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absorption and metabolism of vitamin a

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Vitamin D 3 is made in the skin from 7-dehydrocholesterol under the influence of UV light. These data suggest that retinoids increase bone formation by enhancing the differentiation of osteoblasts. Rothenberg AB, Berdon WE, Woodard JC, Cowles RA. Interestingly, retinoid treatment of APL patients results in reduced numbers of bone marrow microvessels, an effect associated with decreased expression of vascular endothelial growth factor in the tumor cells (168). Similar observations were made in female rats treated with 120 mg/kg ATRA orally for 4 days. Some disorders impair the absorption of fats. The vitamin most closely associated with protein metabolism is. Arch Biochem Biophys. Evaluation of different promoter usage and alternative splicing has shown that there are at least 2 different isoforms for each isotype (26). Structural analyses have revealed that molecules having vitamin E antioxidant activity include four tocopherols (α, β, γ, δ) and four tocotrienols (α, β, γ, δ); see Fig. 1. Under normal conditions of sunlight exposure, no dietary supplementation is necessary because sunlight promotes adequate vitamin D synthesis in the skin. For nutritional Some disorders, called malabsorption disorders, interfere with absorption of fats and thus of fat-soluble vitamins. ATRA levels are lower in the upper portion of the growth plate, which contains resting/proliferating chondrocytes, in comparison to the lower portion, which contains maturing/hypertrophying chondrocytes (31–33). Haider C, Ferk F, Bojaxhi E, Martano G, Stutz H, Bresgen N, Knasmüller S, Alija A, Eckl PM. Although data from ex vivo studies indicate that ATRA up-regulates RANKL and causes significant stimulation of mouse periosteal bone resorption, it has also been observed that ATRA exerts potent inhibition of RANKL-induced differentiation in mouse bone marrow, spleen cell, and human blood osteoclast progenitor cells. Why periosteal osteoclast progenitor cells are not inhibited by ATRA is unclear. It has been reported that bone is the second most important organ for clearance of chylomicron remnants and that vitamins can be delivered to osteoblasts in vivo via chylomicrons (23). Most β-carotene is metabolized by central cleavage at the 15, 15′ double bond, yielding 2 molecules of retinal. Furthermore, the authors found no association between vitamin A or retinol intake from food and supplements, or food only, and the risk of hip or all fractures (97). In the United States, it is not unusual for foods such as cereals to be heavily fortified with vitamin A. If retinol is not needed by the body, it is reesterified and retained in liver stellate cells. Target cell metabolism of retinoids and the different signaling pathways of retinoids involve many different binding proteins and receptors (summarized in Figure 3). Interestingly, isotretinoin (13-cis RA) treatment has been reported to decrease the incidence of heterotopic bone formation in FOP patients, although the use of this particular retinoid caused a variety of side effects and was not considered clinically useful (177). Rejnmark L, Vestergaard P, Charles P, et al. Evidence for delayed plasma clearance of intestinal lipoproteins, Influence of aging on vitamin A transport into the lymphatic circulation, Hypovitaminosis A: contemporary scientific issues, Mawson's Will: The greatest polar survival story ever written, Risk of vitamin A toxicity from candy-like chewable vitamin supplements for children, Water-miscible, emulsified, and solid forms of retinol supplements are more toxic than oil-based preparations, Retinoid-induced bone thinning is caused by subperiosteal osteoclast activity in adult rodents, High dietary intake of retinol leads to bone marrow hypoxia and diaphyseal endosteal mineralization in rats, Subclinical hypervitaminosis A causes fragile bones in rats, Bone changes in children on long-term treatment with etretinate, Rheumatologic effects of etretinate [in French], Central physeal arrests as a manifestation of hypervitaminosis A, Hypervitaminosis A-induced premature closure of epiphyses (physeal obliteration) in humans and calves (hyena disease): a historical review of the human and veterinary literature, Bone changes in a patient with hypervitaminosis A, Hypercalcemia and skeletal effects in chronic hypervitaminosis A, Hypervitaminosis A. Supporting this view, the authors show formation of osteocyte-rich woven bone along the endosteal surfaces, an observation also made by Hough et al (153), but not by Kneissel et al (75). Moreover, prospective studies in humans using peripheral quantitative computed tomography for evaluation of specific effects in cortical and trabecular bone appear to be essential if we are to gain a better understanding of the risk that increased vitamin A intake might pose to skeletal health. Storage • Unlike other water soluble vitamins vitamin B12 is stored in the liver and other tissues. Furthermore, the reliability of the bone particle assay for use in measuring resorptive activity of mature osteoclasts has not been demonstrated. Epub 2019 May 8. In their experiments, plasma with vitamin A added or plasma from fowl treated with high doses of vitamin A was added to explant cultures of either chicken embryonic limb buds obtained from eggs or to fetal mouse long bones. In a substudy of the population-based Nord-Trøndelag Health Study, 3052 Norway women 50–70 years of age had BMD of the forearm measured. Disclosure Summary: The authors have nothing to disclose. Effects on bone were assessed in young (2–3 mo), middle-aged (8–10 mo), and old (18–20 mo) rats. In 31 022 individuals 65 years of age or older (mean age, 76; 91% female) exhibiting 1111 hip fractures and 3770 nonvertebral fractures, median vitamin D intake of 800 IU/d (range, 792–2000) reduced the risk of hip fracture 30% and the risk of nonvertebral fracture 14% (114). Summary of the Effects by Retinoids on Bone Resorption and Osteoclast Formation in Vivo.

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