Although calcium is the fifth most abundant element in the earth’s crust, it is never found free in nature since it easily forms compounds by reacting with oxygen and water.
Calcium carbonate (CaCO3) is one of the common compounds of calcium. It is heated to form quicklime (CaO) which is then added to water (H2O). This forms another material known as slaked lime (Ca(OH)2) which is an inexpensive base material used throughout the chemical industry. Chalk, marble and limestone are all forms of calcium carbonate. Calcium carbonate is used to make white paint, cleaning powder, toothpaste and stomach antacids, among other things. Other common compounds of calcium include: calcium sulfate (CaSO4), also known as gypsum, which is used to make dry wall and plaster of Paris, calcium nitrate (Ca(NO3)2), a naturally occurring fertilizer and calcium phosphate (Ca3(PO4)2), the main material found in bones and teeth.
About 99% of the calcium in the body is found in bones and teeth, while the other 1% is found in the blood and soft tissue and plays key roles in cell signaling, blood clotting, muscle contraction and nerve function.
Calcium plays a role in mediating the constriction and relaxation of blood vessels, nerve impulse transmission, muscle contraction, and the secretion of hormones like insulin. Excitable cells, such as skeletal muscle and nerve cells, contain voltage-dependent calcium channels in their cell membranes that allow rapid changes in calcium concentrations. When a nerve impulse stimulates a muscle fiber to contract, calcium channels in the cell membrane open let calcium ions into the muscle cell. In the cell, these calcium ions bind to activator proteins, and a flood of calcium ions from storage vesicles of the endoplasmic reticulum (ER) is released inside the cell. The binding of calcium to the protein troponin-c initiates a series of steps that lead to muscle contraction. The binding of calcium to the protein calmodulin activates enzymes that break down muscle glycogen to provide energy for muscle contraction. Upon completion of the action, calcium is pumped outside the cell or into the ER until the next activation.
Parathyroid hormone (PTH) and vitamin D control the level of circulating calcium (in the blood and extracellular fluid) within a narrow concentration range for normal physiological functioning. The physiological functions of calcium are so vital to survival that when calcium intake is inadequate, the body will stimulate bone resorption (demineralization) to maintain normal blood calcium concentrations. This is why calcium intake is so important for healthy teeth and bones.
About 40% of the calcium in blood is bound to proteins in blood, mainly albumin. Protein-bound calcium has no function but it acts as a reserve. Low calcium level (hypocalcemia) occurs when the level of unbound calcium is low.
Table 1 shows the recommendations for calcium intake based on the optimization of bone health were released by the Food and Nutrition Board (FNB) of the Institute of Medicine in 2011.
|Infants||0-6 months||200 (AI)||200 (AI)|
|Infants||6-12 months||260 (AI)||260 (AI)|
|Adults||71 years and older||1,200||1,200|
Low dietary intake may cause hypocalcemia. Since the skeleton provides a large reserve of calcium, low levels of calcium usually implies abnormal parathyroid function.
Other causes of abnormally low blood calcium concentrations include chronic kidney failure, vitamin D deficiency, and low blood magnesium levels often observed in cases of severe alcoholism. Some drugs can lower the level of calcium or interfere with its absorption. If you have calcium deficiency, and you are taking chronic therapy, talk to your doctor or your pharmacist.
Magnesium deficiency can impair parathyroid hormone (PTH) secretion by the parathyroid glands and lower the responsiveness of osteoclasts (bone cells) to PTH. Thus, magnesium supplementation is required to correct hypocalcemia in people with low serum magnesium concentrations (see the article on Magnesium). In children and growing individuals, chronically low calcium intakes may prevent the attainment of optimal peak bone mass. Once peak bone mass is achieved, inadequate calcium intake may contribute to accelerated bone loss and ultimately to the development ofosteoporosis.
Calcium levels may be low without any symptoms. If levels of calcium are low for long period you may develop:
- scaly skin, brittle nails, and coarse hair
- muscle cramps involving the back and legs are common
Over time, hypocalcemia can affect the brain and cause neurologic or psychologic symptoms, such as
- memory loss,
- and hallucinations.
These symptoms disappear if the calcium level is restored.
