US Pharm. 2015;40(9):HS-28-HS-32.

Calcium is one of the most important nutritional elements for optimal bone and dental health. Several studies suggest that calcium, along with vitamin D, may have benefits beyond bone health, and it is generally accepted that the heart, muscles, and nerves also need calcium to function properly. Millions of women in the United States take calcium supplements in an attempt to boost bone strength, especially after menopause when the risk of fractures increases. Patients with rheumatoid arthritis and other inflammatory forms of the disease also routinely take calcium supplements.

Most people get enough calcium through their diets. However, those who do not may need to take calcium supplements. It is important for individuals to know how much calcium they need and what types of supplements are the most appropriate.1

Calcium supplements are not for everyone. For instance, people who have a health condition that causes excess calcium in their bloodstream (hypercalcemia) should avoid calcium supplements. Too much or too little calcium, whether through diet or supplements, could be problematic for these individuals.1

In this article, we briefly discuss daily human calcium requirements, types of calcium supplements, nutritional considerations of calcium, and problems with too little or too much calcium intake.

Types of Calcium Supplements

The two main forms of calcium supplements are carbonate and citrate.2 Calcium carbonate is the least expensive and, therefore, is a practical option. Calcium supplements contain several different kinds of calcium salts. Each salt contains varying amounts of elemental calcium. The most common calcium supplements are labeled as calcium carbonate (40% elemental calcium); calcium citrate (21% elemental calcium); calcium lactate (13% elemental calcium); and calcium gluconate (9% elemental calcium).

In addition, some calcium supplements are combined with vitamin D or magnesium. Product labels should be read carefully and the supplement ingredients checked to see which form and amount of calcium are present in the product. This information is important if a person has any health or dietary concerns.2

Administration and Dosage

The daily requirement of calcium depends on age and sex. The body’s bone mass peaks between the ages of 18 and 25 years and declines slowly thereafter. The daily calcium recommended dietary allowance (RDA) of calcium for adult males is as follows: aged 19 to 70 years 1,000 mg, and aged >71 years 1,200 mg. The RDA of calcium for females aged 19 to 50 years is 1,000 mg, while for females aged >51 years the RDA rises to 1,200 mg.

People should not take more than 1,200 mg of calcium a day (in supplement form) unless instructed by a doctor or dietitian. On average, the majority of Americans get between 750 mg and 900 mg of calcium daily through diet alone.

It is now known that vitamin D (calciferol) has a big role in calcium absorption. Before 1997, the RDA of vitamin D taken with calcium was 200 IU (international units) for those up to age 50 years, 400 IU for people aged 51 to 70 years, and 600 IU for those >70 years. The requirements increase with age because older skin produces less vitamin D. These recommendations have since increased, as discussed below.2

Calcium Deficiency

Conditions associated with calcium deficiency include hypoparathyroidism, achlorhydria, chronic diarrhea, vitamin D deficiency, steatorrhea, sprue, pregnancy and lactation, menopause, pancreatitis, renal failure, alkalosis, and hyperphosphatemia. Administration of certain drugs (e.g., some diuretics, anticonvulsants) may sometimes result in hypocalcemia, which may warrant calcium-replacement therapy.3

People who follow vegan diets, have lactose intolerance and limit dairy products, eat large amounts of protein or sodium, have osteoporosis, have undergone long-term treatment with corticosteroids, or have certain bowel or digestive diseases that decrease their ability to absorb calcium, such as inflammatory bowel disease or celiac disease, are also at risk for low calcium intake. In these situations, calcium supplements may help people meet their calcium requirements.3

Calcium Sources

Calcium supports the development and preservation of bone mass to prevent fractures associated with osteoporosis and must be taken from natural sources or supplementation. Calcium is found in dairy products and in a variety of nondairy products, including dark green leafy vegetables, grains, figs, fish with soft bones, and calcium-fortified foods. Even with healthy eating and a balanced diet, one may not get enough calcium daily.

Some other natural sources of calcium are coral calcium and oyster shell calcium. Coral calcium is a form of calcium carbonate that comes from fossilized coral sources. The human body undergoes a natural process known as chelating, in which it combines calcium with another material (e.g., an amino acid) that the body can metabolize. Coral calcium is also used in maxillofacial surgery and bone grafting.2,4

Calcium and Vitamin D: A major role of vitamin D is to help the body absorb calcium and maintain bone density. For this reason, some calcium supplements are combined with vitamin D. This vitamin is available in two forms, vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). The D2 form of the vitamin has a shorter shelf life compared to the D3 form.5

A few foods are known to have small amounts of vitamin D, such as canned salmon with bones and egg yolks. Vitamin D can also be acquired from fortified foods and produced naturally through sun exposure. The RDA for vitamin D is 600 IU a day for persons aged <70 years and for pregnant or breastfeeding women, and 800 IU for those aged >71 years.

