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What is it? Overview Usage Side Effects and Warnings

Calcium Overview

Written by FoundHealth, ColleenO.

Calcium is the most abundant mineral in the body, making up nearly 2% of total body weight. More than 99% of the calcium in your body is found in your bones, but the other 1% is perhaps just as important for good health. Many enzymes depend on calcium in order to work properly, as do your nerves, heart, and blood-clotting mechanisms.

To build bone, you need to have enough calcium in your diet. But in spite of calcium-fortified orange juice and the best efforts of the dairy industry, most Americans are calcium deficient. 1 Calcium supplements are a simple way to make sure you're getting enough of this important mineral.

One of the most important uses of calcium is to help prevent and treat osteoporosis, the progressive loss of bone mass to which menopausal women are especially vulnerable. Calcium works best when combined with vitamin D.

Other meaningful evidence suggests that calcium may have an additional important use: reducing PMS symptoms.


Although there are some variations between recommendations issued by different groups, the official US and Canadian recommendations for daily intake of calcium are as follows:

  • Infants
    • 0-6 months: 210 mg
    • 7-12 months: 270 mg
  • Children
    • 1-3 years: 500 mg
    • 4-8 years: 800 mg
  • Males and Females
    • 9-18 years: 1,300 mg
    • 19-50 years: 1,000 mg
    • 51 years and older: 1,200 mg
  • Pregnant and Nursing Women
    • Under 19 years: 1,300 mg
    • 19 years and older: 1,000 mg

To absorb calcium, your body also needs an adequate level of vitamin D (for more information, see the article on Vitamin D ).

Various medications may impair calcium absorption or metabolism, either directly or through effects on vitamin D. People who use these may benefit by taking extra calcium and vitamin D. Implicated medications include corticosteroids , 2 3 heparin , 4 5 isoniazid , 6 7 and anticonvulsants . 8 9 10 11 12 13 Note:Calcium carbonate might interfere with the effects of anticonvulsant drugs, and for that reason should not be taken at the same time of day. 14 Milk, cheese, and other dairy products are excellent sources of calcium. Other good sources include orange juice or soy milk fortified with calcium, fish canned with its bones (such as sardines), dark green vegetables, nuts and seeds, and calcium-processed tofu.

Many forms of calcium supplements are available on the market, each with its own advantages and disadvantages.

Naturally Derived Forms of Calcium

These forms of calcium come from bone, shells, or the earth: bonemeal, oyster shell, and dolomite. Animals concentrate calcium in their shells, and calcium is found in minerals in the earth. These forms of calcium are economical, and you can get as much as 500 mg to 600 mg in one tablet. However, there are concerns that the natural forms of calcium supplements may contain significant amounts of lead. 15 The level of contamination has decreased in recent years, but still may present a health risk. 16 Calcium supplements rarely list the lead content of their source, although they should. The lead concentration should always be less than 2 parts per million.

Refined Calcium Carbonate

This is the most common commercial calcium supplement, and it is also used as a common antacid. Calcium carbonate is one of the least expensive forms of calcium, but it can cause constipation and bloating, and it may not be well absorbed by people with reduced levels of stomach acid. Taking it with meals improves absorption because stomach acid is released to digest the food. 17 (See the section, Chelated Calcium, below.)

Chelated Calcium

Chelated calcium is calcium bound to an organic acid (citrate, citrate malate, lactate, gluconate, aspartate, or orotate). The chelated forms of calcium offer some significant advantages and disadvantages compared with calcium carbonate.

Certain forms of chelated calcium (calcium citrate and calcium citrate malate) are widely thought to be significantly better absorbed and more effective for osteoporosis treatment than calcium carbonate. However, while some studies support this belief, 18 others do not. 19 The discrepancy may be due to the particular calcium carbonate products used; some calcium carbonate formulations may dissolve better than others.

One study found that calcium citrate malate in orange juice is markedly better absorbed than tricalcium phosphate/calcium lactate in orange juice. 20 A form of calcium called active absorbable algal calcium (AAACa) has also been promoted as superior to calcium carbonate, but the study upon which claims of benefit are founded actually used quite questionable statistical methods (technically, post-hoc subgroup analysis). 21 Chelated calcium is much more expensive and bulkier than calcium carbonate. In other words, you have to take larger pills, and more of them, to get enough calcium. It is not at all uncommon to need to take five or six large capsules daily to supply the necessary amount, a quantity some people may find troublesome.

Therapeutic Dosages

Unlike some supplements, calcium is not taken at extra high doses for special therapeutic benefit. Rather, for all its uses, it should be taken in the amounts listed under Requirements/Sources, along with the recommended level of vitamin D. (See the article on vitamin D for proper dosage amounts.)

Calcium absorption studies have found evidence that your body can't absorb more than 500 mg of calcium at one time. 22 Therefore, it is most efficient to take your total daily calcium in two or more doses.

Use of prebiotics known as "inulin fructans" may improve calcium absorption. 23 It isn't possible to put all the calcium you need in a single multivitamin/mineral tablet, so this is one supplement that should be taken on its own. Furthermore, if taken at the same time, calcium may interfere with the absorption of chromium and manganese . 24 This means that it is best to take your multivitamin and mineral pill at a separate time from your calcium supplement.

Although the calcium present in some antacids or supplements may alter the absorption of magnesium , this effect apparently has no significant influence on overall magnesium status. 25 Calcium may also interfere with iron absorption, 26 27 but the effect may be too slight to cause a problem. Some studies show that calcium may decrease zinc absorption when the two are taken together as supplements; however, studies have found that, in the presence of meals, zinc levels may be unaffected by increases of either dietary or supplemental calcium. 28 29 30 31 32 33

What Is the Scientific Evidence for Calcium?


