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Calcitriol mediates its biological effects by binding to the vitamin D receptor (VDR), which is principally located in the nuclei of target cells. The binding of calcitriol to the VDR allows the VDR to act as a transcription factor that modulates the gene expression of transport proteins (such as TRPV6 and calbindin), which are involved in calcium absorption in the intestine. The vitamin D receptor belongs to the nuclear receptor superfamily of steroid/thyroid hormone receptors, and VDRs are expressed by cells in most organs, including the brain, heart, skin, gonads, prostate, and breast. VDR activation in the intestine, bone, kidney, and parathyroid gland cells leads to the maintenance of calcium and phosphorus levels in the blood (with the assistance of parathyroid hormone and calcitonin) and to the maintenance of bone content. The VDR is known to be involved in cell proliferation and differentiation. Vitamin D also affects the immune system, and VDRs are expressed in several white blood cells, including monocytes and activated T and B cells. Apart from VDR activation, various alternative mechanisms of action are known. An important one of these is its role as a natural inhibitor of signal transduction by hedgehog (a hormone involved in morphogenesis). Adequate intake levels of vitamin D have been established by the Food and
Nutrition
Adequate intake
Nutrition Board at the Institute of Medicine of The National Academies (formerly National Academy of Sciences). Adequate intakes depend only on age (i.e., they are the same regardless of sex, pregnancy, or lactation). These intake levels are based on the assumption that the vitamin is not synthesized by exposure to sunlight. In the United States, typical diets provide about 100 IU/day. The NIH has a "Tolerable Upper Intake Level (UL)" of 2,000 IU/day, while newer studies indicate a UL as high as 10,000 IU/day.
* 14–18 years, 5 µg (200 IU)
* 19–50 years, 5 µg (200 IU)
* 51–70 years, 10 µg (400 IU)
* 71+ years, 15 µg (600 IU)