Calcium

Description

As the most plentiful mineral in the body, calcium plays a key role in the development and maintenance of bones and teeth. Calcium enables the contraction of muscles, including the function of the body's most important muscle, the heart. It is also essential for normal blood clotting, proper nerve impulse transmission, and the appropriate support of connective tissue.

Almost every segment of the population—women, children, teenagers, men, unborn babies, and the elderly—benefit from calcium in their daily diet. The mineral is an important dietary supplement for those who are undergoing significant periods of bone growth, such as in childhood, during pregnancy , and while breast-feeding.

Calcium is an effective weapon for the aging population as they combat osteoporosis . A condition that simply means "porous bones," osteoporosis attacks bones when they are their most vulnerable. As the body ages, bones lose more calcium, and it becomes vital to supplement the diet with calcium in order to encourage bone growth and prevent or slow down the process of osteoporosis.

General use

While the body relies on the presence of calcium for many of its everyday functions, the number of reasons why the mineral should be supplemented in the diet are numerous. Calcium is beneficial to everyone, but re-search has shown that women may benefit more than others. A study in the October 1999 issue of the journal Obstetrics & Gynecology found that pregnant women who do not get enough calcium in their diet can increase the bone mineral content of their fetus by about 15% by taking 1,300 mg of a calcium supplement per day during their second and third trimesters. For those women who already consume enough calcium, the additional supplements do not have this effect. Additional research shows that calcium deficiencies lead to preeclampsia during pregnancy, causing high blood pressure, swelling, and weight gain greater than 1 lb (0.5 kg) per day. The risk of preeclampsia developing lowers by 45–75% for women who receive calcium supplementation.

Premenstrual syndrome (PMS) is another condition women face that may be alleviated by the use of calcium supplements. Researchers at the National Institute of Mental Health (NIMH) concluded that those women who took 1,200 mg of calcium per day reduced their overall PMS symptoms by more than 50%. In the study, calcium supple-mentation led to the reduction of psychological PMS symptoms (such as mood swings) by 45%, food cravings by 54%, and bloating and water retention by 36%.

A 1999 study reported that researchers have found that increasing the amount of daily calcium consumed by women may reduce their risk of stroke . Those women in the Nurses' Health Study who took more than 400 mg of calcium daily were at the lowest risk for a stroke, while those who consumed more than 600 mg each day did not have an increased benefit. Researchers believe that the risk of stroke is reduced by calcium from decreased cholesterol levels, or by stopping the formation of blood clots that cause strokes.

For elderly postmenopausal women, the prevention of osteoporosis becomes critical. In order to maintain bone mass during this time, a study conducted in 1999 concluded that a low-dose hormone replacement therapy (HRT) combined with calcium and vitamin D supple-mentation is an effective therapeutic option for prevention of osteoporosis. Estriol, which is used in HRT, appears to be helpful in controlling menopausal symptoms. Results from research regarding this use of estriol on bone density have been contradictory, according to the Alternative Medicine Review, with the results showing the most effectiveness coming from Japanese studies.

Calcium alone is frequently prescribed with estrogen at the beginning of menopause to treat or prevent osteoporosis. This therapy is recommended to guard against the increased loss of calcium in the bones due to increasing age. As bones lose more calcium they become dense and brittle, and more vulnerable to the attack of osteoporosis. This condition is most common in people over 70, and in women after menopause, where it may increase the risk of broken hips, ribs, and pelvis, and the weakening of other bones. Increased physical exercise is also important for bone strengthening.

On the other hand, although calcium supplementation is useful in lowering the risk of osteoporosis in Western women, more research is needed to determine why the rates of osteoporosis are low in some Eastern societies with low-calcium diets . There is evidence that osteoporosis, like coronary artery disease, is primarily a problem in Western societies. In addition, accumulating evidence that a diet high in fruits and vegetables helps to prevent fractures suggests that the level of calcium in the diet is not the only nutritional factor involved in osteoporosis.

Calcium has been shown to be beneficial to the colon. Among those people taking calcium supplements, research points to a modest reduction in the recurrence of polyps in their colons. Colon polyps are benign tumors that often turn cancerous. Researchers think that calcium binds to carcinogens, preventing abnormal cell growth.

Stemming from its active role in building bone density throughout the body, calcium may prove particularly beneficial for strengthening of the jawbone. Dental researchers at the State University of New York at Buffalo report that calcium supplementation may prevent periodontal disease as it builds a strong jawbone. Periodontal, or gum, disease is an infection caused by bacteria that deposits in pockets between the teeth and gums, and is the leading cause of tooth loss in the United States. As the infection progresses, the jawbone that holds a tooth in place is eventually destroyed, causing the tooth to loosen and fall out. The researchers contend that calcium's overall bone-building role would equal a stronger jawbone that would better fight off gum disease .

While supplements of calcium can be found in many forms, research has shown a promising benefit if it is obtained from dairy foods, rather than supplements or leafy greens—calcium in the form of dairy may actually prevent weight gain. Those in the study who consumed at least 1,000 mg of calcium a day (equaling about 3 cups, or 750 ml of skim milk), gained 6–7 lb (2.7–3.2 kg) less over two years than those with low-calcium diets. Researchers of Purdue University speculate that calcium probably prevents weight gain by increasing the breakdown of body fat and decreasing its formation. It is important to note, however, that dairy products should be consumed in moderation, as other research conducted has indicated that dairy products are not necessarily a good source of absorbable calcium. In addition, other studies indicate that women are often reluctant to increase their intake of dairy products because they dislike milk, suffer from lactose intolerance, or fear that they will gain too much weight.

Calcium is proving essential to those children around the world who are stricken by rickets. Rickets is a deficiency condition in children that affects developing cartilage and newly formed bone throughout the body, causing severe deformities. Often thought to be a result of the inadequate intake of vitamin D from dietary sources or lack of exposure to sunlight, research reported in 2000 has found that children with rickets respond well to calcium supplementation. While rickets is still rare in most developed countries, it is becoming more common in the United States due to lower milk consumption by children; and it remains a problem in many other parts of the world. Researchers conclude that effective treatment for the condition is calcium supplementation alone, or in combination with vitamin D. Osteomalacia, or the adult form of rickets, also responds to calcium supplementation.

Evidence is accumulating in the United States that women are not the only group at risk for insufficient dietary levels of calcium. Children and adolescents are also at risk, according to a 2001 report from the National Institutes of Health. Researchers found that "only 13.5% of girls and 36.3% of boys ages 12 to 19 in the United States get the recommended daily amount (RDA) of calcium, placing them at serious risk for osteoporosis and other bone diseases" in their adult years. The report listed increased consumption of soft drinks and decreased consumption of milk as contributing to the problem.

Preparations

Calcium may be supplemented in the diet in a variety of ways. Numerous foods are rich in calcium, including dairy products (such as milk, yogurt, and cheese) and leafy green vegetables like turnip greens, broccoli, kale, and collards. Canned salmon, sardines, shrimp, and tofu are also high in calcium. More foods are being fortified with calcium, making it easier to ensure the proper amount of the mineral is consumed. Calcium-fortified foods range from cranberry juice cocktail, cereal and waffles, to orange juice and flour. With almost every segment of the population consuming too little calcium, researchers recommend calcium-fortified foods to increase daily calcium intake.

While the types of food calcium may be obtained from continues to increase, most people still lack enough of the essential mineral. For those who are not getting enough calcium from foods, supplements are an acceptable alternative. The chemical form of calcium supplements come in five varieties: carbonate, citrate, lactate, phosphate, chelate, and citrate malate. The supplements are available as tablets, syrup, or suspension form. Calcium supplements should be stored at room temperature and away from moisture and sunlight. It should not be stored in the bathroom, and the liquid forms should not be frozen.

Experts state that calcium is best absorbed from the citrate malate form, or the type of calcium found in some juices, but they recommend calcium carbonate for the overall amount of calcium it offers and its affordability. Calcium carbonate can be found in antacids, and it is absorbed better when taken with meals. Food slows down the time it takes substances to travel through the gut, giving the calcium more time to be absorbed. Absorption is key for the proper functioning of calcium. Sufficient levels of vitamin D and hydrochloric acid in the stomach, and the presence of other minerals, such as magnesium and phosphorous are essential for quick absorption.

