Calcium is the most abundant mineral in the body. An average man contains about one and a half kilograms of calcium and an average woman about one kilogram. Over 99 per cent of the calcium in your body is in bones and teeth. The remaining one per cent is found in the blood, lymph and other body fluids, cell membranes and structures inside cells. The main function of calcium is in the structural development and maintenance of healthy bones and teeth. Bone is made up of both mineral (mostly hydroxyapatite-like crystals) and nonmineral (mostly protein) components. Calcium in the bone is in two forms, one bound tightly and the other more easily removed. Calcium is removed from the tightly bound part of the bone to maintain blood levels only when dietary intake is inadequate and the more mobile stores are exhausted. Bone undergoes a constant remodeling process with 20 per cent of an adult’s bone calcium re-absorbed and replaced every year.

Muscle contraction

Calcium plays a vital role in muscle contraction. It is also necessary for heartbeat regulation through its effects on heart muscle.

Nervous system

Calcium is essential for nerve impulse conduction. It plays a role in the release of neurotransmitters and activates some enzymes which generate neurotransmitters.

Cardiovascular system and blood

Calcium interacts with sodium, potassium and magnesium to regulate blood pressure and water balance. A major class of drugs used to lower high blood pressure blocks the channels which transport calcium across muscle cell membranes. Calcium also plays a role in the activation of prothrombin (which is formed from vitamin K in the liver) which is essential to the blood clotting process.

Other functions

Calcium is essential for cell division, healthy immune function, for enzyme activity and for the production and activity of hormones involved in digestion, energy and fat metabolism, and the production of saliva. It is also involved in the transport of nutrients and other substances across cell membranes.

Absorption and metabolism

On average, adults absorb around 25 to 50 per cent of dietary calcium. Some is absorbed passively while some is transported via a vitamin D-mediated process. Most absorption occurs in the small intestine. The calcium then passes into the exchangeable calcium pool that is in the body fluids. This pool turns over 20 to 30 times a day whereas the calcium in bone turns over every five to six years.

Blood levels of calcium are tightly regulated by the hormones calcitonin, parathyroid hormone and vitamin D. These hormones act together to regulate calcium levels as dietary intake and requirements vary. They control absorption from the gut, excretion in the kidney and the rate of bone formation and breakdown. In the absence of vitamin D, less than 10 per cent of dietary calcium may be absorbed. When intake is inadequate, calcium is removed from storage sites in bone and used to keep blood levels constant. Other hormones which affect calcium levels include estrogens, glucocorticoids, thyroid hormone, insulin and growth hormones.

Absorption is enhanced when calcium intake is low1 and also by moderate exercise. Lactose, vitamin D and adequate (but not excessive) protein improve calcium absorption. High levels of fat reduce absorption. Compounds known as phytates, which are found in dietary fiber; and oxalates, which are found in leafy greens, reduce absorption. The acid environment of the stomach makes calcium salts more soluble, and therefore easier to absorb; and low stomach acid reduces absorption.

Absorption and retention of calcium become less efficient with age,2 partly due to lower estrogen and testosterone levels; and a postmenopausal woman may only absorb 7 per cent of her dietary intake. The ability to absorb and retain calcium improves during pregnancy although it seems that some calcium is drawn from bone stores later in pregnancy. A study done in 1996 in Cincinnati showed that breastfeeding stimulates increases in calcium absorption and these increases become apparent after weaning or after menstrual periods restart.3

Smoking, high refined sugar intake, caffeine, alcohol and excess salt promote calcium excretion, thereby increasing the risk of deficiency. High protein diets also increase calcium excretion, particularly if the protein comes from meat.4


Mild calcium deficiency can cause nerve sensitivity, muscle twitching, brittle nails, irritability, palpitations and insomnia. Signs of severe deficiency include abnormal heartbeat, muscle pains and cramps, numbness, stiffness and tingling of the hands and feet, and depression. Children can suffer from rickets, a disease characterized by excessive sweating of the head; slowness in sitting, crawling and walking; insomnia; bone deformities; and growth retardation. In adults, deficiency can lead to osteomalacia with symptoms of bone pain, muscle weakness and delayed healing of fractures.

Those at risk of calcium deficiency include the elderly, people who don’t eat dairy products or other high calcium foods, athletes, those on high protein or high fiber diets, and those who drink a lot of alcohol. High dietary levels of phosphorus cause calcium to be removed from bone and excreted. Phosphorus is found in many common foods such as meat, cheese, processed foods and soda drinks, and people who consume large amount of these foods are at increased risk of calcium deficiency. People on weight-reducing diets are also at risk as they may avoid high calorie foods, which are often good sources of calcium.

