Calcium

Calcium is an alkaline earth metal (atomic weight 40.08) with valence 2. Abbreviations

CaSR calcium sensing receptor ECaCl epithelial calcium channels 1 (TRPV5) NCX1 Na( 4- )/Ca(2 +) exchanger (SLC8AT) PTH parathyroid hormone PTHrP PTH-related protein

Nutritional summary

Function: Calcium is the major mineral in bone. It is needed for intracellular and hormone-like signaling, neurotransmission, muscle contraction, for the regulation of cell growth and different tat ion. blood clotting, and many oihcr functions. Requirements Adults should consume at least 1000 mg'day; adolescents as well as postmenopausal and lactating women need slightly more.

Sources: Rich sources are all dairy foods, fortified cereals and juices, and lofu set with calcium sulfate. Kale and chard arc more modest sources. Most plant-based foods contain much smaller amounts. The proportion that is absorbed from all sources strongly depends on vitamin D status (which increases calcium absorption and retention) and on phosphate, sodium, and animal protein intakes (excess decreases calcium absorption and retention).

Deficiency: Inadequate calcium availability slows hone grow th and mineralization in childhood and adolescence, and causes bone mineral loss in adults. Low mineral content of bone (osteoporosis) greatly increases the risk of fractures. People with habitually low calcium intake may also have a greater than average risk of colon cancer and elevated blood pressure.

Excessive intake: Intakes over 2500 mg'day increase the risk of renal stone formation in some people.

Dietary sources

Significant calcium sources include dairy products, soybean curd (tofu), a few green leafy vegetables and calcium-fortified foods and beverages. Milk and yoghurt prov ide about 1 mg ml, hard cheeses about 7mg/g. firm tofu contains 2 mgg. and as mucli as 7mg g. if set with calcium sulfate. Other sources are green leafy vegetables, such as Chinese mustard greens (2.5 mg g). kale (0.7mg/g), and chard (0.6mg,'g). Spinach also contains significant amounts of calcium (l,4mg,g), but the high oxalate content minimizes absorption.

Women in the US get around 600 mg calcium from foods per day, and men between SOO and 900mg, most of this from dairy products. Older people consume even less. Many people use dietary supplements in addition to food sources. Nonetheless, combined intakes of many are well below what is considered to be adequate (Kohlmeier et at., 1997).

Digestion and absorption

Calcium is absorbed with 15-70% efficiency in the small intestine. Absorption proceeds mainly by active transport at low doses, and increasingly via paracellular passive diffusion at high doses (several hundred mg). The efficiency of calcium absorption depends on vitamin D status, age, amount ingested, and the concentration of phosphate and other ingredients in a meat (Lemann and Favus, 1999). 1,25-dihydroxy-vitamin D provides a potent stimulus for absorption. Availability for absorption is best for highly-soluble complexes from which the calcium cation can dissociate easily (e.g. calcium citrate malate. Keaney et at., 1999).

Inhibition of absorption tends to be strongest at low calcium doses, when the tran-scellular pathway predominates, file paracellular pathway, which becomes more significant at higher dose, may accommodate some calcium complexes that are excluded from transccllular transport. Phosphate forms poorly soluble complexes with calcium and decreases its fractional absorption. Milk, meals, colas, and many processed foods arc major phosphate sources. Oxalate tightly binds to calcium and strongly inhibits ns absorption. This is the reason why less than 10% of the calcium in rhubarb and spinach is available for absorption (Weaver el at., 1999). Phytate also decreases calcium absorption slightly. Fractional absorption of calcium from green leafy vegetables is better than from dairy products, nonetheless, as long as oxalate content is low. About two-thirds ofthe modest calcium dose in broccoli is absorbed by healthy young men as compared to about a third ofthe calcium in milk I Weaver et ul., 1999). Bioavailability of calcium from mustard greens, kale, and soybeans is still similar or slightly better than from milk. Absorption from other types of beans is slightly less effective.