The body may also be better able to absorb calcium when taken along with ingredients extracted from chicory root. Research indicates that Raftilin inulin and Raftilose oligofructose, both extracts from chicory root, are dietary fibers that are not digested in the stomach or the small intestine. Instead, they are fermented by Bifidobacteria in the colon—beneficially leading to increased calcium absorption throughout the body, with emphasis on bone tissue. Additionally, Oligofructose improves the texture and mouthfeel while improving taste and fruit flavors in low-fat yogurts. Inulin is used for fat replacement and fiber enrichment of reduced-fat and fat-free sour cream and whipped topping.

There are many ways to ensure calcium is part of a daily diet, but it is important that the recommended daily allowance (RDA), or appropriate dosage of the mineral be followed. The RDA of calcium for adults is 800 mg; pregnant women and young adults should be certain their intake equals 1,200 mg per day. Adults over 50 should increase their intake to 1,000 mg per day with supplements that include vitamin D.

Calcium supplements may be taken with a large glass of water during or after a meal. Tablets in chewable form must be chewed thoroughly before swallowing, and effervescent tablets should be diluted in cold water or juice before taking. It is recommended that other medications be taken two hours after any calcium supplement. The simultaneous intake of calcium may interfere with the absorption of other drugs. Do not take more than 500 mg of calcium at one time for the best absorption of the mineral.

Precautions

When adding calcium supplements to the diet, it is recommended that it not be taken within one to two hours of eating bran, or whole grain cereals or breads. Large amounts of alcohol or caffeine containing beverages or tobacco should be avoided. Large amounts of calcium, phosphates, magnesium, or vitamin D in medication or dietary supplements should not be taken unless directed by a physician. Those with diarrhea , stomach trouble, parathyroid disease, sarcoidosis, or kidney stones should consult with their physician before taking calcium.

Side effects

Calcium is typically well tolerated by those who add it to their diets, but if the mineral is taken in high levels it can cause several side effects, including: nausea, vomiting , loss of appetite, constipation , stomach pain , thirst, dry mouth , increased urination, and weakness. While these side effects are rare, it is even more unlikely to experience the life-threatening symptoms of an irregular or very slow heart beat. If these dangerous symptoms appear while taking calcium, use of the mineral should be discontinued and emergency treatment should be sought. An overdose of a calcium supplement may lead to confusion, irregular heartbeat, depression , bone pain, or coma.

Interactions

It is important that all over-the-counter (OTC) or prescription medications are reviewed with a physician before beginning calcium supplement.

According to the Complete Guide to Prescription & Nonprescription Drugs, the following are some of the drugs that may cause possible interactions if taken with calcium:

• alendronate
• anticoagulants
• calcitonin
• calcium-containing medicines
• chlorpromazine
• oral contraceptives
• corticosteroids
• digitalis preparations
• diuretics, thiazide
• estrogens
• etidronate
• iron supplements
• meperidine
• mexiletine
• nalidixic acid
• nicardipine
• nimodipine
• oxyphenbutazone
• para-aminosalicyclic acid (PAS)
• penicillins
• pentobarbital
• phenylbutazone
• phenytoin
• pseudoephedrine
• quinidine
• salicylates

Resources

BOOKS

The Editors of Time-Life Books. "Essential Vitamins and Minerals." In The Medical Advisor: The Complete Guide to Alternative & Conventional Treatments. Richmond, VA: Time-Life Inc., 1996.

Griffith, H. Winter. "Calcium Supplements." In Complete Guide to Prescription & Nonprescription Drugs, 1999 Edition. New York: The Berkley Publishing Group, 1998.

PERIODICALS

"Calcium May Help Prevent Colon Polyps." Environmental Nutrition 22, no. 2 (February 1999): 1.

"Calcium May Help Prevent Gum Trouble." Tufts University Health & Nutrition Letter 17, no. 5 (July 1999): 6.

"Calcium May Reduce Stroke Risk in Women." Stroke (September 1999).

"The Four Supplements You Can't Live Without." Prevention 51, no. 12 (December 1999): 1.

Gulliver, Pauline, and Caroline C. Horwath. "Assessing Women's Perceived Benefits, Barriers, and Stage of Change for Meeting Milk Product Consumption Recommendations. " Journal of the American Dietetic Association 101 (November 2001): 1354–1357.

Head, Kathleen A., n.d. "Estriol: Safety and Efficacy." Alternative Medicine Review 3, no. 2 (April 1998). <http://www.thorne.com>.

Hegsted, D. Mark. "Fractures, Calcium, and the Modern Diet." American Journal of Clinical Nutrition 74 (November 2001): 571.

Liebman, Bonnie. "Calcium Supplements: The Way to Go." Nutrition Action Healthletter 25, no. 3 (April 1998): 5.

Marion, Matt. "Health Bulletin." Men's Health 14, no. 10 (December 1999): 32.

"Using Calcium to Combat PMS Symptoms." Medical Update 22, no. 5 (November 1998): 6.

Wallace, Phil. "NIH Says Calcium 'Crisis' is Affecting Young People." Food Chemical News 43 (December 17, 2001): 27.

ORGANIZATIONS

Food and Drug Administration, Office of Consumer Affairs, HFE–88, Rockville, MD 20857.

Beth Kapes

Rebecca J. Frey, PhD

CALCIUM

CALCIUM. Calcium (Ca²) is a silver-white metallic element of the alkaline-earth group. Ninety-nine percent of calcium in the human body is in bone and teeth. The remaining one percent is in blood and body fluids. In addition to its role in maintaining strength of bone and teeth, calcium is involved in nerve cell function, control of muscle tone, and blood clot formation. Calcium is also necessary in order for many important proteins to properly perform critical metabolic functions throughout the body.

Functions

Cells. Calcium concentrations in the fluids outside cells are much larger than calcium concentrations inside cells (the cytosol). Unequal calcium concentrations in the extracellular fluid and cytosol are required for cells to carry out many crucial functions. For example, when a hormone in the blood binds to a receptor on the cell, calcium pours into the cytosol from extracellular fluid. This change in the amount of calcium in the cytosol signals the cell to perform some critical function. The critical function that is triggered depends on the type of cell. (In muscle cells, for example, a nerve signal triggers the release of calcium into the cytosol, allowing muscle contraction to occur.) After the critical function is performed, calcium is rapidly pumped out of the cell, and the calcium concentration in the cytosol returns to the normal (low) level.

Structural. In addition to cellular functions, calcium's more familiar role is a structural one—as a component of bones and teeth. Blood calcium levels are maintained strictly even if calcium has to be taken from bone. Bone mineral (hydroxyapatite) is made up primarily of calcium, phosphate, and carbonate. Bone constantly changes during growth and throughout adulthood. Changes in bone occur through balancing activities of bone-destroying cells (osteoclasts) and bone-forming cells (osteoblasts), which act together to remove and replace bone, respectively. During growth, bone formation generally exceeds destruction, yielding net bone-mass gain in the whole skeleton.

Bone-mass accumulation continues until peak bone mass is achieved, generally during the third decade of life. The age at which peak bone mass is reached varies by gender and differs by skeletal site. Males achieve peak bone mass later than females and gain more bone during puberty than females, resulting in larger bones. Although peak bone mass at all skeletal sites is generally reached by age thirty, bone accumulation is nearly complete by age twenty in the lumbar spine and in portions of the hip for both males and females. Genetic, environmental (for example, physical activity or mechanical "loading" of the skeleton), hormonal, and nutritional factors interact to influence peak bone-mass levels. Failure of an individual to reach the maximum peak bone mass permitted by his or her genetic makeup can be related to low calcium intake or a sedentary lifestyle without adequate physical activity. Parathyroid dysfunction, genetic or nutritional skeletal disorders, or medication use may affect peak bone-mass accumulation and overall bone health adversely. Smoking and excessive alcohol consumption also are likely to be detrimental to skeletal health.

After an individual reaches peak bone mass, net bone gain in the whole skeleton generally does not occur. Agerelated bone loss occurs in both genders, but the rate of bone loss increases with estrogen loss at menopause in females. Age-related bone loss is caused by increased osteoclast (bone-destroying) activity compared to osteoblast (bone-building) activity. Physical activity during adulthood, combined with adequate overall nutrition and calcium intake, can help to maintain bone strength.

Metabolism

Absorption. Calcium absorption across the intestinal wall into the blood occurs by different mechanisms. Two major mechanisms include passive diffusion and active transport. Vitamin D is required for the active transport mechanism but not for the passive diffusion mechanism. The percent of calcium that is absorbed into blood generally decreases with higher calcium intakes; however, the total amount of calcium absorbed is usually greater with higher calcium intakes. The percent of calcium absorbed into blood is highest in infants, spikes again at the start of puberty, then gradually declines with age. The percent of calcium absorbed into blood also increases during the last two trimesters of pregnancy.

