. Footnotes CONFLICT OF INTEREST: NONE DECLARED.""There is a double origin of vitamin D. It can be ingested orally or it can be formed endogenously in the skin after exposure to sunlight, or better to say, to UVB light (Figure 1)(1, 6). Very briefly, in the skin under the influence of UVB radiation 7-dehydrocholesterol is converted to previtamin D. By a temperature dependent [http://www.selleckchem.com/products/verubecestat-mk-8931.html check details] process previtamin D is very quickly converted to vitamin D3 (cholecalciferol). Vitamin D3 is transported in the blood by vitamin D binding protein (VDBP). Orally ingested vitamin D is vitamin D2 (ergocalciferol) and is also transported by VDBP to the liver. In the liver cholecalciferol, i.e. vitamin D3, and ergocalciferol, vitamin D2, are converted to 25-hydroxivitamin D by the enzyme 25-hydroxylase. This process is not tightly regulated and the half-life of these metabolites is very long, up to 19 days. Therefore, the measurement of these metabolites is [http://www.selleckchem.com/products/liraglutide.html www.selleckchem.com/products/liraglutide.html] used to define vitamin D deficiency and insufficiency (see later). 25-hydroxivitamin D (D represents D 2 and D3) is biologically inert and much less potent than so-called active metabolite, 1.25 dihydroxivitamin D. Under the influence [https://en.wikipedia.org/wiki/Oxymatrine Oxymatrine] of 1 �� hydroxylase, 25-hydroxivitamin D (named calcidiol) is hydroxlated to the above-mentioned 1.25 dihydroxivitamin D, named calcitriol. More than 85% of circulated 25-hydroxivitamin D is hydroxylated in renal tubules and about 5 % is hydroxylated in other cells, e.g. monocytes. Calcitriol from the kidneys has an endocrine function and calcitriol from other peripheral tissues and cells has autocrine or paracrine function. Calcitriol synthesis in kidney is regulated by PTH, calcium, phosphate and fibroblast growth factor 23 (FGF 23). While PTH increases 1 �� hydroxylase activity, the effect of calcium, phosphorus and FGF 23 is the inhibition of calcitriol synthesis. Moreover, FGF 23 increases the activity of 24, 25 hydroxylase, i.e. synthesis of 24, 25 dihydroxivitamin D (Figure 1). Figure 1 Vitamin D metabolism 3. ASSESSMENT OF VITAMIN D STATUS Over the last 25 years, there have been many studies showing the association between decreased calcidiol concentration in the blood and mortality, cardiovascular disease, bone fractures, diabetes mellitus, increased incidence of some malignant diseases and autoimmune disease (6, 7). Therefore, it was obvious that vitamin D status, as a deficiency of insufficiency, should be defined. Several professional societies defined a deficiency as a calcidiol level