A Practical Analysis of Calcium Supplements
April 2000; Volume 2; 28-31
By David I. Levenson, MD, FACE, and Kevin A. Ohayon
Calcium is vital for maintaining skeletal integrity and regulating nerve excitability, muscle contraction, and blood coagulation. In addition to promoting accumulation of bone mass in childhood and early adulthood, calcium ingestion may be effective in preventing osteoporosis later in life. Calcium may also play a protective role in essential hypertension, gestational hypertension, hypercholesterolemia, colon cancer, and other medical conditions.1
Dairy products such as milk, cheese, and yogurt are the most concentrated calcium food sources. Those who avoid dairy products can still consume significant amounts of calcium from bony fish such as sardines, mackerel, and salmon; tofu made with calcium sulfate; and dark-green leafy vegetables such as broccoli, kale, collards, and turnip greens. Also, many brands of orange juice, cereals, flour, and breads are now calcium-fortified.
NHANES III data reveal that most Americans do not consume adequate amounts of calcium;2 older adults and teenagers are at greatest risk for a low calcium intake. The latest guidelines released in 1997 by the Institute of Medicine3 increased the recommended daily calcium intakes for most age groups over the 1994 NIH Consensus Conference recommendations.4 The newer recommendations, called the Dietary Reference Intakes, are: 1,300 mg for ages 9-18, 1,000 mg for ages 19-50, and 1,200 mg after age 50. Recommended intake during pregnancy and lactation is 1,300 mg for those under 18 years of age and 1,000 mg daily for 19 years and older.
Forms of Calcium Supplements
More than a dozen commonly prescribed calcium salts (see Table 1) and hundreds of different preparations are commercially available. Understandably, questions often arise about which supplement is best.1
| Table 1-Calcium content in various preparations | |
| Preparation | % Calcium |
| Carbonate | 40 |
| Tricalcium phosphate | 38 |
| Dicalcium phosphate | 31 |
| Bone meal | 31 |
| Oyster shell | 28 |
| Dolomite | 22 |
| Citrate | 21 |
| Lactate | 13 |
| Gluconate | 9 |
| Glubionate | 6.5 |
| Adapted from: Levenson DI, Bockman RS. A review of calcium preparations. Nutr Rev 1994;52:221-232. | |
Calcium carbonate is generally the least expensive of the available calcium supplements and is the most widely used. Although calcium carbonate contains the highest proportion of elemental calcium by weight (40%), it is a relatively insoluble form of calcium, especially at neutral pH (which may be important in achlorhydric patients). Refined calcium carbonate supplements are made from chalk or calcium hydroxide. Calcium carbonate occurs naturally in limestone and oyster shell, and several supplements use these sources.
Calcium citrate, the second most common form, is available as a tablet or as an effervescent preparation. At 21% by weight, calcium citrate has a lower calcium content than calcium carbonate but is much more soluble (especially important in achlorhydric patients).
Bone meal and dolomite are both considered "natural" sources of calcium and magnesium. Bone meal, a microcrystalline hydroxyapatite, is made up of the ground powder of animal bones. Dolomite is a natural mineral combination of calcium carbonate and magnesium carbonate. Both of these sources have been found to contain trace amounts of lead and other toxic metals.5 Bone meal was also shown to have the lowest absorption and the worst palatability compared to four other preparations.6
Several new chewable chocolate- or fruit-flavored calcium preparations have recently come to market. Some contain vitamin D, vitamin K, potassium, and phosphorus, which may be clinically significant in certain patients.
In addition, several calcium phosphate tablets or wafers, including di- and tricalcium phosphate, are available. These preparations tend to be less soluble. Their phosphate content also limits use in chronic renal failure.1 These products, however, may be less constipating.
Readily soluble forms of calcium include chelates of lactate, gluconate, glubionate, and calcium citrate malate (CCM). Although calcium lactate is well absorbed, the amount of calcium in each tablet is small and many tablets are needed to meet the recommended intake. Calcium gluconate is the preferred intravenous preparation for treatment of life-threatening hypocalcemia. Calcium glubionate is available in syrups for children and for people who have difficulty swallowing pills.
Calcium Solubility vs. Bioavailability
Many studies have demonstrated differences in the bioavailability of various calcium salts. These studies have considered the solubility and ionization of the salts as surrogates for calcium absorption.7 However, a 1990 study by Heaney found a relatively weak association between the solubility of a calcium salt and its absorbability.8 Confirming this finding, a later study that compared calcium citrate to calcium lactogluconate/carbonate concluded that calcium absorption is not determined solely by the solubility of the calcium salt; rather, the factors that affect absorption are more complex.9
Comparisons of Calcium Salts
Studies that compare different calcium salts have provided disparate results. Several groups have shown CCM to be more absorbable than calcium carbonate.4,10 Studies have also shown that calcium citrate is more bioavailable than calcium carbonate.11 However, these studies were done in fasting patients—a condition proven to lead to erratic absorption of the carbonate salt.12 A recent study concluded that calcium carbonate is equally well absorbed as calcium citrate when ingested as part of a meal.13
Factors Affecting Absorption
Many factors, including dose and schedule, influence calcium absorption. (See Table 2.) Heaney studied the influence of dose dependency on calcium absorption and showed an inverse relationship between efficiency of absorption and calcium load.14 A plateau is observed at about 400-500 mg of calcium given in a single dose.15 Therefore, calcium is most efficiently absorbed when given in divided doses of less than 500 mg per dose.
