Calcium supplements most often contain either calcium carbonate or calcium citrate, which are calcium salts. Sometimes, they contain both compounds. Calcium carbonate and calcium citrate have different amounts of elemental calcium, which is the actual amount of usable calcium in a supplement. Specifically, calcium carbonate has about 40 percent elemental calcium, meaning that 500 mg of calcium carbonate actually contains 200 mg of elemental calcium or 20%DV. In contrast, calcium citrate has approximately 21 percent elemental calcium. Therefore, nearly twice as much calcium citrate is needed to obtain the equivalent amount of elemental calcium as in calcium carbonate. Calcium supplements may also contain other calcium salts, but the body may not be able to use the calcium in these compounds. As with food labels, you should look at the Nutrition Facts label on a supplement to determine how much calcium it contains.

Is it safe to take calcium supplements?

For most people, it is safe to eat foods containing calcium and to take calcium supplements that together do not exceed the tolerable upper intake level of 2.5 grams of calcium per day. This upper level for daily calcium intake in adults is the highest level that likely will not pose risks of unwanted side effects in the general population. The upper level of 2.5 grams a day is an average recommendation for all healthy people who are older than a year, regardless of gender.

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Consuming too much calcium—in excess of 5 grams a day, or 3 grams a day in people with existing kidney problems (4)—can lead to several harmful side effects. Most of these side effects result from people taking too many calcium supplements. Rare harmful side effects from excess calcium include kidney stones (5), hypercalcemia (too much calcium in the blood), and kidney failure (1). In addition, excessive consumption of milk (which is high in calcium) and some types of antacids, especially antacids containing calcium carbonate or sodium bicarbonate (baking soda), over a long period of time can cause milk-alkali syndrome, a condition that can also lead to calcium deposits in the kidneys and other tissues and to kidney failure (4, 6, 7).

Is there evidence that calcium may help reduce the risk of colorectal cancer?

The results of epidemiologic studies regarding the relationship between calcium intake and colorectal cancer risk have not always been consistent.

In the American Cancer Society’s Cancer Prevention Study II Nutrition Cohort, the diet, medical history, and lifestyle of more than 120,000 men and women were analyzed (8). Men and women who had the highest intakes of calcium through both their diet and supplement use had a modestly reduced risk of colorectal cancer compared with those who had the lowest calcium intakes. However, the benefit from calcium appeared to plateau, or level off, at an intake of approximately 1200 mg per day. When calcium from the diet was analyzed by itself, no reduction in colorectal cancer risk was found. However, the use of calcium supplements in any amount was associated with reduced risk. This association was strongest (a 31 percent reduction in risk) for people who took calcium supplements of 500 mg per day or more.

A stronger relationship between calcium intake and colorectal cancer risk was found when participants of the Nurses’ Health Study and the Health Professionals Follow-up Study were combined in an analysis that included more than 135,000 men and women (9). Individuals who had a calcium intake of more than 700 mg per day had a 35 percent to 45 percent reduced risk of cancer of the distal (lower) part of the colon than those who had a calcium intake of 500 mg or less per day. No association was found between calcium intake and risk of cancer of the proximal (middle and upper) part of the colon (9). Another large study of Finnish men showed a similar relationship between higher calcium intake and reduced risk of colorectal cancer (10). This study, however, did not evaluate proximal and distal colorectal cancers separately.

In a study that included more than 61,000 Swedish women, colorectal cancer risk was approximately 28 percent lower among individuals who had the highest calcium intakes (approximately 800–1000 mg per day) compared with those with the lowest calcium intakes (approximately 400–500 mg per day) (11). Data from this study also suggested that the benefit associated with calcium was limited to the distal colon (11). In a study that involved more than 34,000 postmenopausal Iowa women, high intakes of calcium (approximately 1280 mg per day or more) compared with lower calcium intakes (approximately 800 mg per day or less) from both the diet and supplements were associated with a 41 percent reduction in risk of rectal cancer (12). Reduced risks of rectal cancer were also observed for dietary calcium alone and supplemental calcium alone, but these associations were not statistically significant (12).

In an analysis involving more than 293,000 men and 198,000 women in the National Institutes of Health-American Association of Retired Persons (NIH-AARP) Diet and Health Study, high intakes of total calcium, dietary calcium, and supplemental calcium were associated with an approximately 20 percent lower risk of colorectal cancer among men and an approximately 30 percent lower risk of colorectal cancer among women (13).