Studies have shown that calcium is deficient in the diets of many women. The National Osteoporosis Foundation estimates that the average adult in the US gets only 500 to 700 mg per day. Calcium deficiency is relatively common in many countries.


Calcium deficiency contributes to osteoporosis, which literally means "porous bones" and in some cases, can be so severe as to cause the bones to break under the weight of the body. Particularly badly affected bones include the spinal vertebrae, the thigh bone and the radius (shorter arm bone). The symptoms of osteoporosis may be absent until fractures occur, although in some cases there may be back pain.

Osteoporosis is most common in elderly white women with a history of borderline calcium intake. Around 35 per cent of women suffer from osteoporosis after menopause and, although it is less common, the problem occurs in a similar way in men. Most of the bone loss seen in osteoporosis occurs in the first five to six years after menopause due to a decline in circulating estrogens and an age-related reduction in vitamin D production.

Good nutrition plays a role in reducing the incidence of osteoporosis by promoting the development of favorable peak bone mass during the first 30 to 40 years of life. Getting enough calcium in early adolescence and early adulthood is vital for bones to reach their maximum density so that they are strong enough to support the body even when they lose density later in life. Studies suggest that calcium intake in adolescence is often below the recommended levels. Researchers involved in a 1994 USDA study measured calcium intake in 51 girls aged 5 to16 years old. They found calcium intake to be below the recommended dietary allowance for 21 out of 25 girls aged 11 or over. These studies suggest that the current calcium intake of American girls during the puberty is not enough to enable bones to develop maximum strength and that increased intakes may be necessary.5

However, it is never too late to slow the bone-loss seen in osteoporosis, and early postmenopausal years are also an important time to ensure optimal intake. A 1997 study done at King’s College Hospital in London suggests that high calcium intakes are linked to bone mineral density in elderly women. Researchers assessed calcium intake in 124 women aged from 52 to 62 and also measured bone mineral density at the spine, hip and foot (oscalcis). Results showed that women with high calcium intakes had higher bone mineral density.6

Results from the Rotterdam Study, which involves 1856 men and 2452 women aged 55 years and over also show that high calcium intakes also protect against bone loss in men.7

Calcium deficiency is only one factor in osteoporosis. There is likely to be a genetic component and other dietary, behavioral and hormonal factors also play a major part. Adequate intakes of vitamin D, magnesium and boron are also necessary to build healthy bones. Body weight is the factor most linked to bone mineral density and, in women, body fat may be at least as important as muscle in maintaining bone mineral content. Weight-bearing exercise, adequate lifelong calcium intake, and moderate alcohol intake all play important roles in preventing osteoporosis. Estrogen replacement therapy is often used to treat osteoporosis.

Bone loss is found to be up to 11 per cent greater during the night. Calcium levels are also lowest during the night and may be affected by the concentration of the hormone, cortisol. These findings may lead to new hormone treatments for osteoporosis.


Calcium deficiency may be linked to an increased risk of colon cancer. Research on animals and some epidemiological studies suggest that people with high calcium intakes are less likely to develop colon cancer. Research findings in humans are inconclusive, with some studies showing protective effects while others have not. The overall results seem to suggest that the protective effect of high calcium intake does exist but that it is not very marked.

The association between calcium intake and deaths from gastrointestinal cancer was assessed in a 28-year follow-up study of 2591 Dutch civil servants and their spouses, aged 40 to 65 years. The researchers found that men and women who died of colorectal cancer had a lower average calcium intake compared to the rest of the population.8

Results from the Iowa Women’s Health Study published in 1998 showed that calcium can decrease the risk of rectal cancer. Researchers analyzed information from 34,702 postmenopausal women who responded to a mailed survey in 1986. After nine years of follow-up, 144 rectal cancer cases were identified. The results showed that high total calcium intake reduced the risk of rectal cancer.9 Other results from this study show a reduced risk of colon cancer in women with high intakes of calcium and vitamin D.

