Calcium, absorption metabolism

A major regulator of bone metabolism and calcium homeostasis, parathyroid hormone (PTH) is stimulated through a decrease in plasma ionised calcium and increases plasma calcium by activating osteoclasts. PTH also increases renal tubular calcium re-absorption as well as intestinal calcium absorption. Synthetic PTH (1-34) has been successfully used for the treatment of osteoporosis, where it leads to substantial increases in bone density and a 60-70% reduction in vertebral fractures. 

Might antagonize verapamil Might induce hypercalcemia with thiazide diuretics Fiber laxatives (variable), oxalates, phytates, and sulfates can decrease calcium absorption if given concomitantiy Phenytoin, barbiturates, carbamazepine, rifampin increase vitamin D metabolism... 

Calcium—Absorption and adsorption— Congresses. 2. Calcium—Metabolism—Congresses. 

Dietary phosphorus also affects calcium metabolism. Polyphosphate decreases calcium absorption in young men while orthophosphate supplement does not (26J. However, in the rat all forms of phosphate decrease calcium absorption about equally (31). However, widely divergent dietary calcium phosphorus ratios do not seem to affect calcium utilization by people as long as there is adequate phosphorus intake (32). In general phosphorus stimulates calcium retention in man (3277... 

Based on limited epidemiologic evidence, fluoride supplements, with or without calcium, estrogen and vitamin D, are used by clinicians for the treatment of osteoporosis. However, knowledge of the effects of fluoride on calcium and phosphorus metabolism in normal animals is limited although Spencer et al. (32) reported that ingestion of fluoride by three osteoporotic men did not affect calcium absorption but caused a decrease in urinary excretion. Moreover, there is a need to determine the long-term effects of fluoride treatment on bone strength and on soft tissues ( ). 

Further, fish protein hydrolysates contain hormone-like peptides and growth factors that accelerate calcium absorption (Fouchereau-Peron et ah, 1999). These peptides are capable of binding to the cell surface receptors on osteoclasts and have a role in calcium metabolism by decreasing the number of osteoclasts. Therefore, these peptides could be used in the treatment of osteoporosis and Paget s disease. Further,... 

There is some evidence, mostly from animal studies, to suggest that high dietary levels of phosphorus, especially if dietary levels of calcium are low, may adversely affect bone mass and calcium metabolism (Greger and Krystofiak 1982). However, in humans there is little direct evidence to indicate that large variations in dietary phosphorus or in the Ca P ratio have any significant influence on calcium utilization or balance (Heaney et al. 1982). Some preliminary findings, however, suggest that the form of phosphorus may influence calcium absorption (Zemel et al. 1982). Hexametaphosphate, as compared with orthophos-... 

Finally, PTH helps increase the absorption of calcium from the gastrointestinal tract. This effect appears to be caused by the interaction between PTH and vitamin D metabolism. PTH increases the conversion of vitamin D to 1,25-dihydroxycholecalciferol (calcitriol).36 Calcitriol directly stimulates calcium absorption from the intestine. 

The well-known dose-related side effects include gingival hyperplasia (due to altered collagen metabolism), cerebellar-vestibular effects (nystagmus, vertigo, ataxia), behavioural changes (confusion, drowsiness, hallucinations), increased seizure frequency, gastrointestinal disturbances (nausea, anorexia), osteomalacia (due to reduced calcium absorption and increased vitamin D metabolism) and megaloblastic anaemia (due to reduced folate absorption). 

Physiological actions of PTH include regulation of bone metabolism, renal tubular reabsorption of calcium and phosphate, and intestinal calcium absorption... 

Not all calcium present in the diet is absorbed by the small intestine and mechanisms are present to ensure only amounts appropriate to body needs are absorbed. These processes are complex and involve the interaction of special transport protein, vitamin D and parathormone. Thus, abnormalities of calcium metabolism may result from many different disease processes. Diseases affecting the bowel may prevent normal absorption, diseases of the parathyroid gland may result in inappropriate levels of parathormone for calcium requirement and a nutritionally inadequate diet may cause vitamin D deficiency with consequent disordered calcium absorption. 

D enhances calcium absorption, regu-lates calcium and phosphoms metabolism, promotes minerali-zation of bones, plays role in cell differentiation insufficient bone mineralization, skeletal deformities (rickets and osteomalacia) protection against certain types of cancer, reduces risk of osteoporosis, psoriasis (topical)... 

To see if monitoring these variables acmally yields any useful information on the folding pathway, multiple simulated annealing runs with all parameter monitoring facihties implemented were carried out on a family of vitamin D molecules (Figure 4) [23]. The classical biological responses for l,25(OH)2-vitamin D3 are now well established. The vitamin D hormone is known to mediate intestinal calcium absorption and bone calcium metabolism via its receptor. More recently, non-traditional activities have been discovered including cell differentiation and prohferation. 

Eastell R, Yergey AL, Vierira NE, Cedel SL, Kumar R, Riggs BL. Interrelationship among vitamin D metabolism, true calcium absorption, parathyroid function, and age in women evidence of an age-related intestinal resistance to 1,25-dEiydroxyvitamin D action. J Bone Miner Res 1991 6 125-32. 

The importance of VDR in bone metabolism arises from studies of polymorphisms of the VDR gene. These show that normal allelic variations in the VDR gene may account for inherited variability in bone mass and density. Specific alleles of the VDR gene predict the density of femoral and vertebral bone in prepubertal American girls of Mexican descent. In the development of osteoporosis, a disorder determined by multiple genetic and environmental factors, variants of the VDR gene and its products may play a significant role. One of the important functions of the VDR protein is calcium absorption. 

Hypocalcemia can result from inadequate dietary intake, decreased fractional calcium absorption (as seen with increasing age), or enhanced calcium excretion. To restore calcium homeostasis after hypocalcemia, PTH concentrations rise, and vitamin D metabolism increases to enhance intestinal calcium absorption (see Fig. 88-3), renal calcium reabsorption, and bone resorption. Fracture risk is greatest with low calcium intake and low fractional calcium absorption." ... 

Neufeld and Boskey 1994 Storey 1962). The skeletal effects of strontium are known to be related to its chemical similarity to calcium, and to its suppression of vitamin D metabolism and intestinal calcium absorption (Armbrecht et al. 1998). Effects are more severe in young rats than in adults, because the rate of skeletal incorporation of strontium is higher in young animals (see Section 3 4.2.2). 

Corradino RA. 1972. Strontium inhibition of the vitamin D-induces calcium-binding protein and the intestinal calcium absorptive mechanism. In International Conference on Strontium Metabolism, ed. Second international conference on strontium metabolism, Glasgow and Strontian, 16-19 August, 1972. 

Beonnee F, Salle BL, Putet G, Rigo J and Sen-TEEEE J (1992) Net calcium absorption in premature infants results of 103 metabolic balance studies. Am J Clin Nutr 56 1037-1044. 

This active vitamin D metabolite (1,25 dihydroxycholecalciferol) is an important cofactor for intestinal calcium absorption, which involves calbindins (calcium binding proteins) in the intestine and kidney. Calcitriol is produced in the kidneys by the conversion of 25-hydroxycholecalciferol (calcidiol) and its formation is stimulated by a reduction of plasma calcium and/or phosphate and increased production of parathyroid hormone and prolactin (Figure 6.3). Calcitriol also inhibits the release of calcitonin and, together with PTH, increases the absorption of calcium and phosphate from the gastrointestinal tract and the kidneys. Growth hormone, glucocorticoids, estrogens, testosterone, and the thyroid hormones also influence calcium metabolism. 

T sodium retention T potassium excretion metabolic alkalosis i Gl calcium absorption J. antibody production 4. inflammatory reaction 4. immunocompetent lymphocytes 4. antigen processing t erythropoesis t neutrophils 4. lymphocytes... 

Its structure was quickly confirmed by chemical synthesis which led to the preparation of radioactive 25-OH-D3 ° Using this preparation it was then possible to show that it, like its vitamin D precursor was rapidly metabolized further and appeared in the target tissues of intestine and bone before those tissues responded by increasing intestinal calcium absorption and the mobilization of calcium from bone In 1971 the most potent and hormonal form of vitamin D was isolated in pure form from chick small intestine and identified as 1,25-dihydroxyvitamin D3 (1,25-(0H)2D3) Its structure was then quickly confirmed by chemical synthesis proving its structure to be la25-dihydroxyvitamin D3 (la,25-(OH)2D3) . This conclusion was finally supported by the synthesis of the epimer ip,25-dihydroxy-vitamin D3 (l/i,25-(OH)2D3) and the demonstration that the natural product was identical with la,25-(OH)2D3 ... 

It is well known that strontium brings about a vitamin D-resistant rickets and markedly reduces intestinal calcium absorption It is now clear that the feeding of strontium represses the renal 25-OH-D3-la-hydroxylase and that the administration of 1,25-(0H)2D3 to strontium-fed animals will restore their ability to absorb calcium Thus at least in part the metabolic basis for strontium-induced rickets has been solved. Recent results suggest that strontium suppresses the 1-hydroxylase by suppressing parathyroid secretion ... 

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