The raised intraluminal concentration altera meal and the electronegative potential of the enteroeytc interior dri\es calcium into enterocytcs (Peng et at., 1999). The microvillous membrane of the proximal small intestine contains several channels through which calcium can enter, including epithelial calcium channels 1 (ECaCl, TRPV5)and 2{HCaC2; Barley etui.. 20011. The calcium ions then move across the cell

Calttirtdm-9K ? (irittucM by 1.25-0)

Enlerocyte

Brush border membrane

Figure 11.11 Intestinal calcium absorption

Cd" 4

PMC Aib

Calttirtdm-9K ? (irittucM by 1.25-0)

Enlerocyte

Brush border membrane

Basóla lera I membrane

Capillary lumen

Capillary endothelium

Figure 11.11 Intestinal calcium absorption with several chaperone-like proteins. Calbindin-9K is one of ihcsc translocating proteins. Its synthesis is strongly induced by 1.25-dihydroxy vitamin I) (1.25-1)}, The driving force for calcium absorption comes from calcium-transporting ATPase lb (plasma membrane calcium-pumping ATPase lb. PMCAlb; EC3.6.3.8) that pumps calcium into the pericapillary space. The intestinal form hydrolyzes one ATP molecule to ADP for each transported calcium ion.

Transport and cellular uptake

Blood circulation Normal blood concentration of calcium is between 2.20 and 2.65 mmol.l. About 40% of calcium in blootl is albumin-bound. 10% bound to citrate, bicarbonate, phosphate, and 50% is present in the ionized form. Calcium affinity for albumin and other proteins decreases with decreasing pit (acidosis). Respiratory alkalosis induced by hyperventilation (decreased bicarbonate concentration in blood) can rapidly lower the concentration of free ionized calcium in blood and cause cramping and other neuromuscular symptoms.

Intracellular (cytosolie) concentration of free calcium is only a minute fraction (around 0.0001 mmol'l) of the concentration in the extracellular lluidand blood. Several calcium-transporting ATPases (EC3.6.3.8) pump calcium out of cells to maintain this steep gradient. In addition, endosomcs. lysosomes. and other intracellular compartments sequester calcium with very high specificity and under tight regulation. The N'a(+ )/Ca{2+) exchanger (NCX1, SLC8AI) is the main transporter for calcium extrusion from heart myocytes. A related form (NCX2. SI.C8A2) operates in brain and skeletal muscle.

Blood■ bram barrier: Calcium concentration is lower in brain than in hi mid. Calcium-transporting ATPase at the luminal side of brain capillary endothelial cells maintains this gradient by pumping calcium into blood (Manoonkitiwongsa el uL. 2(H)(1). Materno-fetal transfer: The mismatch of the large fetal head to the narrow birth canal requires some flexibility of the skull. This may explain why mineralization of the fetal skeleton is relatively light. Nonetheless, about 140mg d calcium have to be transferred from mother to fetus during the third trimester, a total of about 30 g during the entire pregnancy (Lafond et al.. 2001). Information on the pathways for calcium transfer across the placenta is still incomplete. It seems clear that calcium dill uses from the maternal side into the syntrophoblast through calcium channels by passive diffusion, which is made possible by the much lower calcium concentration inside the cell layer than in maternal blood. Transport to the fetal side of the syntrophoblast depends on vitamin D-induced caIbindin-9K. Calcium-transporting ATPase then pumps calcium across the basal membrane towards fetal circulation.

Storage

An infant at birth contains about 21 g of calcium (Crowley et al.. 1998). Young mature males may carry more than 1400g calcium in their bones, mature females more than 1200 g (Anderson, 2000). Turnover and net accretion are under the control of parathyroid hormone, calcitonin, 1,25-dihydroXy-vitamin D. sex hormones, interleukin-6. and many other hormonal factors and mediators.

Osteoblasts deposit hydroxyapatite ¡3 Cad P04)>J.(0l I h. the typical bone mineral. Osteoclasis release calcium from bone by breaking down the protein matrix and creating an acid microcnvironment that dissolves the denuded bone surface. Tartrate-resistant alkaline phosphatase (EC3.1.3.1, requires both zinc and magnesium) also plays an important role in osteoclastic bone digestion. Calcium can then move from the extracellular space into circulation by diffusion along the concentration gradient, since the extracellular calcium concentration may be as low as 0.5mmol I (Karon. 1999).

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