In a 1996 study, Harvard University researchers working on the Health Professionals Study assessed the links between calcium intake and colon cancer in almost 48,000 men aged from 40 to 75. They found that higher intake of calcium from foods and supplements was associated with a lower cancer risk until they adjusted their results to take other factors into account. They concluded that calcium may possibly mildly lower the risk of colon cancer.10 Data from the Nurses Health Study, which involved over 89,000 nurses, also showed a small reduced risk.11

Calcium may exert its protective effects by binding to toxic substances such as bile acids and fats and reduce the chance that these will cause cancerous changes in the gut. Calcium may also normalize the growth of cells in the intestinal wall, thus protecting against cancerous changes. Limited evidence suggests that low calcium intake may also increase the risk of breast, cervical and esophageal cancers.

Taiwanese studies done in 1997 and 1998 showed a protective effect both against gastric and colorectal cancers from high levels of calcium in drinking water.12,13

Blood pressure

Calcium metabolism seems to be altered in people with hypertension. Several studies suggest that low dietary intake of calcium is associated with an increased risk of developing hypertension and cardiovascular disease. Some research suggests studies show that restriction of calcium increases, and supplementation with calcium lowers, blood pressure. Data from the US Health and Nutrition Examination Survey (NHANES I) showed that hypertensive people consumed 18 per cent less dietary calcium than those with normal blood pressure.14

A review published in 1997 in the American Journal of Clinical Nutrition showed that experimental data support the view that when adults meet or exceed the recommended dietary allowances of calcium, potassium, and magnesium, high sodium intakes are not associated with high blood pressure. Thus adequate mineral intake may protect against salt sensitivity.15

Some evidence suggests that a woman who eats a low calcium diet in pregnancy may also increase the chances of her child suffering from high blood pressure.

Muscle cramps

When blood calcium levels drop below normal, the sensitivity of the nerves can increase, leading to muscle cramps. Pregnant women whose diets are deficient in calcium are at greatest risk of muscle cramps.


Severe calcium deficiency can lead to periodontal disease (inflammation and degeneration of the bone and gum structures that support the teeth).


Good sources of calcium include milk and other dairy products, kale, kelp, tofu, canned fish with bones, peanuts, walnuts, sunflower seeds, broccoli, cauliflower and soybeans. Fortified foods such as fruit juices, breads and cereals are also common sources. Calcium in hard water and some mineral waters may be important dietary sources for some people.16

Calcium from milk and milk products is absorbed more easily than that from most vegetables, with the exception of dark green leafy vegetables such as kale, broccoli, turnip and mustard greens. A 1990 study showed that more calcium is absorbed from kale than from milk.17

Green leafy vegetables such as spinach contain large amounts of calcium but also contain oxalic acid which binds calcium and prevents it from being absorbed. Insoluble fiber, such as that found in wheat bran, reduces calcium absorption; but soluble fiber, such as that found in psyllium and fruit pectins, does not seem to affect absorption.18

While dairy products are good sources of calcium, there is concern that their protein content can increase the loss of calcium from bone. Results from the ongoing Nurses Health Study suggest that drinking lots of milk and other dairy foods high in calcium does not protect older women against bone fractures. Researchers analyzed the diets of over 77,000 participants in the study and looked at the rates of bone fractures. Results showed that women who drank two or more glasses of milk per day had around a 45 per cent increased risk of hip fracture and a 5 per cent increased risk of forearm fracture compared to women who drank one glass or less per week. There was also no drop in risk with intake of calcium from other dairy foods.19

In another study done in 1995 at the University of California at Berkeley, researchers assessed the effect of calcium supplementation and drinking milk on pre-eclampsia in over 9000 pregnant women. Results showed that women who drank two glasses of milk per day had the lowest risk. The risk for those drinking one glass of milk per day was similarly low but the risk for those drinking less than one glass of milk per day was substantially higher. Women drinking three or more glasses of milk per day also showed increased risk as did those drinking four or more glasses per day.20

A varied diet which includes nondairy sources of calcium is likely to be more beneficial in protecting against osteoporosis and other disorders of calcium deficiency.

Recommended dietary allowances

Calcium requirements vary throughout a person’s lifetime, with greater needs during periods of rapid growth and later in life. Due to mounting evidence that people are not getting enough calcium to prevent osteoporosis and other bone diseases, in 1997 the US government raised the recommendations for how much calcium people should consume every day.