| Table 2-Factors that influence intestinal calcium absorption | |||||||||||||||||||
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The effect of a complex meal on calcium absorption is extremely difficult to study. Meal components such as fat, fiber, and carbohydrates each influence intestinal absorption. However, several studies have shown that, in general, ingestion of a small meal—especially one with carbohydrates—will enhance absorption.12 Patients with achlorhydria may have difficulty absorbing calcium when fasting. However, this is not a problem when calcium carbonate is taken with a meal. A bedtime dose may inhibit calcium fluxes from bone during the night.16
Interactions with Nutrients
Many dietary factors also can influence calcium absorption. Wheat bran17 and high-phytate soybeans18 bind calcium and reduce absorption. However, the calcium in whole wheat bread is more bioavailable than that in milk. On the other hand, the fiber in green, leafy vegetables19 and in psyllium20 has no influence on absorption. The calcium in beans is only about half as available as the calcium in milk.21 Foods high in oxalic acid, such as spinach and rhubarb, have high concentrations of calcium but are almost totally unabsorbable.22 The oxalate in chocolate, however, does not reduce the absorbability of calcium from chocolate milk.23
Vitamin D has been shown to stimulate renal reabsorption of calcium and phosphate, as well as enhance intestinal absorption of calcium, even in healthy subjects who take an adequate dose of vitamin D.24,25
Calcium inhibits iron absorption, especially when both supplements are taken with meals.26 It is recommended that patients who require both iron and calcium supplementation take these supplements at different times.
Both zinc and magnesium are commonly combined with calcium preparations. High levels of dietary calcium have been shown to have an adverse effect on zinc absorption, and a 5:1 ratio of calcium to magnesium may induce magnesium deficiency.27 However, these issues are generally only a concern in patients who have impaired renal function, diabetes, malabsorption syndromes, or alcoholism.
Interactions with Drugs
Calcium citrate also enhances absorption of aluminum from food sources and products containing aluminum hydroxide. Calcium carbonate has no apparent effect on aluminum absorption.28 Drugs such as sodium cellulose phosphate and tetracycline bind calcium and may reduce absorption.29
Quality and Safety Issues
The higher price of certain brand name preparations may be justified if a better manufacturing process reduces contaminants or makes the calcium more bioavailable. The time it takes a tablet to dissolve can predict absorption of calcium carbonate tablets.30 Consumers can perform a simple "vinegar test" by adding a tablet to a cup of vinegar. If the tablet dissolves within 30 minutes, it will probably dissolve in vivo. Chewable tablets or candies bypass the problem. Calcium supplements can cause constipation in some people. Taking magnesium (half the calcium dose) alleviates this problem.
"Oyster shell" calcium carbonate is actually mined from fossilized limestone quarries.31 Although fresh oyster shells do not incorporate contaminants, the fossilized shells have the potential for heavy metal contamination, especially with lead6 and aluminum.32 Bourgoin and associates performed an extensive study of 70 brands of calcium supplements available in North America.6 They found that calcium chelates and refined calcium carbonate contained only low levels of lead. The highest levels of lead were in the bone meal preparations, followed closely by the fossil shell, and then dolomite. Tums® and OsCal® (calcium carbonate), however, consistently contained less than 3 ppm of lead.1
Conclusion
Calcium supplementation is convenient and often necessary, but is not a substitute for a good diet. Calcium carbonate is the most commonly used and the least expensive formulation. Because of its high calcium content, calcium carbonate requires fewer or smaller tablets to achieve a given dose of calcium. It is equally as bioavailable as the more soluble forms of calcium, such as calcium citrate, when it is taken with meals. Calcium absorption can be enhanced by taking it in divided doses of 400-500 mg, taking it with meals, and avoiding co-ingestion with substances such as oxalate (spinach, rhubarb, nuts) and phytate (wheat bran, soybeans) that interfere with calcium absorption. Achlorhydric patients can increase absorption of calcium by taking the supplement with meals or by taking calcium citrate. One should be careful with "natural" sources of calcium such as bone meal, fossilized oyster shell, and dolomite preparations that may have heavy metal contamination. Chewable calcium carbonate, effervescent calcium citrate, and calcium glubionate syrup are available for those who have difficulty swallowing pills. Adequate vitamin D intake (400-800 IU) is important, especially in elderly, chronically ill, or institutionalized individuals.
