Calcium is the most plentiful mineral in the body; 99 percent is stored in bones and teeth and 1 percent in tissues and blood.¹ It is essential for healthy bones and strong teeth, but it also plays a critical role in regular heartbeat, blood pressure and secretion of hormones and digestive enzymes, as well as proper cell metabolism, muscle contraction, nerve impulse transmission and blood clotting.^1,2 There?s more than just a strong skeleton depending on adequate calcium intake.

The body maintains a calcium balance in the blood, but if levels get low, it draws calcium from bones.3^,4 To offset this loss and prevent unnecessary depletion of skeletal reserves, taking in enough daily calcium is vital. Unfortunately, most Americans older than 12—particularly adolescents, women and seniors—do not get enough calcium from their diets to meet daily requirements.5^,6^,7 The recommended dietary allowance for preteens and teens is 1,300 mg per day; for adults under 50 years old, it?s 1,000 mg; and for adults over 50, it?s 1,200 mg. Calcium supplements and calcium-fortified foods can compensate for inadequate consumption.5^,7 A savvy supplements retailer will be well-versed in calcium?s benefits, the pros and cons of the various forms and what factors affect absorption.

Bone health
Results of most studies indicate supplements, such as calcium carbonate and calcium citrate, improve calcium retention, enhance bone formation, increase bone-mineral density, reduce bone loss, minimize bone fractures and prevent osteoporosis.4^,8 Notably, though, the benefits from calcium supplements are only sustained by continuous daily use. Gains in bone mass and strength disappear when supplementation is discontinued.3

In a two-year study, bone mineral density increased 1 percent to 3.7 percent in healthy menopausal women taking 500 mg calcium (from carbonate) twice daily with food. Those taking a placebo or four 8-ounce glasses of milk per day saw decreased bone mineral densities of 3 percent and 1.8 percent, respectively. The average total calcium intake was 1,678 mg per day for the supplement group and 1,052 mg per day for the dairy group. Researchers noted the higher daily calcium consumption in the supplement group might explain why calcium carbonate worked better than milk in this study.9

Remind your older customers that calcium intake becomes increasingly important with age because absorption decreases. Chronically low calcium intakes create serum imbalances, decreased bone mass and bone mineral density, and eventually osteoporosis.10 Calcium carbonate and calcium citrate malate with vitamin D are particularly useful in preserving bone mineral density, minimizing bone turnover and reducing hip fractures in those with low calcium intakes and absorption.11^,12^,13

Calcium may also ward off periodontal disease by keeping the jaw strong and healthy. By preserving bone density and preventing bone loss in the jaw, calcium may help it withstand the adverse effects of bacteria that cause periodontal disease and tooth loss.14

Colon cancer
Evidence suggests calcium may reduce colon cancer risk. In a four-year trial, researchers showed that 1,200 mg calcium (from carbonate) reduced recurrent colorectal polyps by 19 percent and total tumor count by 24 percent. They believe that because calcium affects early intestinal growths, it also may reduce the risk for more advanced tumors.15 Calcium unabsorbed in the upper intestines travels to the lower intestines and exhibits anticarcinogenic properties by rendering inactive the colon cancer promoters such as fatty acids and bile acids.¹⁶

Weight loss
Calcium promotes weight loss by reducing fat cell development and storage, and increasing fat metabolism.17 Results of five human studies suggest calcium improves weight loss efforts. In one four-year trial, participants did not reduce caloric intake, yet those in the calcium-supplemented group (1,200 mg per day from calcium carbonate) lost 1.48 pounds per year compared with those in the placebo group who lost 0.72 pounds per year. Researchers deduced that diets low in calcium elevate parathyroid hormone, calcitriol (the active hormone form of vitamin D) and intracellular calcium levels in fat cells. Because these conditions impair fat metabolism and increase fat accumulation, maintaining high calcium intakes is important to successfully lose or control weight.18

Supplemental and dietary calcium sources exhibit comparable fat-reducing effects.19 Researchers believe recommended calcium levels can increase the effectiveness of weight loss strategies and dramatically reduce the prevalence of weight gain and obesity.20

Premenstrual syndrome
For someone working to ease premenstrual complaints, point to calcium again. Researchers have found that PMS symptoms such as anxiety, depression, fatigue, personality disturbances and muscle cramps may result from low calcium levels.^21,22 In a three-month study, participants taking 1,200 mg calcium a day from chewable calcium carbonate reduced PMS symptoms such as food cravings, cramps and lower backaches by 54 percent; mood swings, depression and anxiety by 45 percent; and water retention/bloating by 36 percent.23

