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Other common name(s): calcium carbonate, calcium gluconate, calcium citrate

Scientific/medical name(s): Ca, Ca++


Calcium is a mineral that is vital for a number of body functions, such as contracting muscles (including the heart), secreting hormones, and sending messages through the nervous system. Most of the body's calcium is in the bones and teeth, but a small amount circulates in the blood for these important tasks. The human body can't make calcium, so it must be taken in from foods or supplements.

Calcium is found naturally in many dairy products, leafy green vegetables, and fish. In the United States, it is added to many foods, like some brands of orange juice, cereal, and milk substitutes like soy or rice milk. Calcium is the active ingredient in certain antacids.


Many people, especially women, can benefit from keeping up with their calcium intake to help prevent bone problems such as severe bone thinning (osteoporosis). Calcium supplements will not slow the growth of most cancers, but they appear to reduce the risk of pre-cancerous polyps of the colon. Studies of calcium supplements with vitamin D have shown some promise in cancer risk reduction, but results have been mixed. In fact, high doses of calcium have been linked to a higher risk of prostate cancer. Calcium supplements may be important for some people with cancer, depending on their stage, cancer type, and the type of treatments they get.

How is it promoted for use?

Calcium is best known for its role in growing strong bones and teeth in children and teens, and keeping them strong into adulthood and old age. Calcium is less known for helping the heart and other muscles do their work, but some companies promote this function as well.

Calcium is mainly promoted for bone health. The United Sates Food and Drug Administration (FDA) has approved certain limited health claims about calcium supplements. Now, companies that sell calcium supplements can legally make statements such as these:

  • Adequate calcium throughout life, as part of a well-balanced diet, may reduce the risk of osteoporosis.
  • Adequate calcium and vitamin D as part of a healthful diet, along with physical activity, may reduce the risk of osteoporosis in later life.

At one time, certain types of calcium were promoted as cures and preventions for many types of diseases. More makers of calcium now stick to what is better known about calcium, that it can help promote bone density, and protect bone health.

Some makers of calcium promote their formulas as better than others in some way. For example, certain companies say that extra minerals in the calcium make it work better. Others may state that calcium from coral has more helpful health effects or that calcium made from algae is absorbed more easily by the body.

What does it involve?

Calcium intake from healthy foods is the best source, but calcium supplements can be found in drug stores, grocery stores, health food stores, and via the Internet. Supplements are most often made from calcium carbonate, though calcium citrate and other compounds are also sold. Calcium carbonate is also the form found in antacids like Tums® and Rolaids®.

The Institute of Medicine updated its calcium requirements in 2010. The new Recommended Dietary Allowance (RDA) for calcium is 1,000 milligrams per day for men and women aged 19 through 50, and for men between 50 and 70. The RDA is 1,200 milligrams per day for women over 50 and men over 70. Some nutritionists and doctors recommend that calcium supplements be taken with supplements of vitamin D and other important minerals, such as magnesium and potassium. And vitamin D is required to be able to absorb calcium into the body. (See our document, Vitamin D.)

The body's use of calcium is complex. It is affected by many hormones and factors other than how much calcium a person eats or takes in. There are many ways to treat bone problems and calcium imbalance in the body, depending on their cause.

The best source of calcium is a balanced diet, which helps to avoid bone problems and decreases the risk of some types of cancer. Foods and beverages high in calcium include milk and other dairy products (low-fat products are healthier), leafy green vegetables such as broccoli and greens, nuts, seeds, beans, tofu prepared with calcium, cheese, dried figs, kelp, oysters, and canned fish that can be eaten with the bones still in it, such as sardines and salmon. Certain brands of cereals, orange juices, and soy and rice milks are fortified with calcium and are clearly marked as such on the label.

What is the history behind it?

The importance of calcium for maintaining overall health and promoting bone strength has been known for decades. Some scientists believe that humans became accustomed to diets high in calcium as far back as 35,000 years ago. Calcium's role in preventing or slowing the growth of cancer has only become a subject of research within the last 20 years or so.

What is the evidence?

A number of important studies to measure calcium's impact on cancer have been published in peer-reviewed medical journals. Several studies have suggested that foods high in calcium might help reduce the risk for colorectal cancer. These studies also suggest that calcium supplements may somewhat reduce the risk of adenomas, which are a type of polyps in the colon and rectum that can progress to cancer. More recent studies have noted that women with higher dietary calcium intake seemed to have lower risk of breast cancer. This risk was not affected by calcium supplements.

One researcher reviewed several dozen studies of the effects of calcium on various diseases and concluded that long-term deficiencies in calcium and vitamin D may lead not only to fragile bones, but also to colorectal cancer and high blood pressure in some people. Getting enough calcium may help to prevent these conditions in some.

Randomized clinical trials are now starting to study the role calcium may play in cancer prevention. A Women's Health Initiative study published in 2006 put more than 36,000 menopausal women into 2 groups: half got calcium with vitamin D and half got a placebo (sham pill). After 7 years, the researchers looked at colorectal cancer risk in the 2 groups. Cancer risk was not lower in the group that took calcium. Critics of the study noted that many women were not taking their pills at all. The average calcium and vitamin D intake of the women at the start of the study was also about twice as high as the national average, and close to the doses used in the study. And finally, women in the study were allowed to take extra calcium and vitamin D if they wished. This means that many women who were thought to be on placebo may have taken the same or higher doses as those who were in the test group. These factors may have limited the ability of the study to find any differences. Researchers will keep watching the women until late 2010, so there may be more information at that time. A report on this study from 2009 showed no difference in death rates between the test group (women given calcium and vitamin D supplements) and those who were not given them.

