August 3, 2011

Calcium is an important component of bones and teeth. The calcium in bone is constantly removed and replaced, so getting enough calcium is important to maintain strong bones. Calcium is absorbed in the stomach and small bowel, but only about 25% of ingested calcium is absorbed. About half of all adults do not get the recommended daily allowance (RDA) of calcium in their diet, and calcium supplements are needed (see Table).




Optimal daily intake (in mg of calcium)


Birth-6 months 400
6 months-1 year 600


1-5 years 800
6-10 years 800-1,200

Adolescents/young adults

11-24 years 1,200-1,500


25-50 years 1,000
Pregnant and nursing 1,200-1,500

51-65 years (postmenopausal)

Not on estrogen 1,500
On estrogen 1,000
Over 65 years 1,500


25-65 years 1,000
Over 65 years 1,500

Calcium is available as carbonate, citrate, phosphate, gluconate, lactate, oyster shell, bone meal, and dolomite. Consumer Reports has done extensive testing of calcium absorption from hundreds of supplements and found that almost all dissolve well. According to these experts, there is no scientific reason to buy more expensive supplements that make special health claims. They recommended choosing a calcium supplement based on cost. Calcium carbonate, which is found in many antacids, is often the least expensive.

Patients often overestimate the amount of calcium they consume in their supplements. The most important information on the supplement’s label is the amount of elemental calcium. The amount of elemental calcium by weight varies from 9% in calcium gluconate to 40% in calcium carbonate. When reading the label, also check the serving size: the amount of calcium provided is often based on a serving size of two or more pills.

Calcium is most efficiently absorbed when taken in doses of no more than 500-600 mg at one time. Calcium carbonate is absorbed best in an acid environment, and some patients with low stomach acid (e.g., patients who take acid-blocking medication) may not absorb calcium carbonate well. In this circumstance, calcium citrate is preferred since it does not depend on acid for absorption. Calcium carbonate should be taken with meals since food stimulates acid output. For those who cannot take a pill, chewable calcium supplements are available (Tums, Viactiv) as well as liquid preparations (Citracal effervescent tablets). Calcium can cause gastrointestinal (GI) adverse events such as bloating, gas, and constipation. Switching calcium brands or changing from carbonate to citrate can often alleviate this side effect. There has been some concern that calcium supplements may contain trace amounts of lead. However, calcium itself blocks lead absorption into the bloodstream. Also, the amount of lead found in supplements is less than the amount found in many foods. In general, lead levels in calcium tablets are well below levels that are considered unsafe.

Vitamin D is essential for adequate absorption of calcium. The major sources of vitamin D are fortified dairy products (mainly milk) and sunlight. Vitamin D deficiency is common and occurs in up to 20% of people as reported in some studies. Those living in northern latitudes are at higher risk, as are lactose-intolerant people and the elderly. Vitamin D supplementation is often needed. The RDA for vitamin D is 400 IU which is the amount in an adult multivitamin, although many experts believe that 600 IU or, in the elderly, 800 IU is the appropriate dose. Calcium supplements are available with and without vitamin D. People in higher risk groups, especially the elderly, should take vitamin D in a multivitamin or with their calcium supplement.

Several medical conditions affect calcium absorption and excretion. As mentioned above, low stomach acid decreases calcium carbonate absorption, and patients with GI disorders (celiac disease or surgery with removal of portions of the stomach or small bowel) may not absorb enough of the calcium and vitamin D that they take in. Patients taking glucocorticoids like prednisone also have reduced calcium absorption. Parathyroid hormone regulates calcium absorption and excretion, and abnormalities of the parathyroid glands can lead to dangerously high or low levels of calcium in the blood. Some patients lose too much calcium in the urine. This can occur in patients with renal tubular abnormalities and in patients who are on high-sodium diets. Hydrochlorothiazide, a diuretic used for high blood pressure, decreases the excretion of calcium into the urine.

The most obvious and probably the most important benefit of calcium is the prevention and treatment of osteoporosis. There have been many studies of calcium and its effect on bone mass and risk of fracture. Almost all show a beneficial effect on bone mass and several have demonstrated significant reduction in fractures of the spine and hip. It is important to note that calcium alone cannot prevent the accelerated bone loss that occurs in the first 5-6 years of menopause. The relatively sudden loss of estrogen at menopause overwhelms any beneficial effect of calcium. Similarly, calcium and vitamin D are not adequate treatments for patients with established osteoporosis, especially those with fractures. In these patients, other drugs are required in addition to calcium and vitamin D. Some drugs used for osteoporosis block the removal of calcium from the bones, like estrogen, alendronate (Fosamax), risedronate (Actonel), or raloxifene (Evista), while others stimulate new bone formation, like teriparatide (Forteo).

In addition to preventing and treating osteoporosis, calcium supplements appear to lower blood pressure, protect against kidney stones, and may reduce colon cancer. The reduction in kidney stone formation occurs because most stones are made of oxalate. Calcium binds oxalate in the intestines and reduces the amount of oxalate in the urine. People with calcium-containing kidney stones should talk to their doctor before taking calcium supplements.

SEE ALSO: Osteoporosis and osteopenia, Vitamins

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