The hormonal or active form of vitamin D, that is, 1,25(OH)₂D₃, acts through a nuclear receptor (VDR) to carry out its many functions. The receptor has been recognized in a wide variety of tissues such as enterocytes, osteoblasts, distal renal tubular cells, parathyroid gland cells, skin keratinocytes, lymphocytes, prostate, colon, pituitary gland, and ovaries. Many tissues in the body, including osteoblasts, prostate, and colon express the 1-hydroxylase and synthesize 1,25(OH)₂D₃ locally.

Vitamin D can be said to have at least 2 distinct groups of functions. One is classically endocrine: calcitriol synthesis increases in response to parathyroid hormone (PTH); the resulting product circulates in the blood to reach target tissues (gut and bone) and regulates PTH secretion and hence its own production. The other is autocrine (or perhaps paracrine): the cells or tissues concerned make and degrade the hormone locally to regulate their own proliferation and differentiation.

Chronic vitamin D deficiency may have serious adverse consequences, including increased risk of hypertension, multiple sclerosis, rheumatoid arthritis, cancer of the colon, prostate, breast, and ovary, and type 1 and 2 diabetes. Vitamin D supplementation can affect many aspects of health because its metabolites function at many tissues.

The high prevalence of vitamin D deficiency (defined as low serum levels of 25-D) in the general population as well as in patients with chronic kidney disease has been reported.

The main focus of this special issue will be on vitamin D and Kidney. We welcome authors to submit original research articles, review articles, and clinical studies that address these issues. Potential topics include, but are not limited to:

• Vitamin D and chronic kidney disease
• Endocrine functions of vitamin D
• Paracrine functions of vitamin D
• Vitamin D deficiency
• Vitamin D toxicity

Before submission authors should carefully read over the journal's Author Guidelines, which are located at http://www.hindawi.com/journals/ije/guidelines/. Prospective authors should submit an electronic copy of their complete manuscript through the journal Manuscript Tracking System at http://mts.hindawi.com/author/submit/journals/ije/vitnd/ according to the following timetable: