Papers by Immanuel Dillinger
Calcium is the most divalent cation in the human body, averaging about 1.5% of total body weight ... more Calcium is the most divalent cation in the human body, averaging about 1.5% of total body weight or between 1000 and 1200g, bones and teeth contains about 99% of the calcium. The other 1% of the body's calcium is distributed both in intracellular and extracellular fluids. Calcium is concentrated in the organelles and blood, and is very important in the structure of the skeleton and maintenance of the extracellular fluid calcium concentration. The most important source of dietary calcium is milk, which may provide over half of the required intake. The calcium content of milk is 35mg/ 100ml of human milk and 120mg / 100ml for cow's milk. Other important sources of calcium include cheese, nuts, vegetables, eggs, cereals and fruits. DIGESTION OF CALCIUM: Calcium is present in foods, and dietary supplements as relatively insoluble salts. Because calcium is absorbed only in its ionized (ca²+) form, it must first be released from the salts. Calcium is solubilized from most calcium salts in about 1 hour at a mildly acidic ph. Solubilization does not however ensure better absorption, because within the more alkaline ph of the small intestine, calcium may complex with minerals or other selected dietary constituents. Formation of these complexes in the small intestine may limit calcium bioavailability. ABSORPTION OF CALCIUM: Two main transport processes are responsible for the absorption of calcium, which occurs along the length of the small intestine, especially the ileum where food remains for the longest time. 1. One of the transport processes, operative primarily in the duodenum and proximal jejunum, is saturable requires energy, involves a calcium-binding protein and is regulated by calcitriol. The calcitriol dependant calcium transport system is stimulated with ingestion of low-calcium diets, especially intakes <400mg, as well as in condition of growth, pregnancy and lactation in which calcium requirements are increased. For calcium to be absorbed by this active process, three sequential steps, all regulated by vitamin D, must occur, 1. entry at the brush border. 2. intracellular movement. 3. active extrusion at the basolateral membrane. Calcitriol is released in response to an increase in parathyroid hormone (PTH) secretion caused by a reduction in plasma levels of ionized calcium. Calcitriol-induced calcium absorption involves changes in membrane lipid composition and topology and the synthesis of calbindin. Calcium transport through the intestinal cells may occur by diffusion through the intestinal cells may occur by diffusion or via calbindin. Calbindin serves as a transport protein to shuttle the calcium through the cytoplasm of the enterocyte to the basal membrane. The extrusion of calcium from the enterocyte into the extracellular fluid membrane requires ATP and a vitamin D regulated ca²+. Mg²+ ATPase, an enzyme that hydrolyses ATP and release energy for pumping Ca²+ out of the cell as mg²+ moves in. Sodium is also exchanged for Ca²+ in the extrusion process in the basolateral membrane. With age, however, the vitamin D regulated absorption of calcium becomes impaired by decreased efficiency in renal calcitriol production in response to PTH. Estrogen deficiency at menopause also decrease vitaminD mediated calcium absorption. 2. The second of the two processes of calcium absorption occurs throughout the small intestine but mostly in the jejunum and ileum, is non saturable and passive, and appear to be paracellular (i.e absorbed between cells, rather than through them). The amount of calcium absorbed via the book saturable, paracellular mechanisms depends on the supply of calcium in the intestinal lumen up to a threshold level. Increased absorption via this mechanism becomes possible when there is an increased intake of the mineral. Beer calcium absorption from both routes is about 25% to 35%. The larger intestine also appear to play a role in calcium absorption. Bacteria in the Colon may release calcium bound to some fermentable fibres such as pectins upto 4% (or 8mg) of calcium is absorbed by the colon per day. This amount may be higher in people who are absorbing less calcium in the small intestine. FACTORS FAVOURING CALCIUM ABSORPTION: 1. VITAMIN D:
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Papers by Immanuel Dillinger