A number of double-blind, placebo-controlled studies indicate that calcium supplements (especially as calcium citrate, and taken with vitamin D) are slightly helpful in preventing and slowing down bone loss in postmenopausal women. 34 Contrary to some reports, milk does appear to be a useful source of calcium for this purpose. 35 However, the effect of calcium supplementation in any form is relatively mild and may not be strong enough to reduce the rate of osteoporotic fractures. Note that use of calcium and vitamin Dmust be continual. Any improvements in bone density rapidly disappear once the supplements are stopped. 36 A large study of over 3,000 postmenopausal women aged 65-71 years found that three years of daily supplementation with calcium and vitamin D was not associated with a significant reduction in the incidence of fractures. 37 Calcium carbonate may not be effective. 38 One study found benefits for male seniors using a calcium and vitamin D-fortified milk product. 39 (See, however, possible increased risk of prostate cancer discussed in Safety Issues .)

Calcium and vitamin D supplementation may help bones heal that have become fractured due to bone thinning. 40 Calcium supplements may do a better job of strengthening bones when people have relatively high protein intake. 41 Heavy exercise causes increased calcium loss through sweat, and the the body does not compensate for this by reducing calcium loss in the urine. 42 The result can be a net calcium loss great enough so that it presents health concerns for menopausal women, already at risk for osteoporosis. One study found that use of an inexpensive calcium supplement (calcium carbonate), taken at a dose of 400 mg twice daily, is sufficient to offset this loss. 43 Calcium supplementation could, in theory, be useful for young girls as a way to "put calcium in the bank"—building up a supply for the future in order to prevent later osteoporosis. However, surprisingly, the benefits seen in studies have been modest to nonexistent, and this approach may only produce results when exercise is also increased. 44 One study found that in calcium-deficient pregnant women, calcium supplements can improve the bones of their unborn children. 45 Evidence suggests that the use of calcium combined with vitamin D can help protect against the bone loss caused by corticosteroid drugs, such as prednisone. A review of five studies covering a total of 274 participants reported that calcium and vitamin D supplementation significantly prevented bone loss in corticosteroid-treated individuals. 46 For example, in a 2-year, double-blind, placebo-controlled study that followed 65 individuals with rheumatoid arthritis taking low-dose corticosteroids, daily supplementation with 1,000 mg of calcium and 500 IU of vitamin D reversed steroid-induced bone loss, causing a net bone gain. 47 There is some evidence that essential fatty acids may enhance the effectiveness of calcium. In one study, 65 postmenopausal women were given calcium along with either placebo or a combination of omega-6 fatty acids (from evening primrose oil ) and omega-3 fatty acids (from fish oil ) for a period of 18 months. At the end of the study period, the group receiving essential fatty acids had higher bone density and fewer fractures than the placebo group. 48 However, a 12-month, double-blind trial of 42 postmenopausal women found no benefit. 49 The explanation for the discrepancy may lie in the differences between the women studied. The first study involved women living in nursing homes, while the second studied healthier women living on their own. The latter group of women may have been better nourished and already received enough essential fatty acids in their diet.

Premenstrual Syndrome (PMS)

According to a large and well-designed study published in a 1998 issue of American Journal of Obstetrics and Gynecology,calcium supplements are a simple and effective treatment for a wide variety of PMS symptoms. 50 In a double-blind, placebo-controlled study of 497 women, 1,200 mg daily of calcium as calcium carbonate reduced PMS symptoms by half over a period of three menstrual cycles. These symptoms included mood swings, headaches, food cravings, and bloating. These results corroborate earlier, smaller studies. 51

High Cholesterol

In a 12-month study of 223 postmenopausal women, use of calcium citrate at a dose of 1 g daily improved the ratio of HDL (“good”) cholesterol levels to LDL (“bad”) cholesterol levels. 52 The extent of this improvement was statistically significant (as compared to the placebo group), but not very large in practical terms. Similarly modest benefits were seen in a previous, smaller double-blind, placebo-controlled study. 53 A third double-blind, placebo-controlled study failed to find any statistically significant effects. 54

Colon Cancer

Evidence from observational studies showed that a high calcium intake is associated with a reduced incidence of colon cancer , 55 but not all studies have found this association. 56 Some but not all evidence from intervention trials supports these findings. 57 A 4-year, double-blind, placebo-controlled study followed 832 individuals with a history of colon polyps. 58 Participants received either 3 g daily of calcium carbonate or placebo. The calcium group experienced 24% fewer polyps overall than the placebo group. Since colon polyps are the precursor of most colon cancer, this finding strongly suggests benefit. Combing the results for two trials, involving a total of 1,346 subjects also with a history of polyps, researchers found that 1,200 mg or 2,000 mg of daily elemental calcium led to a significant reduction in polyp recurrence compared to placebo over a 3-4 year period. 59 Another, large-sized study found that calcium carbonate at a dose of 1,200 mg daily may have a more pronounced effect on dangerous polyps than on benign ones. 60 However, a gigantic (36,282 participant), very long-term (average 7 years) study of postmenopausal women failed to find that calcium carbonate supplements at a dose of 1,000 mg daily had any effect on the incidence of colon cancer. 61 Given these conflicting results, if calcium supplementation does have an effect on colon cancer risk, it is probably small.


A very large randomized, placebo-controlled trial of over 36,000 postmenopausal women found daily supplementation with 1,000 mg of calcium plus 400 IU of vitamin D did not reduce or prevent hypertension during 7 years of follow-up. These results are possibly limited by non-study calcium use. 62


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