Dr. Levenson is Voluntary Clinical Assistant Professor of Medicine at the University of Miami; Mr. Ohayon is a fourth-year Medical Student at Nova Southeastern University College of Medicine in Davie, FL.
References
1. Levenson DI, Bockman RS. A review of calcium preparations. Nutr Rev 1994;52:221-232.
2. Marwick C. NHANES III health data relevant for aging nation. JAMA 1997;277:100-102.
3. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, DC: National Academy Press; 1997.
4. NIH Consensus Development Panel on Optimal Calcium Intake. Optimal calcium intake. NIH Consensus conference. JAMA 1994;272:1942-1948.
5. Bourgoin BP, et al. Lead content in 70 brands of dietary calcium supplements. Am J Public Health 1993;83:1155-1160.
6. Reid IR, et al. The acute biochemical effects of four proprietary calcium preparations. Aust N Z J Med 1986;16:193-197.
7. Pak CY, Avioli LV. Factors affecting absorbability of calcium from calcium salts and food. Calcif Tissue Int 1988;43:55-60.
8. Heaney RP, et al. Absorbability of calcium sources: The limited role of solubility. Calcif Tissue Int 1990;46:300-304.
9. Hansen C, et al. Intestinal calcium absorption from different calcium preparations: Influence of anion and solubility. Osteoporos Int 1996;6:386-393.
10. Dawson-Hughes B, et al. A controlled trial of the effect of calcium supplementation on bone density in postmenopausal women. N Engl J Med 1990;323:878-883.
11. Harvey JA, et al. Superior calcium absorption from calcium citrate than calcium carbonate using external forearm counting. J Am Coll Nutr 1990;9:583-587.
12. Heaney RP, et al. Meal effects on calcium absorption. Am J Clin Nutr 1989;49:372-376.
13. Heaney RP, et al. Absorption of calcium as the carbonate and citrate salts, with some observations on method. Osteoporos Int 1999;9:19-23.
14. Heaney RP, et al. Calcium absorption in women: Relationships to calcium intake, estrogen status, and age. J Bone Mineral Res 1989;4:469-475.
15. Heaney RP, et al. Influence of calcium load on absorption fraction. J Bone Min Res 1990;5:1135-1138.
16. Blumsohn A, et al. Effect of timing of calcium supplementation on the circadian rhythm of bone collagen degradation abstract. J Bone Miner Res 1993; 8(suppl):S158.
17. Weaver CM, et al. Wheat bran abolishes the inverse relationship between calcium load size and absorption fraction in women. J Nutr 1996;126:303-307.
18. Heaney RP, et al. Soybean phytate content: Effect on calcium absorption. Am J Clin Nutr 1991;53:745-747.
19. Heaney RP. Nutritional factors in osteoporosis. Annu Rev Nutr 1993;13:287-316.
20. Heaney RP, Weaver CM. Effect of psyllium on absorption of co-ingested calcium. J Am Geriatr Soc 1995;43:261-263.
21. Weaver CM, et al. Absorbability of calcium from common beans. J Food Sci 1993;58:1401-1403.
22. Heaney RP, et al. Calcium absorbability from spinach. Am J Clin Nutr 1988;47:707-709.
23. Recker RR, et al. Calcium absorbability from milk products, an imitation milk, and calcium carbonate. Am J Clin Nutr 1988;47:93-95.
24. Mortensen L, Charles P. Bioavailability of calcium supplements and the effect of vitamin D: Comparisons between milk, calcium carbonate, and calcium carbonate plus vitamin D. Am J Clin Nutr 1996;63:354-357.
25. Heaney RP, et al. Calcium absorptive effects of vitamin D and its major metabolites. J Clin Endocrinol Metab 1997;82:4111-4116.
26. Cook JD, et al. Calcium supplementation: Effect on iron absorption. Am J Clin Nutr 1991;53:106-111.
27. Whiting SJ, et al. Calcium supplementation. J Am Acad Nurse Pract 1997;9:187-192.
28. Coburn JW, et al. Calcium citrate markedly enhances aluminum absorption from aluminum hydroxide. Am J Kidney Dis 1991;17:708-711.
29. Breslau NA. Calcium, magnesium, and phosphorus: Intestinal absorption. In: Favus M, ed. Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism. 3rd ed. Philadelphia, PA: Lippencott-Raven Publishers; 1996: 45.
30. Whiting SJ, Pluhator MM. Comparison of in vitro and in vivo tests for determination of availability of calcium from calcium carbonate tablets. J Am Coll Nutr 1992;11:553-560.
31. Whiting SJ. Safety of some calcium supplements questioned. Nutr Rev 1994;52:95-97.
32. Bourgoin BP. Alumino-silicate content in calcium supplements derived from various carbonate deposits. Bull Environ Contam Toxicol 1992;48:803-808.
April 2000; Volume 2; 28-31
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