Homeostasis. The body keeps tight control (homeostasis) of blood calcium concentration by continuously changing various factors. When blood calcium concentration falls below normal, the parathyroid gland releases parathyroid hormone (PTH). PTH stimulates increased removal of phosphate into urine by the kidneys. This increased phosphate removal triggers the kidneys to keep calcium in the blood rather than excrete it in the urine. PTH also stimulates osteoclasts to remove calcium from bone in order to help restore normal blood calcium concentration. Finally, PTH is involved in making certain that enough vitamin D is present in the intestine to allow for increased calcium absorption from the gut into the blood. PTH decreases to normal once calcium homeostasis is reached. Another hormone, calcitonin, is responsible for stopping bone breakdown by osteoclasts when blood calcium concentration is above normal. Thus, the hormones PTH and calcitonin work together to keep blood calcium concentration within a very narrow range.

Dietary Requirements

Bioavailability. Both dairy products and most dietary supplements provide adequate amounts of calcium. Calcium is present in smaller amounts in grains, fruits, and vegetables. Because grains are eaten in high amounts, however, they are an important source of calcium. Other calcium-rich foods include bok choy (Chinese cabbage), kale, cabbage, and broccoli. Calcium from some foods containing high levels of oxalic acid (spinach, sweet potatoes, rhubarb, beans) or phytic acid (unleavened bread, nuts and grains, seeds, raw beans) is absorbed poorly due to formation of insoluble calcium salts. The ability to enhance dietary calcium intake by consuming calcium-fortified food sources is increasingly common.

Although high protein intake temporarily increases urinary calcium excretion, there is no evidence to indicate that calcium intake recommendations should be adjusted according to protein intake. Although caffeine has a slightly negative impact on calcium retention, the modest calcium loss can be offset by a similarly modest increase in calcium intake. High salt (sodium chloride) intake usually results in increased urinary calcium loss because excretion of sodium and calcium at the kidney are linked. High salt intake triggers increased urinary sodium loss and, therefore, increased urinary calcium excretion. However, as with protein and caffeine, there is no evidence to indicate that calcium intake recommendations should be adjusted according to salt intake.

Dietary requirements and bone mass. Because circulating calcium levels are so strictly controlled, blood calcium concentration is a poor indicator of calcium status. Chronic inadequate calcium intakes or poor intestinal absorption leads to reduced bone mass as PTH acts to maintain homeostatic blood calcium at the expense of skeletal strength. Bone mineral content (BMC) and bone mineral density (BMD) are common measures of bone strength and fracture risk. BMC is measured in grams, the amount of bone mineral at the selected site (for example, whole skeleton, lumbar spine, hip, forearm) and BMD (g/cm²) are calculated as BMC divided by bone area in the region of interest. An adult is defined as osteoporotic by the World Health Organization if his or her BMD is more than 2.5 standard deviations below gender-specific normal young adult BMD. Osteoporosis and related spine, hip, and wrist fractures are major public health concerns.

Recommended daily calcium intakes (measured in milligrams) increase from infancy through adolescence. The rate of calcium accretion relative to body size is greatest during infancy. Infants accrete approximately 140 mg of calcium per day during the first year of life. This need for calcium during the first year of life is reflected in the amount of milk consumed by human milk-fed infants. Although evidence indicates that feeding of formula results in greater bone mineral accretion than human milk feeding during the first year of life, there is no indication that this effect is beneficial either short-or long-term.

Calcium accretion continues in childhood, and maximal accretion occurs during puberty. Children of ages one to eight years accrete 60 to 200 mg of calcium per day. Peak calcium accretion occurs during puberty for both males (mean age 14.5 years) and females (mean age 12 years). Accordingly, calcium intake requirements are highest during adolescence.

Calcium retention and bone turnover decline after menarche in females, but the amount of calcium women need does not change because the percentage of calcium absorbed into the blood decreases. In males, bone mineral accretion occurs until mean age 17.5 years. Evidence from clinical trials indicates that calcium supplementation in children can increase BMD, but the effect occurs primarily among populations who usually have low calcium intake, is not apparent at all skeletal sites, and probably does not persist when supplementation is stopped. Apparently the benefit is short-term only.

Dietary calcium requirements decline for both males and females once adulthood is reached and remain constant throughout the reproductive years. Intestinal calcium absorption, however, also decreases with age. At the end of the reproductive years (approximately age fifty), bone-mass loss occurs in both males and females. Bone-mass loss is particularly pronounced in females during the first few years following menopause. The bone loss that occurs with the loss of estrogen at menopause cannot be reversed simply through increased calcium intake. Reductions in age-related bone loss through calcium supplementation have been demonstrated in postmenopausal women, but the effects vary by skeletal site, usual calcium intake, and postmenopausal age. Because of the reduction in intestinal calcium absorption with age in all individuals and the potential of increased calcium intake to offset bone loss due to estrogen depletion, increasing the amount of calcium in one's diet is recommended for all individuals over fifty years of age.

Maternal calcium requirements increase during the third trimester of pregnancy in accordance with fetal growth needs and to prepare for lactation, and the mother's intestinal calcium absorption efficiency increases in order to meet her increased need for calcium. If this need for more calcium is not met, the mother's skeleton will be depleted to meet the calcium demands of the fetus. Furthermore, calcium loss from the mother's skeleton occurs during lactation and cannot be prevented by calcium supplementation. However, evidence indicates that maternal bone density is recovered to prelactation levels within approximately six months after the recurrence of menses.

Toxicity. Calcium toxicity is uncommon but can occur if too much calcium is taken in through dietary supplements. In susceptible individuals, excess calcium intake can lead to the formation of kidney stones (renal calcium deposits); however, dietary calcium is not a common cause of kidney stones. Hypercalcemia from ingestion of large quantities of calcium supplements is rare but the resulting kidney problems and ramifications to cell function affect major tissues and organs. In the United States, the maximum daily calcium intake judged likely to pose no adverse health effects—Tolerable Upper Intake Level (UL)—is set at 2,500 mg per day for all ages beyond one year of age. There are insufficient data to determine a UL for calcium for infants less than one year of age.

Summary. Changes in dietary calcium requirements throughout the lifespan reflect concurrent alterations in growth rate, intestinal absorption efficiency, and reproductive and estrogen status. Because calcium plays vital roles in critical cell responses, plasma calcium levels are strictly homeostatically controlled at the expense of skeletal integrity, if necessary. Homeostatic control of circulating calcium involves PTH, vitamin D, and calcitonin. Appropriate lifestyle choices (for example, physical activity) and adequate calcium nutrition promote optimal bone-mass accretion during growth and young adulthood, possibly resulting in reduced current and future fracture risk. Dairy products and dietary supplements provide similarly adequate amounts of calcium to the body. Grains, fruits, and vegetables contain smaller amounts of calcium, and calcium absorption from foods high in oxalic acid or phytic acid is limited. Calcium-enriched products such as bread and fruit juice are becoming increasingly important sources of dietary calcium.

See also Dairy Products; Lactation; Milk, Human; Nutrition; Phosphorus and Calcium; Trace Elements.

BIBLIOGRAPHY

Abrams, S. A., K. O. O'Brien, and J. E. Stuff. "Changes in Calcium Kinetics Associated with Menarche." Journal of Clinical Endocrinology and Metabolism 81 (1996): 2017–2020.

Aloia, J. F., A. Vswani, J. K. Yeah, P. L. Ross, E. Flaster, and F. A. Dilmanian. "Calcium supplementation with and without Hormone Replacement Therapy to Prevent Post-menopausal Bone Loss." Annals of Internal Medicine 120 (1994): 97–103.

Barger-Lux, M. J., R. P. Heaney, and M. R. Stegman. "Effects of Moderate Caffeine Intake on the Calcium Economy of Premenopausal Women." American Journal of Clinical Nutrition 52 (1990): 722–725.

Bonjour, J. P., G. Theintz, F. Law, D. Slosman, and R. Rizzoli. "Peak Bone Mass." Osteoporosis International 1 (1994): S7–S13.

Dawson-Hughes, B., G. E. Dallal, E. A. Krall, L. Sadowski, N. Sahyoun, and S. Tannenbaum. "A Controlled Trial of the Effect of Calcium Supplementation on Bone Density in Postmenopausal Women." New England Journal of Medicine 323 (1990): 878–883.

