Calcium Forms, Bioavailability Subject of Industry Discussion

November 18, 2002

6 Min Read
Calcium Forms, Bioavailability Subject of Industry Discussion

Calcium Forms, Bioavailability Subject of Industry Discussion

by Kurt Schneider

Calcium is returning to the forefront of the dietary supplementindustry. Much of the recent publicity has centered on coral calcium, includingsupplier claims of higher absorptivity and bioavailability. Coral calcium can beconsidered 'young' calcium carbonate ore, which is an economical, pure andwidely 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 leveleffective in fighting osteoporosis. To do so, there are many forms of calciumcurrently available, such as calcium carbonate, calcium citrate, calciumgluconate, oyster shell, scallop shell, eggshell, milk products and, the latest,coral calcium. While there has been much debate over which type is best, it isclear that whatever form is taken, it needs to be absorbed by the body toprovide 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 breaksdown into carbon dioxide. At this point the calcium is in its ionic, orpositively charged, state. In this state, the calcium can pass through theintestinal tract, into the bloodstream, and into the bones. It is this rate ofcalcium absorption, or bioavailability, that determines how much of the ingestedcalcium actually can be used by the body. The higher the bioavailability, themore calcium ions can travel into the body. In addition, the faster thesupplement can obtain this ionic state, the faster it becomes available forabsorption. Since time is critical, it is important to ensure as much calcium isabsorbed before it passes through the body. Studies have shown that as long asthe body absorbs enough calcium, the rate of absorption is not indicative of thequality of the calcium. The bioavailability or absorption and uptake of calciumare partly a function of pH. Acidity favors absorption while alkalinity hindersabsorption. An increase in acidophilic flora (i.e., L. acidophilus) inthe intestine favors calcium absorption.1 With normal gastricfunction, almost all calcium forms become soluble and bioavailable. Further,studies have shown that solubility of calcium forms does not significantlyaffect absorption.2 Calcium carbonate is not soluble in water; it issoluble in acid and is well-absorbed in normal gastric function.

Levels of calcium in various products also vary. Calciumpercentages range from 9 percent for calcium gluconate to nearly 40 percent forcalcium carbonate.3 It would appear that the higher starting level ofcalcium would provide better absorption and bioavailability; this is notnecessarily the case. As the supplement passes through the body, studies haveshown there is little difference in bioavailability between all major types ofcalcium. When taken with meals, calcium carbonate has been shown to be as ormore absorbable (39%) as five other forms of calcium, including calcium acetate(32%), calcium lactate (32%), milk (31%), calcium citrate (30%) and calciumgluconate (27%).4

There are other factors that play a role in how well the bodyabsorbs calcium. "Nutrients in foods interact with each other, and theabsorption of any nutrient depends, in part, upon the foods that are eaten withit and the nutrients and other substances those foods contain," wroteSuzanne Havala, M.S., R.D., in the Vegetarian Journal. "It alsodepends upon the body's need for the particular nutrient. Our bodies can adaptto varying dietary conditions; when we need more of many nutrients, the body maycompensate 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 depositsources: 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 layerof newly forming deposits. Others are millions of years old. Recent deposits aresubjected to all elements in the surrounding water, and, with the state of therivers and oceanic margins becoming more polluted, it may be wise to find acalcium carbonate source that is from a high quality mine, further inland, thatis much less susceptible to environmental pollutants. A reef, for example, ismade up almost exclusively of animals that use dissolved calcium carbonate inthe water to make either an internal or external calcium carbonate structure. Astime goes on, the deposit accumulates to various levels. Some deposits arethousands of feet thick while others are just a few. It all depends on theinitial environment and time."

The majority of the coral calcium products sold in the UnitedStates is made of calcium carbonate from coral reefs, which, apart from age, isno different than calcium carbonate from other sources such as limestone. Somestudies have even stated the claimed benefits of coral calcium are derived froma separate compound found in the water, calcium aspartate, and not the coralitself.7

If all calcium carbonate sources are basically the same inregard to source and bioavailability, what is the best way for manufacturers tochoose a source? Five criteria should be considered: grade, lead level, quality,color and cost. For grade, there are basically just two--food andpharmaceutical--with numerous permutations. For lead level, CaliforniaProposition 65 has set the standard for dietary supplement lead intake, and hashad a great impact on the calcium carbonate industry. Tablet manufacturers arerequiring lower lead levels, with some sources moving into the low parts perbillion range. Basically, the lower the lead level, the more options forformulation.

Quality of the material has come under scrutiny lately. Besidesensuring the material meets all chemical specifications (including lead), amanufacturer needs to pay close attention to the microbiological activity of thematerial. Laboratory tests conducted by Silliker Laboratories in Carson, Calif.,for Nutri Granulations showed micro counts as high as 31,000 APC in some coralsamples. With a USP standard specification of less than 3,000 APC, diligence inraw material testing is critical. Color has also become an important criterionfor tableting companies. The whiter the source material, the more formulationoptions for visual appeal of the tablet. Finally, costs of powdered calciumcarbonate can range from less than $0.10/kg to more than $30/kg based primarilyon source.

Overall, selecting a calcium carbonate raw material depends onhow it meets the manufacturer's needs. Given the similarities of the varioussources, as well as similar bioavailability levels, it is apparent that the endbenefits for the consumer are the same, regardless of the origin of the calcium.

Kurt C. Schneider, general manager of La Mirada, Calif.-basedNutri Granulations, has a 16-year career in foods, nutritional foods, anddietary supplements. He currently specializes in granulation technologies ofcalcium-based products and can be reached at [email protected].


  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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