Increasing Dietary and Supplemental Calcium CME/CE

Release Date: June 16, 2006; Valid for credit through June 16, 2007

Authors: Robert P. Heaney, MDAuthor Information and Disclosures

Credits Available

Physicians - maximum of 0.25 AMA PRA Category 1 Credit(s)™

Nurses - 0.3 ANCC continuing nurse education contact hours (0.0 contact hours are in the area of pharmacology)

Supported by an independant educational grant from Sanofi-Aventis

Dietary Calcium Sources

Calcium-rich foods include the dairy group (milk, hard cheese, yogurt, cottage cheese), broccoli, Chinese cabbage (bok choy), green leafy vegetables (kale, mustard, collards), sardines with bones (canned), dried fruit, nuts and seeds (figs, almonds, soy nuts), and pulses (peas, beans, and lentils). A number of calcium-fortified foods and drinks are also now available, including breakfast bars, cereals, breads, juices, and milk substitutes. An extensive list of the calcium content of foods is available online from the US Department of Agriculture.[1]

Patients often erroneously believe that they are obtaining sufficient calcium through their diet. However, it is known that the median calcium intake in postmenopausal women in North America is substantially below existing recommendations.[2] Because milk and milk products provide the majority of dietary calcium in the United States, if a person is lactose-intolerant, a vegan (consuming no animal products), or avoids dairy products for other reasons, it may be especially challenging to obtain adequate amounts of calcium solely through diet.[3]

In addition to being aware of the amount of dietary calcium they are ingesting, it is important that patients understand that absorption from foods can be affected by a number of factors, many of which have already been well described. These include age, vitamin D, pregnancy, and plant substances in the diet.

Oxalates (found in chocolate and spinach) and phytate (found in whole grains) are among dietary substances that impair absorption. Therefore, 8 cups of spinach are needed to obtain the same amount of calcium obtained from an 8-ounce serving of milk or 1 cup of yogurt, which contain calcium in an easily absorbable form.[4]

Calcium From Supplements

For many patients, calcium supplements are the most appropriate choice to ensure adequate intake. A number of different calcium compounds are used in supplements; the 2 main forms are calcium carbonate and calcium citrate. Although absorption of calcium citrate is similar to calcium carbonate, a calcium carbonate supplement contains 40% calcium vs the 21% found in calcium citrate.[5] The percentage of available calcium in commonly used supplements is: 9% in gluconate; 13% in lactate; 21% in citrate; 38% in tricalcium phosphate; and 40% in carbonate.[4] Because formulations may contain different amounts of calcium, the number of tablets needed to obtain a recommended dose may vary.

When beginning supplementation, gradual intake is best -- taking less than 500 mg a day for a week -- followed by slowly adding calcium to achieve the recommended amount. Most supplements should be taken with food, as the slow gastric emptying following a meal optimizes calcium absorption. Although most brand-name calcium supplements are absorbed easily in the body, chewable and liquid calcium supplements dissolve most easily because they are broken down prior to entering the stomach.

Patients should also be told that calcium supplements can interact with prescription and over-the-counter medications. Medications that may interact with calcium include[4]:

  • digoxin;
  • fluoroquinolones;
  • levothyroxine;
  • antibiotics in the tetracycline family;
  • tiludronate disodium;
  • anticonvulsants such as phenytoin;
  • thiazide, type of diuretic;
  • glucocorticoids;
  • mineral oil or stimulant laxatives; and
  • aluminum- or magnesium-containing antacids.

Ingesting too much calcium can cause hypercalcemia, kidney stones, milk-alkali syndrome, or interfere with absorption of other minerals (iron, zinc, magnesium, and phosphorus). Calcium citrate may be preferred as a calcium source if a patient is at high risk for stone formation.[4] Although some believe that calcium supplementation causes constipation, the evidence base to support this is scant.

Choosing a Supplement

Patients frequently ask healthcare providers for a specific recommendation for which type of supplement to take. General considerations include purity and tolerance.[6]

Purity. Supplements with familiar brand names are typically best. Labels on supplements should include the word "purified" or have the United States Pharmacopoeia (USP) symbol. Calcium from unrefined oyster shell, bone meal, or dolomite without the USP symbol should be avoided, because high levels of lead or other toxic metals may be present.

Tolerance. A calcium supplement may be associated with side effects such as constipation, though constipation may also be a problem associated with aging. If simple measures (such as increasing intake of fluids and fiber) do not resolve the problem, another calcium supplement should be tried.

Specific questions to be considered by patients in choice of a supplement include[6]:

  • Is it convenient -- can they remember to take it as frequently as recommended?
  • Is the cost of the supplement within budget?
  • Is it widely available?

Good online sources of information for patients about calcium and or bone health include the National Institutes of Health, Office of Dietary Supplements [4]; American Osteoporosis Foundation[7]; and the National Institute of Arthritis and Musculoskeletal and Skin Diseases.[6]

Other Factors Important for Bone Health

In addition to calcium there are at least 3 other factors critical for bone health, particularly after mid-life: vitamin D, protein, and exercise.

Vitamin D. It has long been recognized that vitamin D is important for calcium absorption, and recent studies have demonstrated that absorption efficiency increases with improving vitamin D status up to serum 25(OH)D levels of about 80 nmol/L (32 ng/mL).[8,9] Postmenopausal women, as reported in many studies, tend to have average serum 25(OH)D values ranging from 50 to 55 nmol/L (20 to 22 ng/mL)[8,10] and are therefore absorbing the calcium they ingest with reduced efficiency.[8,11]

In the sole fracture study that has evaluated this issue, raising serum 25(OH)D from the typical postmenopausal range up to 75 nmol/L resulted in a 33% reduction in all osteoporotic fractures combined.[10] Among persons in this age range, vitamin D may be acting in several different ways in addition to promoting calcium absorption.[12] Nevertheless, it is clear that inadequate vitamin D status reduces the benefit potentially achievable from an adequate calcium intake.

Protein. As with vitamin D, protein plays an important role in its own right. Although North Americans are considered to consume generous amounts of protein it is also true that many fragile elderly individuals have low protein intake. If these same individuals are our osteoporosis patients, then they will probably not respond well to pharmacotherapy until their nutritional status is repaired. If deficient in calcium, vitamin D, and protein, many will be unresponsive to monotherapy, whether nutritional or pharmacologic. This is seen most obviously in patients with hip fracture, whose outcomes have been shown to improve dramatically with protein supplementation.[12]

Exercise. Bones are designed to bear loads and to resist mechanical forces. Maintenance of adequate bone mass requires continued mechanical loading. Nutrition alone may slow the progress of disuse bone loss, but it will not block its full, ultimate expression. Optimal exercise regimens are uncertain, but impact loading appears to be more osteotrophic than, for example, weight lifting or swimming. In general, patients with osteoporosis need to maintain as vigorous an exercise program as is compatible with their bone fragility status.


The totality of the evidence indicates that high calcium intake is important both for prevention and management of osteoporosis, and recent negative trials do not refute the much larger body of positive studies. The challenge is not to haggle over exactly how much is enough but, as the Surgeon General's report[13] on osteoporosis puts it, to recognize that current intake is far below optimal values. It is imperative to take action to augment calcium intake, both in the general population and particularly in persons being treated for osteoporosis.



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