Heaney, R. P. "Protein Intake and Bone Health: The Influence of Belief Systems on the Conduct of Nutritional Science." American Journal of Clinical Nutrition 73 (2001): 5–6.

Heaney, R. P., R. R. Recker, M. R. Stegman, and A. J. Moy. "Calcium Absorption in Women: Relationships to Calcium Intake, Estrogen Status, and Age." Journal of Bone and Mineral Research 4 (1989): 469–475.

Heaney, R. P., R. R. Recker, and C. M. Weaver. "Absorbability of Calcium Sources: The Limited Role of Solubility." Calcified Tissue International 46 (1990): 300–304.

Heaney, R. P., P. D. Saville, and R. R. Recker. "Calcium Absorption as a Function of Calcium Intake." Journal of Laboratory and Clinical Medicine 85 (1975): 881–890.

Heaney, R. P., and T. G. Skillman. "Calcium Metabolism in Normal Human Pregnancy." Journal of Clinical Endocrinology and Metabolism 33 (1971): 661–670.

Kalkwarf, H. J., B. L. Specker, D. C. Bianchi, J. Ranz, and M. Ho. "The Effect of Calcium Supplementation on Bone Density during Lactation and after Weaning." New England Journal of Medicine 337 (1997): 523–528.

Kurtz, T. W., H. A. Al-Bander, and R. C. Morris. "'Salt Sensitive' Essential Hypertension in Men." New England Journal of Medicine 317 (1987): 1043–1048.

Lu, P. W., J. N. Briody, G. D. Ogle, K. Morley, I. R. Humphries, J. Allen, R. Howman-Giles, D. Sillence, and C. T. Cowell. "Bone Mineral Density of Total Body, Spine, and Femoral Neck in Children and Young Adults: A Cross-Sectional and Longitudinal Study." Journal of Bone and Mineral Research 9 (1994): 1451–1458.

Martin, A. D., D. A. Bailey, and H. A. McKay. "Bone Mineral and Calcium Accretion during Puberty." American Journal of Clinical Nutrition 66 (1997): 611–615.

Prince, R. L., M. Smith, I. M. Dick, R. I. Price, P. G. Webb, N. K. Henderson, and M. M. Harris. "Prevention of Post-menopausal Osteoporosis: A Comparative Study of Exercise, Calcium Supplementation, and Hormone-Replacement Therapy." New England Journal of Medicine 325 (1991): 1189–1195.

Recker, R. R., K. M. Davies, S. M. Hinders, R. P. Heaney, M. R. Stegman, and D. B. Kimmel. "Bone Gain in Young Adult Women." Journal of the American Medical Association 268 (1992): 2403–2408.

Riis, B., K. Thomsen, and C. Christiansen. "Does Calcium Supplementation Prevent Postmenopausal Bone Loss?" New England Journal of Medicine 316: 173–177.

Specker, B. L., A. Beck, H. Kalkwarf, and M. Ho. "Randomized Trial of Varying Mineral Intake on Total Body Bone Mineral Accretion during the First Year of Life." Pediatrics 99 (1997): e12.

Wallace, B. A., and R. G. Cumming. "Systematic Review of Randomized Trials of the Effect of Exercise on Bone Mass in Pre- and Postmenopausal Women." Calcified Tissue International 67 (2000): 10–18.

World Health Organization. Assessment of Fracture Risk and Its Application to Screening for Postmenopausal Osteoporosis. Geneva, Switzerland: World Health Organization, 1994.

World Health Organization, Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorous, Magnesium, Vitamin D, and Fluoride. Washington, D.C.: National Academy Press, 1997.

Wosje, K. S., and B. L. Specker. "Role of Calcium in Bone Health during Childhood." Nutrition Reviews 58 (2000): 253–268.

Karen S. Wosje

Calcium

Definition

Purpose

Description

Precautions

Interactions

Complications

Parental concerns

Resources

Definition

Calcium (Ca) is the most abundant mineral in the body. About 99% of calcium in the body is in bones and teeth. The remaining 1% is in blood and soft tissue. Calcium in body fluids is an electrolyte with a charge of + 2. Humans must meet their need for calcium through diet.

Purpose

Calcium is essential for:

• building and maintaining strong bones and teeth.
• muscle contraction.
• blood vessel contraction and relaxation.
• nerve impulse transmission.
• regulating fluid balance in the body.

Calcium .

(Illustration by GGS Information Services/Thomson Gale.)

Description

Most calcium in the body is stored in bones and teeth. Here it combines with phosphate to form strong, stable crystals. The remaining 1% is dissolved in body fluids and much of it forms Ca 2+ ions. In the body, these electrically charged particles are called electrolytes . Calcium and other electrolytes are not distributed evenly throughout the body. Dissolved calcium is found mainly in the fluid outside cells (extracellular fluid). Metabolic events cause the movement of calcium across cell membranes result in muscle contraction, nerve impulse transmission, and various chemical reactions. The cell then uses energy to restore the balance of calcium between the inside and outside of the cell membrane, so that the event can be repeated.

To remain healthy, the amount of calcium dissolved in body fluids must be stay within a very narrow range. Bone acts like a calcium bank. Bone is constantly being broken down by cells called osteoclasts and built up again by cells called osteoblasts. This process is called bone remodeling, and it continues throughout an individual’s life. When excess calcium is present in the blood, osteoblasts deposit calcium into bones. When too little calcium is in the blood, osteoblasts dissolve calcium from bones and move it into the blood. This process is controlled by parathyroid hormone (PTH) secreted by the parathyroid glands. The parathyroid glands are extremely sensitive to the level of calcium in the blood, and in a healthy individual they are able to maintain the concentration of calcium ions fluctuates very little.

Normal calcium requirements

The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for many vitamins and minerals . The DRIs consist of three sets of numbers. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97-98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.

The IOM has not set RDA s for calcium, but instead it has set AI levels for all age groups based on observed and experimental information. However, many studies show that Americans across almost all age groups are not meeting these AI levels. One large study, the Continuing Survey of Food Intakes of Individuals, found that in children ages 6-11 44% of boys and 58% of girls were getting lower than recommended amounts of calcium. In adolescents ages 12-19, the rate of deficiency was higher—64% for boys and a huge 87% for girls. Adults over age 20 did not do much better with 55% of men and 78% of women taking in less calcium than recommended. IAs and ULs for calcium are measured in milligrams (mg). The following list gives the recommended AL and UL levels of calcium for each age group.

• children birth-6 months: AI 210 mg; UL not established;.
• children 7-12 months: AI 270 mg; UL not established.
• children 1-3 years: AI 500 mg; UL 2,500 mg
• children 4-8 years: AI 800 mg; UL 2,500 mg
• children 9-13 years: RDA 9 mg; UL 2,500 mg
• adolescents 14-18 years: IA 1,300 mg; UL 2,500 mg

KEY TERMS

Dietary supplement— A product, such as a vitamin, mineral, herb, amino acid, or enzyme, that is intended to be consumed in addition to an individual’s diet with the expectation that it will improve health.

Diuretic— A substance that removes water from the body by increasing urine production.

Electrolyte— Electrically charged particles (ions) that form when salts dissolve in water or fluids. Electrolytes regulate water balance in the body and play a critical role in almost every metabolic reaction.

Enzyme— Proteins that change the rate of a chemical reaction within the body without themselves being used up in the reaction.

Mineral— An inorganic substance found in the earth that is necessary in small quantities for the body to maintain a health. Examples: zinc, copper, iron.

• adults age 19-50: RDA 1,000 mg; UL 2,500 mg
• adults over age 50: RDA 1,200 mg; UL 2,500 mg
• pregnant women 18 years and younger: RDA 1,300 mg; UL 2,500 mg
• pregnant women over age 18: RDA 1,000 mg; UL 2,500 mg
• breastfeeding women 18 years and younger: RDA 1,300 mg; UL 2,500 mg
• breastfeeding women over age 18: RDA 1,000 mg;2,500 mg

Sources of calcium

In the United States, dairy products—milk, yogurt, and cheese—are the main sources of dietary calcium. Low-fat dairy products, such as skim milk or reduced-fat cheese, contain about the same amount of calcium as whole milk products. Other sources of calcium include canned fish with bones, dark green leafy vegetables, and tofu made with calcium sulfate. Other types of tofu do not contain significant amounts of calcium. Processed foods such as orange juice, breakfast cereal, instant breakfast drinks, and bread are often fortified with calcium. This will be indicated on the label.

