research over the last two decades has indicated that vitamin D may also be a critical modulator of several nonskeletal systems and related diseases. Vitamin D refers to two biological precursors -ergocalciferol (D 2 ) and cholecalciferol (D 3 ). Vitamin D 3 constitutes around 80-90% of the circulating metabolites and is synthesized mainly in the skin by the action of ultraviolet (UV) light (280-315 nm). Vitamin D 3 can also be obtained exogenously from animal sources such as fish oils or

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... ied dairy products and vitamin supplements. Sun-dried mushrooms, UV-B irradiation of the yeast sterol ergosterol, as well as vitamin supplements are considered as main sources of vitamin D 2 [2] . Vitamin D (D 2 , D 3 ) has no biological activity without a two-step hydroxylation process. The first step, in the liver, requires P450 enzymes such as CYP2R1 and CYP27A1 (25-hydroxylases) to form the major circulating form 25(OH)D. The second step, in the kidneys, requires the P450 enzyme, CYP27B1 (1α,25-hydroxylase) to form the main active metabolite -1,25(OH)D or calcitriol. The second hydroxylation reaction is stimulated mainly by parathyroid hormone (PTH), and inhibited by calcium, phosphate and fibroblast growth factor-23 (FGF-23) [3] . The biological action of 1,25(OH) 2 D is mediated through the vitamin D receptors (VDRs) and this receptor as well as CYP27B1 are expressed widely in several tissues [4] ( fig. 1 ) . Vitamin D deficiency has been defined by the Institute of Medicine (IOM) as a measured serum 25(OH)D of <30 nmol/l (12 ng/ml), vitamin D insufficiency when the se- Abstract In recent years there has been increasing interest in the nonskeletal effects of vitamin D. It has been suggested that vitamin D deficiency may influence the development of diabetes, cardiovascular dysfunction and autoimmune diseases. This review focuses on the current knowledge of the effects of vitamin D and its deficiency on cardiovascular function, glucose homeostasis and immune function, with a particular focus on children. Although, there is good evidence to show that there is an association between vitamin D deficiency and an abnormality of the above systems, there is little evidence to show that vitamin D supplementation leads to an improvement in function, especially in childhood. Vitamin D Overview Most plants and animals that are exposed to sunlight have the ability to produce vitamin D. In humans, the active vitamin D metabolite, 1,25-dihydroxyvitamin D (1,25(OH) 2 D), has been well recognized for its role in calcium and phosphate homeostasis [1] . However, intensive 365 deficiency [6, 9] . In this review, PubMed and the Web of Knowledge were used to identify studies that explored a link between vitamin D, glucose homeostasis and cardiovascular and immune function in children. Studies conducted within the last 7 years, which involved large paediatric samples and those reported in English, were selected and are summarized in tables 1-3 as appropriate.