Recommended intakes for pregnant and breastfeeding women are no longer greater than those for other women. This is partly based on recent studies which suggest that changes in calcium metabolism and absorption during pregnancy and breastfeeding are enough to meet the extra demands placed on a woman’s body by her baby. A 1998 British study suggests that bone mineral density changes seen during breastfeeding seems to be unrelated to dietary calcium intake.21

Two randomized, placebo-controlled trials of calcium supplementation were done on new mothers in 1997 in Cincinnati, Ohio. Researchers tested the effect of 1000 mg of calcium per day on bone density, measured at enrolment and after three and six months. The results showed no effect of either lactation or calcium supplementation on bone density in the forearm, and also no effect of calcium supplementation on the calcium concentration in breast milk.22

In another study published in 1998, researchers studied calcium metabolism in 14 pregnant women from before conception to five months after their periods restarted. When the women were pregnant the increased needs were met by improved absorption, and then during the early breastfeeding period calcium excretion decreased. Some calcium was drawn from bone but this was recovered after menstruation restarted, although not to pre-pregnancy levels.23

The women involved in this study were all consuming adequate levels of calcium and it is possible that women whose calcium intake is lower than 1300 mg per day may benefit from extra calcium.


Calcium and magnesium

If you take a calcium supplement, you should also take a magnesium supplement. This helps to avoid constipation and to balance the effect of calcium on the electrical impulses in the nerves and muscles. Calcium and magnesium work together as mild neuromuscular relaxants. Some experts recommend taking calcium and magnesium in a 2:1 ratio while others suggest 1:1.

Toxic effects of excess intake

Toxic effects are rare as the body can excrete excess calcium with doses up to 2500 mg per day considered safe. Some people may suffer constipation at these doses. Daily intakes above 2500 mg may cause kidney stones and other problems. At very large doses, such as 25,000 mg, vomiting, nausea and loss of appetite can occur. If taken with high levels of vitamin D for long periods, deposition of calcium in the kidneys, heart and other soft tissues can occur. High levels may also impair vitamin K metabolism, reduce iron and zinc absorption, and affect the activity of neurons in the brain which control mood and emotion.

Calcium forms part of the plaque laid down in the arteries in atherosclerosis, although this problem is likely to be due to abnormalities in calcium metabolism rather than excess dietary calcium.

Results from the Health Professionals Follow-Up study which involved 47,781 men suggest that high calcium intakes from both food and supplements increase the risk of prostate cancer.28

Therapeutic uses of supplements


Research suggests that taking calcium supplements later in life can slow the bone loss associated with osteoporosis. Treatment which combines calcium and estrogen is likely to be better at building bone than treatment with estrogen alone, according to a review published in 1998 in the American Journal of Clinical Nutrition.

Researchers analyzed the results of 31 studies and found that the postmenopausal women who took estrogen alone had an average increase in spinal bone mass of 1.3 per cent per year, while those who took estrogen and calcium supplements had an average increase of 3.3 per cent. Increases in bone mass in the forearm and upper thigh were also greater in women taking supplements. The added benefit from the calcium was seen when the women increased their intake from an average of 563 mg per day to 1200 mg per day.29

Another study done in 1997 at Tufts University in Boston showed reduced rates of bone loss and fractures in men and women over 65 who took calcium and vitamin D supplements. Researchers assessed the effects of calcium (500 mg per day) and vitamin D (700 IU per day) on 176 men and 213 women aged 65 years or older. After a three-year period, those taking the supplements had higher bone density at all body sites measured. The fracture rate was also reduced by 50 per cent in those taking the supplements.30

Calcium supplements have also been shown to increase bone mass in children, although a 1996 study done in Hong Kong found that when the supplements were stopped, the beneficial effects disappeared.31

Protection against the side effects of corticosteroid drugs

One of the side effects of corticosteroid drugs, which are often used to treat arthritis, asthma and other chronic diseases, is a loss of bone mineral density and therefore an increased risk of osteoporosis. In a study done in 1996 at the Medical College of Virginia, researchers showed that calcium and vitamin D supplements can help prevent this loss. In the two-year study, 96 patients with rheumatoid arthritis, 65 of whom were taking corticosteroid drugs, were given 1000 mg calcium and 500 IU vitamin D per day or placebo. The researchers analyzed the bone mineral density of the lumbar spine and femur for one year. In those patients taking corticosteroid drugs and placebo losses of bone mineral density were seen. In those taking the supplements, gains were seen and in those not taking corticosteroids, the supplements did not appear to affect bone mineral density.32

Blood pressure

Some studies have shown that calcium supplements lower blood pressure in mildly hypertensive patients, while others have shown no effect.