An extremely low calcium level may cause
- tingling (often in the lips, tongue, fingers, and feet),
- muscle aches,
- spasms of the muscles in the throat (leading to difficulty breathing),
- stiffening and spasms of muscles (tetany),
- and abnormal heart rhythms.
Sources of calcium
Total dietary intakes of calcium in the US are well below the RDA for every age and gender group, especially in youth and women.
The bioavailability of the calcium must be taken into consideration. The calcium content in calcium-rich plants in the kale family (broccoli, bok choy, cabbage, mustard, and turnip greens) is as bioavailable as that in milk. But some food components have been found to inhibit the absorption of calcium.
- Oxalic acid, also known as oxalate, is the most potent inhibitor of calcium absorption and is found at high concentrations in spinach and rhubarb and somewhat lower concentrations in sweet potatoes and dried beans.
- Phytic acid (phytate) is a less potent inhibitor of calcium absorption than oxalate. Yeast possess an enzyme (phytase) that breaks down phytate in grains during fermentation, lowering the phytate content of breads and other fermented foods. Only concentrated sources of phytate, such as wheat bran or dried beans, substantially reduce calcium absorption
Table 2 Some Food Sources of Calcium
|Tofu prepared with calcium sulfate (raw)||½ cup||434|
|Yogurt, plain, low-fat||8 ounces||415|
|Sardines, canned||8 ounces||325|
|Cheddar cheese||1.5 ounces||303|
|White beans (cooked)||½ cup||81|
|Chinese cabbage (Bok choy/Pak choi, cooked)||½ cup||79|
|Figs (dried)||¼ cup||61|
|Kale (cooked)||½ cup||47|
|Pinto beans (cooked)||½ cup||39|
|Broccoli (cooked)||½ cup||31|
|Red beans (cooked)||½ cup||25|
Supplements of calcium
Vitamin D helps the absorption of calcium.
Calcium supplements may be necessary for those who have difficulty consuming enough calcium from food. No multivitamin/mineral tablet contains 100% of the recommended daily value (DV) for calcium because it is too bulky, and the resulting pill would be too large to swallow.
Calcium preparations used as supplements include
- calcium carbonate; calcium carbonate is generally the most economical calcium supplement. To maximize absorption, take no more than 500 mg of elemental calcium at one time.
- calcium citrate, the preferred calcium formulation for individuals who lack stomach acids (achlorhydria) or those treated with drugs that limit stomach acid production (H2 blockers and proton-pump inhibitors)
- calcium citrate malate,
- calcium lactate, and
- calcium gluconate.
To determine which calcium preparation is in your supplement, you may have to look at the ingredient list. Most calcium supplements should be taken with meals, although calcium citrate and calcium citrate malate can be taken anytime.
Is taking too much calcium harmful?
Malignancy and primary hyperparathyroidism are the most common causes of elevated calcium concentrations in the blood (hypercalcemia). Also high doses of calcium supplements can cause elevated calcium levels.
Increased dietary calcium intake has been associated with a decreased risk of kidney stones. Studies have shown that kidney stones are not usually related to calcium intake, but rather to increased absorption of calcium in the intestine or increased excretion by the kidneys.
There are studies that show that high calcium intake increase the risk of prostate cancer. Until the relationship between calcium and prostate cancer is clarified, it is reasonable for men to consume a total of 1,000 to 1,200 mg/day of calcium (diet and supplements combined), which is recommended by the Food and Nutrition Board of the Institute of Medicine.
Also there are concerns that high calcium intake increases the risk of cardiovascular disease. Because the results of many studies are contradictory, the experts support the use of supplemental calcium for generally healthy individuals who do not consume enough calcium-rich food to meet the current daily recommendations.
Mild hypercalcemia may be without symptoms or may result in loss of appetite, nausea, vomiting, constipation, abdominal pain, fatigue, frequent urination (polyuria), and hypertension. More severe hypercalcemia may result in confusion, delirium, coma.
Table 3 shows tolerable upper intake level, updated in 2011 by the Food and Nutrition Board of the Institute of Medicine.
|Age Group||UL (mg/day)|
|Infants 0-6 months||1,000|
|Infants 6-12 months||1,500|
|Children 1-8 years||2,500|
|Children 9-13 years||3,000|
|Adolescents 14-18 years||3,000|
|Adults 19-50 years||2,500|
|Adults 51 years and older||2,000|