A randomized clinical trial reported in 2007 found lower risk for all cancers combined in women given calcium supplements than in women taking the placebo. Cancer risk was even lower among women taking both calcium and vitamin D. Doses in this study were higher than in the Women's Health Initiative study above.

While further research is needed to clarify the role of calcium in preventing or reversing cancer growth, there is little doubt that adequate calcium intake is required for preventing bone shrinkage and weakening. For people who have cancer, calcium and vitamin D intake may help keep bones strong.

Some chemotherapy medications can reduce appetite, create swallowing difficulties, cause nausea and vomiting, and result in osteopenia (thinning bones). The chemotherapy drugs methotrexate and doxorubicin may directly damage bones. Some hormonal therapies used in the treatment of breast or prostate cancer can also weaken bones. Radiation therapy can cause osteopenia within the area being treated, and the combination of radiation therapy and chemotherapy can cause even greater damage to bone structure. Some cancers also can harm bones. Adequate calcium intake is important for bone health in the general public and in cancer survivors, and especially those with osteoporosis, but calcium intake is not the only factor that affects bone health. Physical activity is also important in keeping bones strong. And there are several kinds of medicines available to treat osteoporosis.

Calcium has also been found helpful in reducing certain symptoms of premenstrual syndrome. There is some early evidence that calcium may play a role in helping to prevent heart disease and reducing insulin resistance in diabetic patients.

Because calcium plays a crucial role in bone mass and strength, its greatest benefit to people with cancer may be to lower the risk of thinning bones or reduced bone mass (osteopenia). This can worsen until the bones get fragile and break easily (osteoporosis). Both conditions are linked closely to aging, and osteoporosis is a common problem for women after menopause. Weak bones can also result from poor nutrition, prolonged drug therapy, disease, and poor mobility, all of which may apply to people who have cancer.

Are there any possible problems or complications?

This product is sold as a dietary supplement in the United States. Unlike companies that produce drugs (which must be tested before being sold), the companies that make supplements are not required to prove to the Food and Drug Administration that their supplements are safe or effective, as long as they don't claim the supplements can prevent, treat, or cure any specific disease.
Some such products may not contain the amount of the herb or substance that is on the label, and some may include other substances (contaminants). Actual amounts per dose may vary between brands or even between different batches of the same brand. In 2007, the FDA wrote new rules to improve the quality of manufacturing for dietary supplements and the proper listing of supplement ingredients. But these rules do not address the safety of the ingredients or their effects on health.

Most such supplements have not been tested to find out if they interact with medicines, foods, or other herbs and supplements. Even though some reports of interactions and harmful effects may be published, full studies of interactions and effects are not often available. Because of these limitations, any information on ill effects and interactions below should be considered incomplete.

For most people, the greatest risk with calcium comes from not getting enough of it. In rare cases, taking in large amounts of calcium (usually more than 2,400 milligrams a day) from dairy foods, supplements, and antacids can lead to a condition called milk alkali syndrome. It throws off the balance between acids and bases in the blood, with high levels of calcium (hypercalcemia), dehydration, and kidney problems. Weakness, mental confusion, irritability, vomiting, and kidney damage can result. Over the long term, too much calcium can also cause kidney stones, muscle pain, red eyes, and deposits of calcium in the muscle tissue.

It is also possible to get high calcium levels from taking in too much vitamin D, usually from supplement overuse. But for people with cancer, high calcium levels are most often caused by the cancer itself, especially in its later stages. Hypercalcemia from any cause is treated as a serious medical problem.

Calcium can keep other minerals such as iron, zinc, magnesium, and phosphorus from being absorbed. Calcium can also prevent or reduce the absorption of certain medicines, so talk with your doctor or pharmacist about all medicines and supplements that you are taking.

Some studies have found that a high calcium intake, mainly through supplements, was linked with a higher risk for prostate cancer. This was especially true for prostate cancers that were more aggressive.

Both men and women should try to take in recommended levels of calcium, mainly through food sources. Dairy products are excellent sources of calcium, as are some leafy vegetables and greens. People who get much of their calcium from dairy products may want to select low-fat or fat-free choices to reduce their intake of saturated fat.

Some health problems can keep calcium from being absorbed from the stomach. Poor vitamin D intake and inactivity decrease calcium absorption. Low stomach acid can cause calcium carbonate to be poorly absorbed, although it doesn't affect other forms of calcium the same way. For those who either have low stomach acid or who take medicine to block stomach acid, calcium citrate absorbs better.

Those who have kidney stones, kidney failure, or high parathyroid levels should talk with their doctors before taking calcium supplements. People who are having treatment for cancer should talk to their doctors or cancer team before taking vitamins, minerals, or other supplements that might interact with the cancer drugs prescribed.

Relying on this type of treatment alone and avoiding or delaying conventional medical care for cancer may have serious health consequences.

Additional resources

More information from your American Cancer Society

The following information on complementary and alternative therapies may also be helpful to you. These materials may be found on our Web site (www.cancer.org) or ordered from our toll-free number (1-800-227-2345).

Dietary Supplements: What Is Safe?

Complementary and Alternative Methods and Cancer

Vitamin D

Placebo Effect

Learning About New Ways to Treat Cancer

Learning About New Ways to Prevent Cancer

The ACS Operational Statement on Complementary and Alternative Methods of Cancer Management


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Note: This information may not cover all possible claims, uses, actions, precautions, side effects or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for consultation with your doctor, who is familiar with your medical situation.

Last Medical Review: 10/21/2010
Last Revised: 12/02/2010