In an eight-week randomized, placebo-controlled study done in 1985 in the US, researchers assessed the effect of 1000 mg per day of calcium supplements on the blood pressure of 48 people with hypertension and 32 without. Compared with placebo, calcium significantly lowered both systolic and diastolic blood pressures, but only in those with high blood pressure.33

Results from the University of Pittsburgh Trials of Hypertension Prevention (TOHP) showed calcium supplements (100 mg per day) to have little effect on blood pressure. The participants were healthy adult men and women (both white and African American) aged 30 to 54 years with high-normal diastolic blood pressure. However, the supplements did seem to lower blood pressure in white women, who are at particular risk of low calcium intakes.34 Supplements may be beneficial in cases where calcium intake is insufficient, which may be relatively common. Whether calcium can lower blood pressure in cases where there is no apparent deficiency is not clear. Increasing calcium intake may lower blood pressure by increasing the excretion of sodium and calcium supplements may be most useful in those who are salt sensitive.

The results of a study, reported in 1997 in the British Medical Journal, suggest that women who take calcium supplements in pregnancy have children with lower blood pressures. Researchers measured the blood pressures of almost 600 children of women who had previously been involved in a double-blind trial of the effects of calcium on blood pressure during pregnancy. The results showed that, overall, systolic blood pressure was lower in the calcium group, particularly among overweight children.35

Muscle cramps

Calcium can be used to control the incidence of leg cramps in pregnant women, possibly by decreasing nerve irritability. It has also been used to reduce the incidence of menstrual cramps and symptoms associated with premenstrual syndrome.


Use of calcium supplements during pregnancy may lower a woman’s risk of pre-eclampsia, a disorder which occurs in one in every 20 pregnant women. Symptoms of pre-eclampsia are high blood pressure, headache, protein in the urine, blurred vision and anxiety. It can lead to eclampsia, a seizure disorder which can cause complications with pregnancy and even death. There is some evidence that abnormalities in calcium metabolism are involved in pre-eclampsia. Many pregnant women do not consume enough calcium to ensure optimal blood pressure regulation and the results of several clinical trials have suggested that calcium supplements reduce the incidence of pre-eclampsia.36

A 1996 analysis of clinical trials which looked at the effects of calcium intake on pre-eclampsia and pregnancy outcomes in 2500 women found that those who consumed 1500 to 2000 mg of calcium supplements per day were 70 per cent less likely to suffer from high blood pressure in pregnancy.37

However, in a study published in 1997 in the New England Journal of Medicine, researchers found that calcium supplements did not prevent pre-eclampsia. The study, the largest ever done on the subject, involved 4589 healthy, first-time mothers. Half of the subjects received 2000 mg of calcium per day and the other half received a placebo. The researchers then assessed the incidence of high blood pressure and protein excretion in the urine. No significant differences in the groups were found. Supplements did not reduce other complications associated with childbirth or increase the incidence of kidney stones.38

The results of this study still leave open the possibility that calcium supplements may be useful as the women included in the study were already consuming higher than average levels of calcium than is typical even before they took the supplements. Women at high risk of pre-eclampsia were also not included in the investigation.

Other uses

Calcium supplements can be useful in congestive heart failure as they increase the contractility of heart muscle. Calcium salts are used intravenously to treat heart attack associated with high potassium and magnesium levels and low calcium levels. They are also used in cases of calcium antagonist drug overdose.

Calcium supplements have also been used to treat allergy complaints, depression, panic attacks, arthritis, hypoglycemia, muscle and joint pains. Calcium salts are a major component of antacids which are used to treat indigestion and ulcers. Taken with magnesium, they may have neuromuscular relaxing effects and may be useful in insomnia.

Interactions with other nutrients

Calcium and phosphorus work together to form healthy bones and teeth. High phosphorus intakes lead to increased calcium excretion. The intake ratio for calcium to phosphorus should be 1:1.

Calcium competes with zinc, manganese, copper and iron for absorption in the intestine, and a high intake of one mineral can reduce absorption of the others. This is of particular concern in the case of iron. Calcium reduces both heme and nonheme iron absorption.39 The practical implications of the inhibitory effect of calcium mean that addition of milk or cheese to common meals such as pizza or hamburgers can reduce iron absorption by 50-60%. Some experts recommend eating foods that provide most of the daily iron intake at a different time to foods which provide most of the daily calcium intake. Thus it is advisable to reduce the intake of dairy products with the main meals providing most of the dietary iron.