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Other name(s):

calcium carbonate, calcium citrate, calcium glubionate, calcium gluconate, calcium lactate, calcium phosphate, tricalcium phosphate

General description

Calcium is the most abundant mineral in the human body. It’s also the main component in bone. Most calcium is found in the bones in a mineral form. This is called calcium hydroxyapatite.

Calcium is a building block of bone and tooth enamel. It’s in bone and enamel as calcium hydroxyapatite and other forms. Calcium is also needed for muscle contraction and relaxation. It plays a role in sending nerve impulses and controlling nerve irritability. It also helps to clot blood. Calcium is also an enzyme cofactor. It aids the endocrine and exocrine glands.

Medically valid uses

Calcium is extra important during times of bone growth. These include childhood and adolescence. It’s also needed during pregnancy, when breastfeeding, and after menopause.

You need calcium to make breast milk. A fetus also needs it to form.

Not getting enough calcium during puberty and adulthood may lead to osteopenia and osteoporosis. These conditions cause weak bones that may break easily.

Unsubstantiated claims

Please note that this section reports on claims that have not yet been substantiated through scientific studies.

Calcium may lower the risk of colorectal cancer. It may also help reduce the risk of heart disease and lower blood pressure. It may also ease menstrual cramps and help manage weight. Calcium is also said to prevent premature labor and birth.

Recommended intake

Calcium is measured in milligrams (mg). It’s absorbed by the small intestine. However, only part of the calcium in foods and supplements is absorbed.

The Recommended Dietary Allowance (RDA) by age is as follows:



Infants (0 to 6 months)

200 mg

Infants (6 months to 1 year)

260 mg

Children (1 to 3 years)

700 mg

Children (4 to 8 years)

1,000 mg

Youth (9 to 18 years)

1,300 mg

Adults (females 19 to 50 years; males 19 to 70 years)

1,000 mg

Adults (70+ years)

1,200 mg


50 years and older

1,200 mg

Pregnant or lactating, 18 years or younger

1,300 mg

Pregnant or lactating, 19 to 50 years

1,000 mg

Calcium citrate is more easily absorbed than other forms of calcium. People with low stomach acid should take this form. This includes seniors.

Calcium is available in oral tablets. Strengths range from 250–1,200 mg. It also comes as oral chews, capsules, powders, wafers, and liquids.

Calcium exists in nature only with other substances called compounds. These compounds contain different amounts of elemental calcium. This is the actual amount of calcium in the supplement. Calcium is found in many commercial products. Each form has a different percentage of calcium. Calcium carbonate and calcium phosphate contain the highest concentrations of calcium. Calcium gluconate and calcium glubionate have the lowest.

Calcium-rich foods


Calcium (mg)

Fortified oatmeal, 1 packet


Sardines, canned in oil, with edible bones, 3 oz.


Cheddar cheese, 1.5 oz. shredded


Milk, nonfat, 1 cup


Milkshake, 1 cup


Yogurt, plain, low-fat, 1 cup


Soybeans, cooked, 1 cup


Tofu, firm, with calcium, 1/2 cup


Orange juice, fortified with calcium, 6 oz.


Salmon, canned, with edible bones, 3 oz.


Pudding, instant (chocolate, banana, etc.) made with 2% milk, 1/2 cup


Baked beans, 1 cup


Cottage cheese, 1% milk fat, 1 cup


Spaghetti, lasagna, 1 cup


Frozen yogurt, vanilla, soft-serve, 1/2 cup


Ready-to-eat cereal, fortified with calcium, 1 cup


Cheese pizza, 1 slice


Fortified waffles, 2


Turnip greens, boiled, 1/2 cup


Broccoli, raw, 1 cup


Ice cream, vanilla, 1/2 cup


Soy or rice milk, fortified with calcium, 1 cup

80–500 (varies)

Calcium is stable in foods. It doesn’t deteriorate with different types of storage or cooking.