Calcitriol (Rocaltrol) is the biologically active form of vitamin D that is used to treat and prevent low levels of calcium in the blood of patients whose kidneys or parathyroid glands are not functioning normally.

Calcium and Vitamin K2: Vitamin K2 has several isoforms or analogues called MK-4 to MK-10. This vitamin provides major protection from osteoporosis and pathologic calcification of the arteries and soft tissues—a major known consequence of aging. Vitamin K2 is found in animals and bacteria, including beneficial probiotic bacteria from the gastrointestinal tract. Antibiotics interfere with normal growth of healthy bacteria and impact vitamin K2 production.4,5

Although vitamin D3 has been known as the bone vitamin because it puts the osteocalcin gene into action and acts swiftly on bones, the slower-acting vitamin K2 has been recognized as being just as important for bone maintenance. The human skeleton is fully replaced every 8 to 10 years with good, dense bone, and these two vitamins play a large role in the process. The oral osteoporosis treatment dose of vitamin K2 is 45 mg a day.4

Nutritional Considerations

The following factors must be considered in selecting a calcium supplement.5,6

Elemental Calcium: Elemental calcium is what the body absorbs for bone growth and other health benefits; therefore, the actual amount of calcium in the supplement is very important. The label on calcium supplements is helpful in determining how much calcium is contained in one serving (number of tablets). For example, 1,250 mg of calcium carbonate contains 500 mg of elemental calcium (40%).

Supplement Choice: Some people cannot tolerate certain calcium supplements owing to side effects such as gas, constipation, and bloating. One may need to try a few different brands or types of calcium supplement to find the one that he or she can tolerate best. In general, calcium carbonate is the most constipating supplement, but it contains the highest amount of calcium and is the least expensive. Calcium phosphate does not cause gas or constipation, but it is more expensive than calcium carbonate. Calcium citrate is the most easily absorbed and does not require stomach acid for absorption, but it is expensive and does not contain much elemental calcium. Women should meet their calcium needs through both their diet and supplements.

Calcium supplements are available in a variety of dosage forms, including chewable tablets, capsules, liquids, and powders. Individuals who have trouble swallowing tablets can use chewable or liquid calcium supplements.

Drug Interactions: Calcium supplements may interact with many different prescription medications, including blood pressure medications (calcium channel blockers), synthetic thyroid hormones, bisphosphonates, and antibiotics. Pharmacists are the best professionals to consult about possible drug interactions and for calcium supplement recommendations.

Bioavailability: The human body must be able to absorb calcium so that it is bioavailable and effective. Calcium supplements should be taken in small doses (500 mg at a time) and preferably at mealtime to increase absorption. Calcium citrate is absorbed equally with or without food and is a form recommended for individuals with inflammatory bowel disease or people who have low stomach acid (individuals aged >50 years or those who are taking antacids or proton pump inhibitors).

Cost and Quality: The Federal Trade Commission holds supplement manufacturers responsible for ensuring that their supplements are safe and their claims are truthful. Many companies may have their products independently tested based on the U.S. Pharmacopeia (USP) standards. Supplements that bear the USP abbreviation meet standards for quality assurance.

Calcium Supplementation and Cardiovascular Effects

Some concerns have been raised about the potential adverse effects of high calcium intake on cardiovascular health among the elderly due to calcification of the arteries and veins. There are several possible pathophysiological mechanisms for these effects, which include effects on vascular calcification, function of vascular cells, and blood coagulation. However, newer studies have found no increased risk of heart attack or stroke among women taking calcium supplements during 24 years of follow-up.7

Some scientists believe that because calcium supplements produce small reductions in fracture risk and a small increase in cardiovascular risk, there may be no net benefits from their use. They claim that since food sources of calcium appear to produce similar benefits on bone density and have not been associated with adverse cardiovascular effects, they may be preferable to supplements. More studies are required to prospectively analyze the effect of calcium or calcium plus vitamin D supplementation beyond bone health. The medical community is still uncertain as to the effects of calcium supplements in women.8

Scoring Coronary Artery Calcium Levels

Calcium deposits can be found in many parts of the body at higher ages. A coronary calcium scan is typically done to check for the buildup of calcium in plaque on the walls of the arteries of the heart. Coronary calcium scan scores range from 0 to more than 400. A calcium score of zero means no identifiable plaque, while a score of above 400 indicates extensive atherosclerotic plaque and significant coronary narrowing.9

Calcification of the artery walls is common at age >65 years. Calcification of the breast is often seen in women above the age of 50 years. Calcium deposits are easily detected by x-ray images because calcification is composed of calcium phosphate, similar to that in bone.

