Calcium
PKGhatak,MD
We have about 4 lbs. of calcium in our body, almost entirely in bones and teeth. Blood levels of calcium are 8.5 to 10.5 mg/dl; 50% of blood calcium is in ionized form, and the rest is bound with serum albumin and immunoglobulin. The intracellular calcium concentration is 400 ng/dl. We lose about 250 mg of calcium daily in urine, feces, and sweat. We consume about 500 mg of calcium daily in food.
Calcium not only provides strength and stability to bones but also is the immediate source of ionized calcium in the blood. Calcium is essential for muscle functions, heartbeats, nerve conduction, clotting of blood, secretion of all glands, and vascular wall contraction and relaxation.
Bones and Calcium.
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The calcium in bone is a loosely bound crystalline hydroxyapatite form of calcium phosphate. Both deposition and resorption of calcium in bone are under the direct influence of parathyroid hormone (PTH). Bone is active living tissue and the daily turnover of calcium between blood and bone is around 250 to 500 mg/day. After reaching midlife we begin to lose bone calcium about 1% a year. If for any reason blood level of ionizes calcium drops, even slightly, calcium is mobilized from the bone. The calcium sensors are located on the parathyroid glands; the effect of stimulation of these sensors is the immediate release of the preformed PTH hormone. PTH receptors are present on osteoblast cells but not on osteoclasts. The action of PTH on osteoblasts is to promote calcium deposition and release cytokines which in turn activate osteoclasts thereby increasing the resorption of calcium from bone. Only in cases of prolonged calcium deficiency does the osteoclast activity predominate. Short intermittent administration of PTH increases bone density.
Gut and Calcium.
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The digestion of food by gastric acid liberates calcium from food. The released calcium is absorbed in the small intestine, mostly under the influence of active vitamin D and also about 20% directly without vitamin D. If a meal contains a large amount of calcium, a higher proportion of calcium will not be absorbed; on the other hand, if calcium content is adequate, and meals are taken two or three times a day, a much higher portion of calcium will be absorbed from the gut. The daily absorption of calcium in the gut is about 400 mg.
Calcium present in the gastrointestinal secretions is not available for absorption and about 150 mg a day is lost in the stool. This is an obligatory loss and must be replaced in the diet.
Kidney and Calcium
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The kidneys filter about 10 grams of calcium a day, of which only 100 mg is lost in the 24 hour urine. 65% of the filtered calcium is reabsorbed in the proximal tubules; 20% in the cortical thick part of the ascending loop of Henley (cTAL) by the influence of locally present calcium sensors (CaSR), the remaining 10% in the distal convoluted tubules (DCT) under influence of PTH. In cTAL Thiazide diuretic promotes Na+ secretion and absorbs Ca+, resulting in lowering calcium in the urine. Furosemide prevents calcium reabsorption in DTC.
Blood and Calcium.
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The ionized calcium level in blood is tightly regulated by calcium sensors located on parathyroid glands. Any fall of ionized calcium will be normalized within minutes by the PTH action of osteoclasts of the bone. A sustained low level of calcium increases PTH production. PTH increases calcium reabsorption by the kidney and increased the absorption of calcium in the gut by the activation of vitamin D by PTH. As ionized calcium and vitamin D levels raise the PTH level returns to normal.
Tissues and Calcium
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All cells have calcium in the cell sap, called intracellular calcium. The normal levels are 1.1 to 1.3 mmol/L. Compared with blood level this is about 10, 0000 times lower. There is a tendency for calcium to move from the blood into the cells. The cellular entry and exit of calcium are very closely regulated and take place along calcium channels. Various hormones, proteins, metabolites and nerve impulses modulate calcium movement across the cells.
Food rich in Calcium.
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Milk and milk products are a good source of dietary calcium. Leafy vegetables like turnip greens, kale, Chinese cabbage, spinach, nuts, beans, broccoli and soy products contain varying amounts of calcium. A good plant source of calcium does not automatically mean all the calcium will be absorbed. The phytic acid present in beans, nuts, and whole grains bread binds with calcium and calcium is lost in the stool. The oxalic acid present in rhubarb, spinach, and collard green must be avoided by people who have had kidney stones. In the malabsorption of fat, the undigested fatty acids bind with calcium in the gut and are not absorbed. Our daily intake of calcium in food is between 500 to 1000 mg.
Calcium fortified food.
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Only orange juice and soy products are regularly fortified with calcium, and some brands of cereals also contain added calcium. Products made of whole grain though contain a modest amount of calcium is a good source of calcium because of the amount consumed.
High and Low Intake of Calcium.
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About 5 to 20% of the calcium in the food is absorbed in the small intestine directly without the assistance of vitamin D. In other words, this path of absorption is not regulated. A 20% of 4gm calcium supplement will result in 800 mg of calcium absorption and overwhelm the regulatory system and will produce calcium toxicity and lead to kidney stones and kidney failure among other symptoms if continued for weeks.
A diet deficient in calcium leads to overproduction of PTH, bone demineralization, osteopenia, bone fractures, cardiac arrhythmias, muscle weakness, muscle cramps and other symptoms.
