All About Vitamin D

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Vitamin D (Calciferol) is a fat soluble steroid prohormone that was first identified as a vitamin in the 20th century (Holicks, et al, 2011 ). It has two major forms- D2(ergocalciferol) and D3 (cholecalciferol) (Holicks et al, 2011, BMJ). The sources of vitamin D in human include sunlight (80-90%) and dietary source (10-20%) (Mithal et al, 2009). Dietary sources of vitamin D includes – salmon, canned fishes (sardines, tuna, mackerel) , cod liver oil, shitake mushroom, egg yolk, fortified mils and orange juice, infant formulas, fortified yoghurt and butter, fortified cereal as well as supplemental oral vitamin D. The Institute of Medicine (IOM) and the United States Endocrine society recommends a daily intake of 600 IU/day for adults <50, and 600 IU to 800IU for adults >50, with an upper limit of 4000IU/day. [69] Holicks et al(2011). The National Osteoporosis Foundation (2013) further recommends a daily intake of 400 IU to 800 IU in adults <50 and 800 IU to 1,000 IU of vitamin D per day for adults>50 (due to the increased prevalence of osteopenia and osteoporosis in adults >50). In adults with a BMI>30, the Endocrine society (Holicks et al, 2011) also recommends a maintainence dose of 10,000 iu/day due to storage and sequestering of vitamin D by increased adipose tissue in obese individuals. In humans, vitamin D plays an essential function in maintaining calcium and phosphate balance, which is important for the maintenance of bone (formation, remodeling, and healing). Holicks et al 2011 asserts that “sufficient vitamin D enhances calcium and phosphorus absorption by 30–40% and 80%, respectively”. However the definition of 25(OH) D sufficiency and deficiency varies. According to the Endocrine Society (Horlicks et al (2011) the I... ... middle of paper ... ...ances the expression of intestinal epithelial calcium channels and calcium-binding proteins throughout the entire length of the intestine, with its greatest activity in the duodenum and jejunum. This increases the absorption of ingested calcium (dietary and supplementary) and increases plasma levels of calcium. Calcitrol also mobilizes calcium from the bones, by facilitating the formation of osteoclasts and stimulating the secretion of a protein called “receptor activator for nuclear factor κ B (RANK) ligand” (BMJ). The formation of osteoclast and secretion of κ B (RANK) ligand enhances osteoclastogenesis, increases bone reabsorption and hence increases serum calcium level (BMJ). Additionally, calcitriol effect on the vitamin d receptor (VDR) of the parathyroid gland causes suppression of parathyroid gene expression as well as proliferation of the parathyroid cells

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