Coronary calcium is part of the development of atherosclerosis; it occurs exclusively in atherosclerotic arteries and is absent in normal vessel walls. The amount of calcium in the walls of the coronary arteries, assessed by a calcium score, appears to be a better cardiovascular disease risk predictor than standard factors.9

Achieving Balance

Risks of Low Calcium Intake: As mentioned above, calcium is important for healthy bones and teeth, as well as for normal muscle and nerve function. There are health problems associated with low calcium levels: Children may not reach their full potential adult height, and adults may have low bone mass, which is a risk factor for osteoporosis and hip fracture. Normal blood calcium levels are maintained through the actions of parathyroid hormone, the kidneys, and the intestines. The normal adult value for serum calcium is 4.5 to 5.5 mEq/L.10  

Approximately 40% of serum calcium is ionized (free), while the other 60% is complexed, primarily to albumin. Only ionized calcium is transported into cells and metabolically active. Decreases in the ionized (free) fraction of calcium cause various symptoms. Hypocalcemia, or low-level calcium, most commonly occurs with low calcium absorption, vitamin D or K2 deficiency, chronic renal failure, and hypoparathyroidism.10

Risks of High Calcium Intake: Many factors can increase blood calcium levels. Although the body has a built-in regulatory process for calcium absorption and maintenance, underlying diseases, medication interactions, or overuse of supplements can cause high calcium levels.

An abnormally high calcium concentration can cause damaging health problems and requires medical treatment. Although dietary calcium is generally safe, excessive calcium does not provide extra bone protection. In fact, if calcium from diet and supplements exceeds the tolerable upper limit, it could cause kidney stones, prostate cancer, constipation, calcium buildup in blood vessels, and impaired absorption of iron and zinc.

Taking calcium supplements and eating calcium-fortified foods may increase calcium above normal levels. As a result, it is very important to stick to the RDA and not exceed the recommended dosage.10


The best way to treat calcium deficiency is to prevent its occurrence. Modification of risk factors is imperative, and pharmacists can play a large role in this area. They can recommend appropriate calcium and vitamin D supplements. Individuals, particularly women, at risk of low calcium should take foods and drinks rich in calcium and vitamin D, quit smoking, and increase weight-bearing and muscle-strengthening exercise. Monitoring one’s body mass index at higher ages is also critical to reducing bone fractures.


1. Bailey RL, Dodd KW, Goldman JA, et al. Estimation of total usual calcium and vitamin D intakes in the United States. J Nutr 2010;140(4):817-822.
2. Straub DA. Calcium supplementation in clinical practice: a review of forms, doses, and indication, Nutr Clin Pract. 2007;22(3):286-296.
3. Xiao Q, Murphy RA, Houston DK, et al. Dietary and supplemental calcium intakes in relation to mortality from cardiovascular diseases in the NIH-AARP Diet and Health Study. JAMA Intern Med. 2013;173(6):639-648.
4. Bunyardatavej N, Buranasinsup S. Calcium supplements: humanity’s double-edged sword. J Med Assoc Thai. 2011;94(suppl 5):S56-S58.
5. Saljoughian M. The emerging role of vitamin K2. US Pharm. 2012;37(1):HS-12–HS-14.
6. Baun L, Russell TM. Overview of the management of osteoporosis in women. US Pharm. 2011;36(9):30-36.
7. Hsia J, Heiss G, Ren H, et al. Calcium/vitamin D supplementation and cardiovascular events. Circulation. 2007;115(7):846-854.
8. Persy V, D’Haese P. Vascular calcification and bone disease: the calcification paradox. Trends Mol Med. 2009;15:405-416.
9. Otton JM, Lonborg JT, Boshell D, et al. A method for coronary artery calcium scoring using contrast-enhanced computed tomography. J Cardiovasc Comput Tomogr. 2012;6:37-46.
10. National Institutes of Health, Office of Dietary Supplements. Dietary supplement fact sheet: calcium. Accessed May 30, 2015.

To comment on this article, contact