The following list gives the approximate manganese content for some common foods:

• milk, any type, 1 cup (8 ounces): 300 mg
• yogurt, plain, 8 ounces: 415 mg
• yogurt with fruit, 8 ounces: 245-384 mg
• cheddar cheese, 1.5 ounces: 305 mg
• mozzarella cheese, 1.5 ounces: 372 mg
• sardines with bones, canned in oil, 3 ounces: 324
• salmon with bones, canned, 3 ounces: 181 mg
• tofu, firm, made with calcium sulfate, 1/2 cup: 204 mg
• pinto or red beans, cooked, 1/2 cup: 43 mg
• white beans, cooked, 1/2 cup: 113 mg
• bok choy, 1/2 cup cooked: 61 mg
• spinach, cooked, 1/2 cup: 120 mg
• bread, whole wheat, 1 slice 20 mg
• orange juice, fortified, 6 ounces: 200–260 mg
• instant breakfast drink, powder prepared with water, 105–250 mg
• breakfast cereal, fortified, 1 cup: 100–1,000 mg

Although experts recommend that people meet as many of their vitamins and minerals needs through diet as possible, it is difficult for many people to get enough calcium from food alone. This is especially true for vegans, who eat no dairy products, adolescent girls who are very calorie conscious and tend to avoid milk and replace it with diet sodas, and people with lactose intolerance who cannot easily digest dairy products. Pregnant women and older individuals may also have a hard time eating enough to meet their calcium needs. People who do not get enough calcium through diet can benefit from taking a dietary supplement containing calcium.

Calcium supplements are available over–the–counter. The most common supplements supply calcium in the form of calcium carbonate or calcium citrate. Calcium carbonate is usually the most economical calcium supplement. People who are taking medications to reduce stomach acid may more easily absorb calcium citrate. Some supplements combine calcium and vitamin D because vitamin D helps the body absorb calcium. No calcium supplement contains enough calcium meet the entire daily adequate intake, because the pill would be too large to swallow. In addition, the body absorbs calcium best in doses of 500 mg or less. People who need more than 500 mg of supplemental calcium should divide the dose in half to be taken morning and evening.

Calcium deficiency

Calcium deficiency, called hypocalcemia, can occur because of inadequate calcium intake, excess calcium excretion by the kidney (usually caused by kidney damage), the inability to adequately absorb calcium, or because of interactions between calcium and some prescription drugs. People at highest risk of calcium deficiency are teenagers, women past the age of menopause, individuals who are lactose intolerant, vegans, and people with kidney (renal) damage.

Calcium deficiency rarely shows up in blood tests because calcium is withdrawn from the bones to maintain blood levels of calcium. The bones then become less dense, weaker, and more likely to break. This condition is called osteoporosis and it is most noticeable in the elderly who have a high rate of broken bones resulting from falls. Osteoporosis is a part of aging, but eating a healthy diet high in calcium, getting adequate vitamin D, and doing weight–bearing exercises regularly can delay its onset. Severe calcium deficiency, is usually caused by a medical condition rather than inadequate calcium intake. It causes symptoms such as muscle cramps, tingling in the fingers, lethargy, convulsions, heart rhythm abnormalities, and death. These symptoms can also be caused by many other diseases.

Calcium excess

Calcium excess is called hypercalcemia. It usually results from poor kidney function (renal failure) or from a malignant cancer tumor It can also be caused by very large supplemental doses of vitamin D. Very rarely is hypercalcemia caused by too much calcium from food or dietary supplements . High levels of calcium interfere with the absorption of other minerals such as iron, zinc, magnesium, and phosphorous. People with hypercalcemia usually have multiple medical problems and are under the supervision of a physician.

Precautions

People of all ages, races, and gender need to be alert to getting enough calcium in their diet. Building strong, dense bones begins in childhood and adolescence, even though the results cannot be seen until old age. People mentioned above as being at especially high risk of low dietary calcium intake should investigate taking a calcium supplement.

Interactions

Absorption of calcium is affected by several conditions.

• Age. Infants absorb as much as 60% of the calcium in their digestive system. This decreases to 15–20% in adulthood, and even less in old age.
• Pregnancy. Pregnancy increases the efficiency of calcium absorption in the intestine to meet the demands of fetal development.
• Amount of calcium consumed. The more calcium consumed at one time, the less efficient absorption becomes. Calcium from supplements should be spaced out during the day for maximum absorption.
• Vitamin D The presence of vitamin D improves calcium absorption. Vitamin D deficiency can worsen calcium deficiency.
• Plant products. Phytic found in beans and oxalic acid found in spinach and leafy greens decrease the amount of calcium absorbed from those foods, but does not affect the absorption of calcium from other foods present at the same time in the intestine. Fiber such as wheat bran also reduced calcium absorption.

Prescription medications can also affect or be affected by the absorption of calcium. These include:

• digoxin
• fluroquinolones
• levothyroxine
• tetracycline antibiotics
• anticonvulsants
• thiazide–type diuretics
• glucacorticoids
• mineral oil
• stimulant laxatives
• antacids

People taking these drugs should check with their healthcare provider or pharmacist about potential adjustments in their medications or calcium intake.

Complications

No complications are expected when healthy people take calcium in amounts equal to the AI level and less than the UL level. Some people experience gas, nausea, and abdominal discomfort from calcium supplements. Taking the supplement with meals, taking smaller doses spread out over the day, or changing the type of supplement usually solves this problem. Complications of excess calcium and calcium deficiency are discussed above.

Parental concerns

Building strong bones starts in childhood, and parents should be aware of how much calcium their child needs compared to how much he or she is getting. As children get older they tend to replace milk in their diet with juice, bottled water, and especially carbonated soft drinks. This leads to large calcium deficiencies during adolescence. Parents should monitor their child’s diet and encourage calcium supplements if they cannot induce their adolescents to eat more dairy products and other calcium–rich food.

Resources

BOOKS

Fragakis, Allison. The Health Professional’s Guide to Popular Dietary Supplements Chicago: American Dietetic Association, 2003.

Gaby, Alan R., ed. A–Z Guide to Drug–Herb–Vitamin Interactions Revised and Expanded 2nd Edition: Improve Your Health and Avoid Side Effects When Using Common Medications and Natural Supplements Together New York: Three Rivers Press, 2006.

Lieberman, Shari and Nancy Bruning. The Real Vitamin and Mineral Book: The Definitive Guide to Designing Your Personal Supplement Program, 4th ed. New York: Avery, 2007.

Pressman, Alan H. and Sheila Buff. The Complete Idiot’s Guide to Vitamins and Minerals, 3rd ed. Indianapolis, IN: Alpha Books, 2007.

Rockwell, Sally. Calcium Rich & Dairy Free: How to Get Calcium Without the Cow Pomeroy, WA: Health Research Books, 2005.

Rucker, Robert B., ed. Handbook of Vitamins. Boca Raton, FL: Taylor & Francis, 2007.

PERIODICALS

Familydoctor.org. “Vitamins and Minerals: What You Should Know.” American Family Physician. December 2006. <http://familydoctor.org/>

ORGANIZATIONS

American Dietetic Association. 120 South Riverside Plaza, Suite 2000, Chicago, Illinois 60606–6995. Telephone: (800) 877–1600. Website: <http://www.eatright.org>

International Food Information Council. 1100 Connecticut Avenue, NW Suite 430, Washington, DC 20036. Telephone: 202–296–6540. Fax: 202–296–6547. Website: <http://ific.org>

Linus Pauling Institute. Oregon State University, 571 Weniger Hall, Corvallis, OR 97331–6512. Telephone: (541) 717–5075. Fax: (541) 737–5077. Website: <http://lpi.oregonstate.edu>

National Institutes of Health Osteoporosis and Related Bone Diseases National Resource Center. 2 AMS Circle, Bethesda, MD 20892–3676 Telephone: (800) 624–BONE or (202)223–0344. TTY: (202) 293–2356. Fax: (202)466–4325. Website: <http://www.niams.nih.gov/bone/>

Office of Dietary Supplements, National Institutes of Health. 6100 Executive Blvd., Room 3B01, MSC 7517, Bethesda, MD 20892–7517 Telephone: (301)435–2920. Fax: (301)480–1845. Website: <http://dietary–supplements.info.nih.gov>

OTHER

Harvard School of Public Health. “Calcium Milk.” Harvard University, December 13, 2004. <http://www.hsph.harvard.edu/nutritionsource/calcium.html>

Mayo Clinic Staff. “Calcium supplements: Do Men Need Them Too?” MayoClinic.com, January 4, 2007. <http://www.mayoclinic.com/health/calcium–supplements/AN00420>

Medline Plus. “Calcium.” U. S. National Library of Medicine, March 14, 2007. <http://www.nlm.nih/gov/medlineplus/calcium.html>

Office of Dietary Supplements. “Dietary Supplement Fact Sheet: Calcium.” National Institutes of Health, September 23, 2005. <http://ods.od.nih.gov/factsheets/calcuim.asp>

National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS). “Calcium Supplements: What to Look For.” NIAMS, April 2005. <http://www.niams.nih.gov/bone/hi/calcium_supp.htm>

Tish Davidson, A.M.

