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Sources of calcium

Food sources

Average dietary intakes of calcium in the U.S. are well below the adequate intake (AI) recommendation for every age and gender group, especially in females. Only about 25% of boys and 10% of girls ages 9 to 17 are estimated to meet the AI recommendations. Dairy foods provide 75% of the calcium in the American diet. However, it is typically during the most critical period for peak bone mass development that adolescents tend to replace milk with soft drinks (1, 3). Dairy products represent rich and absorbable sources of calcium, but certain vegetables and grains also provide calcium. However, the bioavailability of that calcium must be taken into consideration. While the calcium rich plants in the kale family (broccoli, bok choy, cabbage, mustard, and turnip greens) contain calcium that is as bioavailable as that in milk, some food components have been found to inhibit the absorption of calcium. Oxalic acid, also known as oxalate, is the most potent inhibitor of calcium absorption, and is found in high concentrations in spinach and rhubarb and somewhat lower concentrations in sweet potato and dried beans. Phytic acid is a less potent inhibitor of calcium absorption than oxate. Yeast possess an enzyme (phytase) which breaks down phytic acid in grains during fermentation, lowering the phytic acid content of breads and other fermented foods. Only concentrated sources of phytate such as wheat bran or dried beans substantially reduce calcium absorption (1). The table below lists a number of calcium rich foods, along with their calcium content, and the number of servings of that food required to equal the absorbable calcium from one glass of milk (8). For more information on the nutrient content of foods you eat frequently, search the United States Department of Agriculture (USDA) food composition database. 

Supplements

Most experts recommend obtaining as much calcium as possible from foods because calcium in foods is accompanied by other important nutrients that assist the body in utilizing calcium. However, calcium supplements may be necessary for those who have difficulty consuming enough calcium from foods. No multivitamin/multimineral tablet contains 100% of the recommended daily value (DV) for calcium because it is too bulky, and the resulting pill would be too large to swallow.  The "Supplement Facts" label, now required on all supplements marketed in the U.S., lists the calcium content of the supplement as elemental calcium. Calcium preparations used as supplements include calcium carbonate, calcium lactate, calcium gluconate, calcium citrate, and calcium citrate malate. To determine which calcium preparation is in your supplement, you may have to look at the ingredient list. Calcium carbonate is generally the most economical calcium supplement. To maximize absorption, take no more than 500 mg of elemental calcium at one time. Most calcium supplements should be taken with meals, although calcium citrate and calcium citrate malate can be taken anytime (36).

Lead in calcium supplements: Several years ago concern was raised regarding the lead levels in calcium supplements obtained from natural sources (oyster shell, bone meal, dolomite). In 1993, investigators found measurable quantities of lead in most of the 70 different preparations they tested (37). Since then, manufacturers have made an effort to reduce the amount of lead in calcium supplements to less than 0.5 micrograms (mcg)/1,000 mg of elemental calcium. The federal limit is 7.5 mcg/1,000 mg elemental calcium. Because lead is so widespread and long lasting on earth, no one can guarantee entirely lead-free food or supplements. A recent study found measurable lead in 8 out of 21 supplements, in amounts averaging between 1 and 2 mcg/1,000 mg of elemental calcium (38). Calcium inhibits intestinal absorption of lead, and adequate calcium intake is protective against lead toxicity, so trace amounts of lead in calcium supplementation may pose less of a risk of excessive lead exposure than inadequate calcium consumption. While most calcium sources today are relatively safe, looking for supplements that are labeled "lead-free," and avoiding large doses of supplemental calcium (more than 1,500 mg/day) are ways to avoid incidental lead exposure.

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