Calcium (chemical symbol: 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.


Calcium is essential for:


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 occurs in the form of positively charged calcium ions, Ca2+. 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 resulting 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.

CT scan of thoracic spine with osteoporosis.

CT scan of thoracic spine with osteoporosis.
(Callista Images/CULTURA/Science Source)



Recommended dietary allowance (mg)

Tolerable upper intake level (mg)

Children 0-6 mos.

200 (AI)


Children 7-12 mos.

260 (Al)


Children 1-3 yrs.



Children 4-8 yrs.



Children 9-13 yrs.



Adolescents 14-18 yrs.



Adults 19-50 yrs.



Adults, male, 51-70 yrs.



Adults, female, 51-70 yrs.



Adults 71 + yrs.



Pregnant women 18≤ yrs.



Pregnant women 19≥ yrs.



Breastfeeding women 18≤ yrs.



Breastfeeding women 19≥ yrs.



AI = Adequate intake

mg = milligram

SOURCE: Office of Dietary Supplements. National Instutes of Health. “Calcium: Fact Sheet for Health Professionals” U.S. National Institutes of Health. U.S. Department of Health and Human Services. (accessed April 13, 2018).

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 a 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 lactating women. The Food and Nutrition Board (FNB) of the IOM regularly reviews and, as necessary, revises the recommended RDAs, AIs, and ULs for all categories of men and women, boys and girls.

The current RDAs for calcium, as established by the FNB, are as follows:

The U.S. RDAs tend to be higher than international recommended values. For example, the requirement for adults in the United Kingdom is 700 mg per day. Recommended dietary intakes in Australia are similar for adults but less for children (500 mg for children aged 1–3, 700 mg for children 4–8, and 1,000 mg for children 9–11).

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 fact will be indicated on the label.

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

Sources of calcium.

Sources of calcium.

It is important to note that although a food may contain a significant amount of calcium, there is a difference between the total calcium content of a food and the amount that is available to the body (bioavailability). Other components present in foods may inhibit the body's access to those foods' calcium supplies. For example, oxalic acid, which is present in spinach, limits the absorption of calcium. So, although spinach is relatively high in calcium, not all of that calcium is absorbed by the body. Kale, on the other hand, contains less calcium than spinach but its calcium content is more easily absorbed in the body.

Although experts recommend that people meet as many of their vitamin and mineral needs as possible through diet, it is difficult for many people to get enough calcium from food alone. This is especially true for vegans, who eat no dairy products, adolescents who may choose to drink sodas rather than milk, 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 absorb calcium citrate more easily. 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 in the morning and evening.

Calcium deficiency

Calcium deficiency, called hypocalcemia, can occur because of inadequate calcium intake, excess calcium excretion by the kidneys (usually caused by kidney damage), the inability to adequately absorb calcium, or 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 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.


People of all ages, races, and genders 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 who are at an especially high risk of low dietary calcium intake should investigate taking a calcium supplement.


Absorption of calcium is affected by several conditions.

Dietary Reference Intakes (DRI)—
A system of nutritional recommendations used in the United States and Canada that includes adequate intakes (AI) and recommended dietary allowances (RDAs). The DRIs are determined by the Institute of Medicine (IOM) of the U.S. National Academy of Sciences.
Any of several chemicals dissolved in blood and other bodily fluids that are capable of conducting an electric current. The most important electrolytes in humans and other animals are sodium, potassium, calcium, magnesium, chloride, phosphate, and hydrogen carbonate. Electrolytes control the fluid balance of the body and are important in muscle contraction, energy generation, and almost all major biochemical reactions in the body.
A chemical messenger produced by the body that is involved in regulating specific bodily functions such as growth, development, reproduction, metabolism, and mood.
A condition marked by abnormally high levels of calcium in the blood.
Thinning of the bones with reduction in bone mass due to depletion of calcium and bone protein. Osteoporosis predisposes a person to fractures, which are often slow to heal and heal poorly. It is more common in older adults.

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

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



Calcium supplements may cause gas, nausea, and abdominal discomfort. Taking the supplement with meals, taking smaller doses spread out over the day, or changing the type of supplement usually solves this problem.

Parental concerns

Building strong bones starts in childhood, and parents should be aware of how much calcium their children need compared to how much they are getting. As children get older, they tend to replace milk in their diet with juice, bottled water, and especially carbonated soft drinks. This practice leads to large calcium deficiencies during adolescence. Parents should monitor children's diet and encourage them to take calcium supplements if they cannot induce them to eat more dairy products and other calcium-rich food.

See also Adolescent nutrition ; Adult nutrition ; Arthritis diet ; Childhood nutrition ; Diet and disease prevention ; Dietary supplements ; Menopause diet ; Osteoporosis diet ; Senior nutrition ; Vitamin D .



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Academy of Nutrition and Dietetics, 120 South Riverside Plz., Ste. 2000, Chicago, IL, 60606-6995, (312) 899-0040, (800) 877-1600,, .

International Food Information Council Foundation, 1100 Connecticut Ave. NW, Ste. 430, Washington, DC, 20036, (202) 296-6540,, .

NIH Osteoporosis and Related Bone Diseases National Resource Center, 2 AMS Circle, Bethesda, MD, 20892-3676, (202) 223-0344, TTY: (202) 466-4315, (800) 624-BONE (2663), Fax: (202) 293-2356,, .

Office of Dietary Supplements, National Institutes of Health, 6100 Executive Blvd., Rm. 3B01, MSC 7517, Bethesda, MD, 20892-7517, (301) 435-2920, Fax: (301) 480-1845,, .

Tish Davidson, AM
Revised by David Newton

  This information is not a tool for self-diagnosis or a substitute for professional care.