CALCIUM (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Calcium is an alkaline earth metal. The alkaline earth metals make up Group 2 (IIA) of the periodic table, a chart that shows how the elements are related. They include beryllium, magnesium, strontium, barium, and radium. The alkaline earth metals are more chemically active than most metals. Only the alkali metals in Group I (IA) are more reactive.

Calcium compounds are common and abundant in the Earth's crust. Humans have used calcium compounds for hundreds of years in construction, sculpture, and roads.

Calcium metal was not prepared in a pure form until 1808 when English chemist Humphry Davy (1778-1829) passed an electric current through molten (melted) calcium chloride.

Metallic calcium has relatively few uses. However, calcium compounds are well known and widely used. They include chalk, gypsum, limestone, marble, and plaster of paris.

SYMBOL
Ca

ATOMIC NUMBER
20

ATOMIC MASS
40.08

FAMILY
Group 2 (IIA)
Alkaline earth metal

PRONUNCIATION
CAL-cee-um

Discovery and naming

It is impossible to say when humans first knew about or used compounds of calcium. Whenever they used limestone to build a structure, they were using a compound of calcium. Limestone is the common name for calcium carbonate (CaCO₃). Whenever humans built a statue or monument out of marble, they were using calcium carbonate in another form. Ancient Egyptians and early Greeks used mortar, a cement-like material that holds stones and bricks together. Early mortar was made by roasting or heating limestone for long periods of time. Water was then mixed with the powder, which would then dry to form a strong bond.

Humphry Davy | English chemist

H umphry Davy (1788-1829) was a major contributor to the field of electrochemistry. This is the science involving the relation of electricity to chemical changes. He is best known for discovering calcium, sodium, potassium, magnesium, strontium, and barium. He also discovered nitrous oxide and two types of lamps.

Davy grew up in Cornwall, England, in a poor family. His father, who died when Davy was a boy, had lost money in unwise investments, so Davy helped his mother pay off the debts. He disliked being a student, though he liked reading about science. In later life, he said he was happy he did not study too hard because he had more time to think on his own.

With no money for further education, the 17-year-old Davy began to work for a surgeon-pharmacist. He also started learning on his own about other subjects that interested him, such as geography, languages, and philosophy. He even wrote poems that later earned him the respect and friendship of William Wordsworth, Samuel Coleridge, and other leading English poets of his time!

At 19, Davy read a chemistry book by the famous French scientist Antoine-Laurent Lavoisier (1743-94). That book convinced him to concentrate on chemistry. For the rest of his life, Davy's career was marked by brilliant scientific explorations in chemistry and electrochemistry.

Davy discovered nitrous oxide after testing the effects of hydrogen and carbon dioxide on himself. (He liked to use himself as a human guinea pig!) Nitrous oxide is a gas consisting of nitrogen and oxygen. While studying nitrous oxide gas, he discovered that its effects often made him feel very happy or very sad. The feeling of happiness eventually gave nitrous oxide another name: laughing gas. Most importantly, though, Davy recognized that it could be used as an anesthetic. An anesthetic is a chemical used to dull pain during minor surgery.

In 1808, Davy invented the carbon arc lamp. He had proposed using carbon as the electrode material instead of metal. (Electrodes are conductors used to establish electrical contact with a nonmetallic part of a circuit.) With carbon electrodes, he made a strong electric current leap from one electrode to the other. This created an intense white light. Davy's invention marked the beginning of the era of electric light. Arc lamps are still used today.

Using his knowledge of electricity, Davy built a large battery which he used to break down substances most scientists thought were pure elements. In 1807, he discovered the element potassium. He created this by using electrolysis. Electrolysis produces chemical changes by passage of an electric current through an electrolyte. An electrolyte is a nonmetallic electric conductor. Within a week he isolated sodium in a similar way. Then in 1808, he used a slightly modified method to isolate calcium, magnesium, barium, and strontium. Davy was only 29 by the time he had discovered all of these elements!

Davy later invented the miner's lamp (now known as the Davy lamp). He learned that methane was the mine gas that caused explosions. But he realized it ignited only at high temperatures. So he designed a lamp in which the flame was surrounded by wire gauze. This reduced the heat and prevented flammable gases from igniting. This made coal-mining safer by reducing the number of explosions in mines.

Davy was rewarded by many honors and medals for his discoveries and inventions. He died of a stroke in 1829 at the age of 49.

Another calcium compound used by early civilizations was plaster of paris. Plaster of paris is made by heating gypsum, or calcium sulfate (CaSO₄), to remove the water that makes it crystallize. Water was added and it hardened into a brittle, cement-like substance. Until recently, it was most often used to make casts to protect broken bones. However, it has largely been replaced by fiberglass, which is lighter, yet stronger. The first mention of plaster of paris to protect broken bones can be found in a book written by Persian pharmacist Abu Mansur Muwaffaw in about 975 A.D.

By the 1700s, chemists had learned a great deal about calcium compounds. They knew that limestone, gypsum, marble, and many other commonly occurring compounds all contain a common element. They called the element calx. That word comes from the Latin term for lime. In 1807, Davy isolated the new element.

Davy invented a system for melting compounds of elements that were difficult to separate by usual methods. He passed an electric current through the compound, causing the compounds to break into parts. One of those parts was calx. He created the name calcium by adding the suffix -ium to calx; -ium is the ending used for almost all metallic elements. Davy was also able to produce free sodium, potassium, strontium, magnesium, and barium.

Physical properties

Calcium is a fairly soft metal with a shiny silver surface when first cut. The surface quickly becomes dull as calcium reacts with oxygen to form a coating of white or gray calcium oxide.

Calcium's melting point is 850°C (1,560°F) and its boiling point is 1,440°C (2,620°F). It has a density of 1.54 grams per cubic centimeter.

Chemical properties

Calcium is a moderately active element. It reacts readily with oxygen to form calcium oxide (CaO):

Calcium reacts with the halogens—fluorine, chlorine, bromine, iodine, and astatine, The halogens are the elements that make up Group 17 (VIIA) of the periodic table. Calcium also reacts readily with cold water, most acids, and most nonmetals, such as sulfur and phosphorus. For example, calcium reacts with sulfur:

Occurrence in nature

Calcium is the fifth most common element in the Earth's crust. Its abundance is estimated to be about 3.64 percent. It is also the fifth most abundant element in the human body.

Calcium does not occur as a free element in nature. It is much too active and always exists as a compound. The most common calcium compound is calcium carbonate (CaCO₃). It occurs as aragonite, calcite, chalk, limestone, marble, and travertine, and in oyster shells and coral.

Shellfish build their shells from calcium dissolved in the water. When the animals die or are eaten, the shells sink. Over many centuries, thick layers of the shells may build up and be covered with mud, sand, or other materials. The shells are squeezed together by the heavy pressure of other materials and water above them. As they are squeezed together, the layer is converted to limestone. If the limestone is squeezed even more, it can change into marble or travertine.

Isotopes

Six naturally occurring isotopes of calcium exist: calcium-40, calcium-42, calcium-43, calcium-44, calcium-46, and calcium-48. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Radioactive isotopes of calcium have also been made. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

Two radioactive isotopes of calcium are used in research and medicine. Calcium-45 is used to study how calcium behaves in many natural processes. For example, it can be used to see how various types of soil behave with different kinds of fertilizers. The calcium-45 is used as a tracer in such studies. A tracer is a radioactive isotope whose presence in a system can easily be detected. The isotope is injected into the system at some point. Inside the system, the isotope gives off radiation. That radiation can be followed by detectors placed around the system. Calcium-45 can also be used as a tracer in the study of glassy materials, detergents, and water purification systems.