People with certain health issues need more calcium. These include malabsorption syndromes, such as sprue, celiac disease, pancreatitis, and cirrhosis. People with lactose intolerance or milk allergies need other dietary sources of calcium. People who consume moderate to heavy amounts of alcohol or caffeine need more calcium. People with anorexia also need more.

Calcium is absorbed best by the body when it’s taken several times a day in amounts of 500 mg or less. But taking it all at once is better than not taking it at all. Calcium carbonate is absorbed best when taken with food. Calcium citrate can be taken anytime.

Calcium supplements are used to treat calcium-deficiency problems. These include:

  • Tetany of the newborn

  • Kidney problems, including end-stage kidney disease

  • Thyroid issues

  • Osteoporosis and softening of the bone (osteomalacia)

  • Rickets

Chronic calcium deficiency leads to poor bone mineralization. It also causes decreased bone growth and repair. In young adults, this may lead to osteomalacia. In older adults and post-menopausal women, it may lead to osteoporosis. In children, it’s linked with rickets. This is often due to both vitamin D and calcium deficiency.

Tetany occurs when the serum level of ionized calcium becomes too low. This causes muscles to contract and stay contracted. This condition happens due to an acute decrease in ionized calcium in the blood. It’s often a result of hyperventilation.

Side effects, toxicity, and interactions

High levels of calcium in the blood is called hypercalcemia. This rarely happens from eating too much calcium from foods. It’s more likely to be caused by calcium supplements. 

Excess calcium intake (2 grams or more) may cause calcium deposits in muscles. This may cause stiffness and pains. Calcium deposits can also show up on heart valves. This can cause fatal heart damage. Calcium deposits are more likely to happen when you also have a high vitamin D intake. Be careful not to take more than 800 mcg/day of vitamin D, especially when taking calcium supplements.

Consuming large amounts of calcium with milk or an antacid can cause milk-alkali syndrome. This can cause hypercalcemia. It can also harm your kidneys.

You shouldn’t take calcium supplements if your serum calcium levels are too high. You also shouldn’t take them if your phosphate levels are too low. Don’t take calcium supplements if you have kidney failure or plan to have dialysis. You also shouldn’t take them if you’re taking thiazide diuretics. These include HCTZ, hydrochlorothiazide, and indapamide.

Calcium from supplements may cause kidney stones. You shouldn’t take calcium if you’re prone to kidney stones.

Women who are pregnant or breastfeeding should talk to their healthcare providers before taking any supplements.

Calcium also interacts with certain medicines. These include:

  • Tetracycline antibiotics

  • Norfloxacin

  • Verapamil

Calcium also reduces how well the body absorbs iron. You shouldn’t take a calcium supplement at the same time as an iron supplement. You can only do so if the calcium supplement is calcium citrate, or if you take the iron supplement with vitamin C. You shouldn’t take any medicine that needs to be taken on an empty stomach with calcium supplements. Calcium competes for absorption with magnesium, iron, and zinc.  High levels of vitamin D may cause you to absorb more calcium.

Additional information

Calcium is the most commonly used supplement. It’s the easiest one to use. It also helps prevent osteoporosis.

Over 40 million people in the U.S. have osteoporosis or have a high risk for the condition.  The following factors increase the risk for osteoporosis in women:

  • Being thin or underweight

  • Having an eating disorder, such as anorexia nervosa

  • Chronic dieting

  • High alcohol or caffeine intake

  • A diet lacking in calcium

Keeping a healthy weight during adolescence can help prevent osteoporosis as an adult. Getting regular exercise can also help. Choose high impact activities, such as running or lifting weights.

Women who get their first menstrual period at a younger age or go through menopause at a later age have a lower risk of osteoporosis. Making sure you get enough calcium and vitamin D throughout your life can also lower your risk of the condition.

Online Medical Reviewer: Brittany Poulson, RD
Online Medical Reviewer: Wilkins, Joanna, R.D., C.D.
Date Last Reviewed: 8/1/2016