Calcium Supplements

Calcium Forms, Bioavailability Subject of Industry Discussion

by Kurt Schneider

Calcium is returning to the forefront of the dietary supplement industry. Much of the recent publicity has centered on coral calcium, including supplier claims of higher absorptivity and bioavailability. Coral calcium can be considered 'young' calcium carbonate ore, which is an economical, pure and widely available source of calcium.

Calcium is a mineral often taken for granted in our diets, usually consisting of a daily dose of a multi-vitamin or a calcium supplement. Despite supplementation, osteoporosis--or bone density loss--remains prevalent. The human body needs a source of calcium that can be absorbed at a level effective in fighting osteoporosis. To do so, there are many forms of calcium currently available, such as calcium carbonate, calcium citrate, calcium gluconate, oyster shell, scallop shell, eggshell, milk products and, the latest, coral calcium. While there has been much debate over which type is best, it is clear that whatever form is taken, it needs to be absorbed by the body to provide any benefit.

As calcium is ingested, the acid in the stomach begins to breakdown the molecule into its atomic components. In the case of calcium carbonate, the components are elemental calcium and carbonic acid, which further breaks down into carbon dioxide. At this point the calcium is in its ionic, or positively charged, state. In this state, the calcium can pass through the intestinal tract, into the bloodstream, and into the bones. It is this rate of calcium absorption, or bioavailability, that determines how much of the ingested calcium actually can be used by the body. The higher the bioavailability, the more calcium ions can travel into the body. In addition, the faster the supplement can obtain this ionic state, the faster it becomes available for absorption. Since time is critical, it is important to ensure as much calcium is absorbed before it passes through the body. Studies have shown that as long as the body absorbs enough calcium, the rate of absorption is not indicative of the quality of the calcium. The bioavailability or absorption and uptake of calcium are partly a function of pH. Acidity favors absorption while alkalinity hinders absorption. An increase in acidophilic flora (i.e., L. acidophilus) in the intestine favors calcium absorption.1 With normal gastric function, almost all calcium forms become soluble and bioavailable. Further, studies have shown that solubility of calcium forms does not significantly affect absorption.2 Calcium carbonate is not soluble in water; it is soluble in acid and is well-absorbed in normal gastric function.

Levels of calcium in various products also vary. Calcium percentages range from 9 percent for calcium gluconate to nearly 40 percent for calcium carbonate.3 It would appear that the higher starting level of calcium would provide better absorption and bioavailability; this is not necessarily the case. As the supplement passes through the body, studies have shown there is little difference in bioavailability between all major types of calcium. When taken with meals, calcium carbonate has been shown to be as or more absorbable (39%) as five other forms of calcium, including calcium acetate (32%), calcium lactate (32%), milk (31%), calcium citrate (30%) and calcium gluconate (27%).4

There are other factors that play a role in how well the body absorbs calcium. "Nutrients in foods interact with each other, and the absorption of any nutrient depends, in part, upon the foods that are eaten with it and the nutrients and other substances those foods contain," wrote Suzanne Havala, M.S., R.D., in the Vegetarian Journal. "It also depends upon the body's need for the particular nutrient. Our bodies can adapt to varying dietary conditions; when we need more of many nutrients, the body may compensate and become more efficient at absorbing them."5

Even similar forms of calcium may come from different sources. For example, calcium carbonate can come from a variety of fossilized deposit sources: limestone, precipitated limestone, oyster shell, scallop shell, eggshell, fish bones and coral. According to geologist Jose N. Peralta Villar, the age of the deposit is the only major difference between the sources.6" Some deposits were literally formed yesterday such as those oyster shells, scallops and corals currently living and dying now, adding to the topmost layer of newly forming deposits. Others are millions of years old. Recent deposits are subjected to all elements in the surrounding water, and, with the state of the rivers and oceanic margins becoming more polluted, it may be wise to find a calcium carbonate source that is from a high quality mine, further inland, that is much less susceptible to environmental pollutants. A reef, for example, is made up almost exclusively of animals that use dissolved calcium carbonate in the water to make either an internal or external calcium carbonate structure. As time goes on, the deposit accumulates to various levels. Some deposits are thousands of feet thick while others are just a few. It all depends on the initial environment and time."

The majority of the coral calcium products sold in the United States is made of calcium carbonate from coral reefs, which, apart from age, is no different than calcium carbonate from other sources such as limestone. Some studies have even stated the claimed benefits of coral calcium are derived from a separate compound found in the water, calcium aspartate, and not the coral itself.

If all calcium carbonate sources are basically the same in regard to source and bioavailability, what is the best way for manufacturers to choose a source? Five criteria should be considered: grade, lead level, quality, color and cost. For grade, there are basically just two--food and pharmaceutical--with numerous permutations. For lead level, California Proposition 65 has set the standard for dietary supplement lead intake, and has had a great impact on the calcium carbonate industry. Tablet manufacturers are requiring lower lead levels, with some sources moving into the low parts per billion range. Basically, the lower the lead level, the more options for formulation.

Quality of the material has come under scrutiny lately. Besides ensuring the material meets all chemical specifications (including lead), a manufacturer needs to pay close attention to the microbiological activity of the material. Laboratory tests conducted by Silliker Laboratories in Carson, Calif., for Nutri Granulations showed micro counts as high as 31,000 APC in some coral samples. With a USP standard specification of less than 3,000 APC, diligence in raw material testing is critical. Color has also become an important criterion for tableting companies. The whiter the source material, the more formulation options for visual appeal of the tablet. Finally, costs of powdered calcium carbonate can range from less than $0.10/kg to more than $30/kg based primarily on source.

Overall, selecting a calcium carbonate raw material depends on how it meets the manufacturer's needs. Given the similarities of the various sources, as well as similar bioavailability levels, it is apparent that the end benefits for the consumer are the same, regardless of the origin of the calcium.

Kurt C. Schneider, general manager of La Mirada, Calif.-based Nutri Granulations, has a 16-year career in foods, nutritional foods, and dietary supplements. He currently specializes in granulation technologies of calcium-based products.


  1. Harper's Review of Biochemistry, 17th ed., p. 575-6, 1979; Normal and Therapeutic Nutrition, 14th ed., p. 105-6, 1972.
  2. Modern Nutrition in Health and Disease, p. 151, 1994.
  3. Sheikh MS et al. "Gastrointestinal absorption of calcium from milk and calcium salts." NEJM, 317, 9:532-6, 1987.
  4. ibid.
  5. Havala S, "Calcium content in tahini." Vegetarian Journal, July/August 1996.
  6. Personal communication, Oct. 25, 2002.

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