Both calcium-45 and calcium-47 can be used to study how calcium is used in the body. A doctor may think that a person's body is not using calcium properly in making bones or regulating nerve messages. The doctor can use calcium-45 or calcium-47 to find out more about this problem. The radioactive isotope is injected into the person's bloodstream. Then its path can be followed by the radiation it gives off. The doctor can then tell if the calcium is being used normally in the body.

Extraction

Pure calcium metal can be made by the same method used by Davy. An electric current is passed through molten calcium chloride:

There is not much demand for pure calcium. Most calcium is used in the form of limestone, gypsum, or other minerals that can be mined directly from the earth.

Shellfish build their shells from calcium dissolved in the water.

Uses

Calcium metal has relatively few uses. It is sometimes used as a "getter." A getter is a substance that removes unwanted chemicals from a system. Calcium is used as a getter in the manufacture of evacuated glass bulbs. Calcium is added to the bulb while it is being made. It then combines with gases left in the glass in the final stages of manufacture. Calcium is also used as a getter in the production of certain metals, such as copper and steel. The calcium removes unwanted elements that would otherwise contaminate the metal.

Calcium is also used to make alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals. An alloy of calcium and cerium is used in flints found in lighters (the elements that create sparks).

Compounds

The starting point for the manufacture of most calcium compounds is limestone. Limestone occurs naturally in large amounts in many parts of the world. It is usually mined from open-pit quarries. A quarry is a large hole in the ground from which useful minerals are taken.

Limestone is first heated to obtain lime, or calcium oxide (CaO):

Lime is one of the most important chemicals in the world. It usually ranks in the top five chemicals produced in the United States. In 1996, about 19 billion kilograms (42 billion pounds or 21 million tons) of lime was produced in the United States.

Lime is used in the production of metals. It is used during the manufacture of steel to remove unwanted sand, or silicon dioxide (SiO₂), present in iron ore:

The product formed in this reaction, calcium silicate (CaSiO₃), is called slag.

Another important use of lime is in pollution control. Many factories release harmful gases into the atmosphere through smokestacks. Lining a smokestack with lime allows some of these gases to be captured. The lime is known as a scrubber. Lime captures one harmful gas, sulfur dioxide (SO₂), which is a contributor to acid rain (a form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes):

Calcium sulfite (CaSO₃) is a solid that can be removed from the inside of the smokestack.

"In the limelight"

At one time, lime was used as a source of light in theaters. When lime is heated to a high temperature, it gives off an intense white light. Pots of hot lime were often used to line the front of the stage. The light the pots gave off helped the audience see the performers. As a result, the performers were said to be "in the limelight." That phrase is still in use today, but lime is no longer used as a source of light in theaters.

Lime is also used in water purification and waste treatment plants. When water combines with water, it forms slaked lime, or calcium hydroxide (Ca(OH)₂):

Slaked lime traps impurities present in the water as it forms. It carries the impurities with it as it sinks to the bottom of the tank.

Lime is used to make more than 150 different industrial chemicals. Some examples of these chemicals with their uses are:

Milk is a good source of calcium.

calcium alginate: thickening agent in food products such as ice cream and cheese products; synthetic fibers

calcium arsenate (Ca₃(AsO₄)₂): insecticide

calcium carbide (CaC₂): used to make acetylene gas (for use in acetylene torches for welding); manufacture of plastics

calcium chloride (CaCl₂): ice removal and dust control of dust on dirt roads; conditioner for concrete; additive for canned tomatoes; provides body for automobile and truck tires

calcium cyclamate (Ca(C₆H₁₁NHSO₄)₂): sweetening agent (cyclamate), no longer permitted for use because of suspected cancer-causing properties

calcium gluconate (Ca(C₆H₁₁O₇)₂): food additive; vitamin pills

calcium hypochlorite (Ca(OCl)₂): swimming pool disinfectant; bleaching agent; deodorant; algicide and fungicide (kills algae and fungi)

calcium permanganate (Ca(MnO₄)₂): liquid rocket propellant; textile production; water sterilizing agent; dental procedures

calcium phosphate (Ca₃(PO₄)₂): supplement for animal feed; fertilizer; commercial production of dough and yeast products; manufacture of glass; dental products

calcium phosphide (Ca₃P₂): fireworks; rodenticide (kills rats); torpedoes; flares

calcium stearate (Ca(C₁₈H₃₅O₂)₂): manufacture of wax crayons, cements, certain kinds of plastics, and cosmetics; food additive; production of water resistant materials; production of paints

calcium tungstate (CaWO₄): luminous paints; fluorescent lights; X-ray studies in medicine

Health effects

Calcium is essential to both plant and animal life. In humans, it makes up about two percent of body weight. About 99 percent of the calcium in a person's body is found in bones and teeth. Milk is a good source of calcium. The body uses calcium in a compound known as hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) to make bones and teeth hard and resistant to wear.

Calcium has many other important functions in the human body. For example, it helps control the way the heart beats. An excess (too much) or deficiency (not enough) of calcium can change the rhythm of the heart and cause serious problems. Calcium also controls the function of other muscles and nerves.

Calcium

Definition

Calcium (Ca) is the most abundant mineral in the body. About 99% of calcium in the body is in bones and teeth. The remaining 1% is in blood and soft tissue. Calcium in body fluids is an electrolyte with a charge of +2. Humans must meet their need for calcium through diet .

Purpose

Calcium is essential for

• building and maintaining strong bones and teeth
• muscle contraction
• blood vessel contraction and relaxation
• nerve impulse transmission
• regulating fluid balance in the body

Description

Most calcium in the body is stored in bones and teeth. Here it combines with phosphate to form strong, stable crystals. The remaining 1% is dissolved in body fluids and much of it forms Ca 2+ ions. In the body, these electrically charged particles are called electrolytes. Calcium and other electrolytes are not distributed evenly throughout the body. Dissolved calcium is found mainly in the fluid outside cells (extracellular fluid). Metabolic events cause the movement of calcium across cell membranes result in muscle contraction, nerve impulse transmission, and various chemical reactions. The cell then uses energy to restore the balance of calcium between the inside and outside of the cell membrane, so that the event can be repeated.

To remain healthy, the amount of calcium dissolved in body fluids must be stay within a very narrow range. Bone acts like a calcium bank. Bone is constantly being broken down by cells called osteoclasts and built up again by cells called osteoblasts. This process is called bone remodeling, and it continues throughout an individual's life. When excess calcium is present in the blood, osteoblasts deposit calcium into bones. When too little calcium is in the blood, osteoblasts dissolve calcium from bones and move it into the blood. This process is controlled by parathyroid hormone (PTH) secreted by the parathyroid glands. The parathyroid glands are extremely sensitive to the level of calcium in the blood, and in a healthy individual they are able to maintain the concentration of calcium ions fluctuates very little.

Normal calcium requirements

The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for many vitamins and minerals. The DRIs consist of three sets of numbers. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97–98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects.

The IOM has not set RDAs for calcium, but in-stead it has set AI levels for all age groups based on observed and experimental information. However, many studies show that Americans across almost all age groups are not meeting these AI levels. AIs and ULs for calcium are measured in milligrams (mg). The following list gives the recommended AL and UL levels of calcium for each age group.

• adults age 19–50: RDA 1,000 mg; UL 2,500 mg
• adults over age 50: RDA 1,200 mg; UL 2,500 mg

KEY TERMS

Dietary supplement —A product, such as a vitamin, mineral, herb, amino acid, or enzyme, that is intended to be consumed in addition to an individual's diet with the expectation that it will improve health.

Diuretic —A substance that removes water from the body by increasing urine production.

Electrolyte —Electrically charged particles (ions) that form when salts dissolve in water or fluids. Electrolytes regulate water balance in the body and play a critical role in almost every metabolic reaction.

Enzyme —Proteins that change the rate of a chemical reaction within the body without themselves being used up in the reaction.

Mineral —An inorganic substance found in the earth that is necessary in small quantities for the body to maintain a health. Examples: zinc, copper, iron.

Sources of calcium

In the United States, dairy products—milk, yogurt, and cheese—are the main sources of dietary calcium. Low-fat dairy products, such as skim milk or reduced-fat cheese, contain about the same amount of calcium as whole milk products. Other sources of calcium include canned fish with bones, dark green leafy vegetables, and tofu made with calcium sulfate. Other types of tofu do not contain significant amounts of calcium. Processed foods such as orange juice, breakfast cereal, instant breakfast drinks, and bread are often fortified with calcium. This will be indicated on the label.

The following list gives the approximate manganese content for some common foods:

• milk, any type, 1 cup (8 ounces): 300 mg
• yogurt, plain, 8 ounces: 415 mg
• yogurt with fruit, 8 ounces: 245–384 mg
• cheddar cheese, 1.5 ounces: 305 mg
• mozzarella cheese, 1.5 ounces: 372 mg
• sardines with bones, canned in oil, 3 ounces: 324
• salmon with bones, canned, 3 ounces: 181 mg
• tofu, firm, made with calcium sulfate, 1/2 cup: 204 mg
• pinto or red beans, cooked, 1/2 cup: 43 mg
• white beans, cooked, 1/2 cup: 113 mg
• bok choy, 1/2 cup cooked: 61 mg
• spinach, cooked, 1/2 cup: 120 mg
• bread, whole wheat, 1 slice 20 mg
• orange juice, fortified, 6 ounces: 200–260 mg
• instant breakfast drink, powder prepared with water, 105–250 mg
• breakfast cereal, fortified, 1 cup: 100–1,000 mg

Although experts recommend that people meet as many of their vitamins and minerals needs through diet as possible, it is difficult for many people to get enough calcium from food alone. This is especially true for vegans, who eat no dairy products, adolescent girls who are very calorie conscious and tend to avoid milk and replace it with diet sodas, and people with lactose intolerance who cannot easily digest dairy products. Pregnant women and older individuals may also have a hard time eating enough to meet their calcium needs. People who do not get enough calcium through diet can benefit from taking a dietary supplement containing calcium.

Calcium supplements are available over-the-counter. The most common supplements supply calcium in the form of calcium carbonate or calcium citrate. Calcium carbonate is usually the most economical calcium supplement. People who are taking medications to reduce stomach acid may more easily absorb calcium citrate. Some supplements combine calcium and vitamin D because vitamin D helps the body absorb calcium. No calcium supplement contains enough calcium to meet the entire daily adequate intake, because the pill would be too large to swallow. In addition, the body absorbs calcium best in doses of 500 mg or less. People who need more than 500 mg of supplemental calcium should divide the dose in half to be taken morning and evening.

Calcium deficiency

Calcium deficiency, called hypocalcemia, can occur because of inadequate calcium intake, excess calcium excretion by the kidney (usually caused by kidney damage), the inability to adequately absorb calcium, or because of interactions between calcium and some prescription drugs. People at highest risk of calcium deficiency are teenagers, women past the age of menopause , individuals who are lactose intolerant, vegans, and people with kidney (renal) damage.

Calcium deficiency rarely shows up in blood tests because calcium is withdrawn from the bones to maintain blood levels of calcium. The bones then become less dense, weaker, and more likely to break. This condition is called osteoporosis and it is most noticeable in the elderly who have a high rate of broken bones resulting from falls . Osteoporosis is a part of aging, but eating a healthy diet high in calcium, getting adequate vitamin D, and doing weight-bearing exercises regularly can delay its onset. Severe calcium deficiency is usually caused by a medical condition rather than inadequate calcium intake. It causes symptoms such as muscle cramps, tingling in the fingers, lethargy, convulsions, heart rhythm abnormalities, and death . These symptoms can also be caused by many other diseases.

Calcium excess

Calcium excess is called hypercalcemia. It usually results from poor kidney function (renal failure) or from a malignant cancer tumor. It can also be caused by very large supplemental doses of vitamin D. Very rarely is hypercalcemia caused by too much calcium from food or dietary supplements . High levels of calcium interfere with the absorption of other minerals such as iron, zinc , magnesium, and phosphorous. People with hypercalcemia usually have multiple medical problems and are under the supervision of a physician.

Precautions

People of all ages, races, and gender need to be alert to getting enough calcium in their diet. Building strong, dense bones begins in childhood and adolescence, even though the results cannot be seen until old age. People mentioned above as being at especially high risk of low dietary calcium intake should investigate taking a calcium supplement.

Interactions

Absorption of calcium is affected by several conditions.

• Age. Infants absorb as much as 60% of the calcium in their digestive system. This decreases to 15–20% in adulthood, and even less in old age.
• Amount of calcium consumed. The more calcium consumed at one time, the less efficient absorption becomes. Calcium from supplements should be spaced out during the day for maximum absorption.
• Vitamin D. The presence of vitamin D improves calcium absorption. Vitamin D deficiency can worsen calcium deficiency.
• Plant products. Phytic found in beans and oxalic acid found in spinach and leafy greens decrease the amount of calcium absorbed from those foods, but does not affect the absorption of calcium from other foods present at the same time in the intestine. Fiber such as wheat bran also reduces calcium absorption.

Prescription medications can also affect or be affected by the absorption of calcium. These include:

• digoxin
• fluroquinolones
• levothyroxine
• tetracycline antibiotics
• anticonvulsants
• thiazide-type diuretics
• glucacorticoids
• mineral oil
• stimulant laxatives
• antacids

People taking these drugs should check with their healthcare provider or pharmacist about potential adjustments in their medications or calcium intake.

Complications

No complications are expected when healthy people take calcium in amounts equal to the AI level and less than the UL level. Some people experience gas, nausea, and abdominal discomfort from calcium supplements. Taking the supplement with meals, taking smaller doses spread out over the day, or changing the type of supplement usually solves this problem.

Resources

BOOKS

Fragakis, Allison. The Health Professional's Guide to Popular Dietary Supplements. Chicago: American Dietetic Association, 2003.

Gaby, Alan R., ed. A-Z Guide to Drug-Herb-Vitamin Interactions Revised and Expanded 2nd Edition: Improve Your Health and Avoid Side Effects When Using Common Medications and Natural Supplements Together. New York: Three Rivers Press, 2006.

Lieberman, Shari, and Nancy Bruning. The Real Vitamin and Mineral Book: The Definitive Guide to Designing Your Personal Supplement Program, 4th ed. New York: Avery, 2007.

Pressman, Alan H., and Sheila Buff. The Complete Idiot's Guide to Vitamins and Minerals, 3rd ed. Indianapolis, IN: Alpha Books, 2007.

Rockwell, Sally. Calcium Rich & Dairy Free: How to Get Calcium Without the Cow. Pomeroy, WA: Health Research Books, 2005.

Rucker, Robert B., ed. Handbook of Vitamins. Boca Raton, FL: Taylor & Francis, 2007.

PERIODICALS

Familydoctor.org. “Vitamins and Minerals: What You Should Know.” American Family Physician. December 2006. http://familydoctor.org/

ORGANIZATIONS

American Dietetic Association. 120 South Riverside Plaza, Suite 2000, Chicago, Illinois 60606-6995. Telephone: (800) 877-1600. Website: http://www.eatright.org

International Food Information Council. 1100 Connecticut Avenue, NW Suite 430, Washington, DC 20036. Telephone: (202) 296-6540. Fax: (202) 296-6547. Web site: http://ific.org

Linus Pauling Institute. Oregon State University, 571 Weniger Hall, Corvallis, OR 97331-6512. Telephone:(541) 717-5075. Fax: (541) 737-5077. Website: http://lpi.oregonstate.edu

National Institutes of Health Osteoporosis and Related Bone Diseases National Resource Center. 2 AMS Circle, Bethesda, MD 20892-3676 Telephone: (800) 624-BONE or (202) 223-0344. TTY: (202) 293-2356. Fax: (202) 466-4325. Website: http://www.niams.nih.gov/bone/

Office of Dietary Supplements, National Institutes of Health. 6100 Executive Blvd., Room 3B01, MSC 7517, Bethesda, MD 20892-7517 Telephone: (301) 435-2920. Fax: (301) 480-1845. Website: http://dietary-supplements.info.nih.gov

OTHER

Harvard School of Public Health. “Calcium & Milk.” Harvard University, December 13, 2004. http://www.hsph.harvard.edu/nutritionsource/calcium.html.

Mayo Clinic Staff. “Calcium Supplements: Do Men Need Them Too?” MayoClinic.com, January 4, 2007. http://www.mayoclinic.com/health/calcium-supplements/AN00420.

Medline Plus. “Calcium.” U. S. National Library of Medicine, March 14, 2007. http://www.nlm.nih/gov/medlineplus/calcium.html.

Office of Dietary Supplements. “Dietary Supplement Fact Sheet: Calcium.” National Institutes of Health, September 23, 2005. http://ods.od.nih.gov/factsheets/calcuim.asp.

National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS). “Calcium Supplements: What to Look For.” NIAMS, April 2005. http://www.niams.nih.gov/bone/hi/calcium_supp.htm.

Tish Davidson A.M.