Blood pressure
If you need another reason not to neglect calcium supplements, consider that this mineral lowers blood pressure, which reduces coronary heart disease and stroke risk.24 In a six-month study, both 1,000 mg and 2,000 mg doses of calcium (from carbonate) decreased systolic blood pressure 3.5 mmHg and diastolic blood pressure 1.6 mmHg.25 (Researchers combined the data for the two calcium groups because of the similar outcomes.) Calcium with vitamin D is even more effective than calcium alone. Results of an eight-week study showed 600 mg calcium (from carbonate) plus 400 IU vitamin D3, taken twice daily with meals, reduced systolic blood pressure 13.1 mmHg and diastolic blood pressure 7.2 mmHg. Calcium by itself only lowered systolic blood pressure 5.7 mmHg and diastolic blood pressure 6.9 mmHg.26

Kidney stones
Calcium helps reduce kidney stone formation by binding with oxalate and preventing its absorption in the intestines.27 High oxalate levels cause kidney calcifications.¹⁶ Some studies suggest dietary calcium protects against kidney stone formation better than supplemental calcium.28^,29 However, in these studies 57 percent of the women28 and 51 percent of the men²⁹ took calcium carbonate without meals or food. This decreases its absorption, raises urinary calcium excretion and discourages intestinal oxalate binding.27^,28^,29

Calcium absorption and bioavailability
Calcium supplements are absorbed at similar rates as dietary calcium, with calcium citrate malate having the highest absorption rate of all sources.^1,10 Average absorption rates from calcium citrate malate, calcium carbonate, tricalcium phosphate and milk are 35 percent, 27 percent, 25 percent and 29 percent, respectively.¹ Because calcium from food sources and supplements are comparable, health benefits most likely depend on total calcium intake and absorption.4

One factor that greatly influences intestinal absorption is digestive pH. Calcium is mainly absorbed in the upper intestine?s more acidic environment, where the mineral is more soluble. The pH becomes more alkaline further down the intestinal tract, which decreases absorption.^2,30 It follows then that disintegration time and dissolution rate also influence absorption. If a supplement doesn?t disintegrate before it gets too far through the intestines, absorption will be reduced. Look for the U.S. Pharmacopeia on a supplement?s label to ensure it contains the standard amount of elemental calcium and will dissolve as it should after ingestion. The USP standard for calcium supplements indicates they must contain 90 percent to 110 percent of the elemental calcium listed on the label and dissolve within 30 to 40 minutes.²

Calcium absorption is improved when taken in two or three smaller doses throughout the day rather than one large dose. Advise customers to take no more than 500 mg elemental calcium at a time.^2,3 Because absorption is saturable, larger calcium intakes may not produce additional health benefits.3 More is not better if taken all at one time in the case of calcium supplements.

Calcium supplement forms
Cost and total tablets needed are important factors for customers choosing supplements. A cost-benefit analysis of calcium citrate and calcium carbonate revealed both supplements are equally bioavailable, and so favored the less expensive carbonate form, which costs 40 percent less per 1,000-mg dose on average.31 Here is a rundown of the common calcium forms so you can answer questions from customers making comparisons.

Calcium carbonate, the most common form of supplement and antacid, comes from limestone and contains the highest amount of elemental calcium, generally 40 percent. However, its alkaline pH makes it insoluble. Calcium carbonate needs acidity, such as stomach acid formed after eating, to make it more soluble and absorbable. It is most effective taken with foods or after meals, especially if tablets are broken up.² Coral calcium also comes from limestone; the fossilized remains of coral?s protective shell, live coral, is not used.

Oyster shell, dolomite and bone meal are all calcium magnesium carbonate. These supplements have higher amounts of lead than refined supplements such as calcium citrate.3^,32,33 Fortunately, their high calcium content offsets these small lead quantities. Calcium blocks intestinal lead absorption and reduces access to lead deposits stored in bones from previous exposures.³² Microcrystalline calcium hydroxyapatite is a variation of bone meal derived from bovine bones.

Calcium citrate and calcium citrate malate are extremely soluble. Because they include acids (citric and malic), they can be taken without foods and are ideal for those with poor acid secretion (generally older individuals). They usually contain 21 percent elemental calcium and, of all calcium supplements, they are the best-absorbed forms.²

Calcium ascorbate is a form of vitamin C (ascorbic acid) that has been buffered with calcium carbonate to reduce upset stomach.

Calcium lactate and calcium gluconate are soluble but have very low elemental calcium, requiring many tablets to achieve adequate daily calcium levels.²

Other absorption factors
Calcium absorption can vary between 17 percent and 58 percent among healthy individuals, depending upon various lifestyle and dietary factors such as consumption of vitamin D, fiber and fat.34.

Vitamin D is necessary to metabolize and absorb calcium in the intestine.3 A vitamin D deficiency, caused by inadequate sunlight or problems with its synthesis, can cause a calcium deficiency—hypocalcemia—and impair parathyroid hormone secretion, which affects bone strength and turnover.3^,12

Fiber (particularly insoluble) and fat influence intestinal digestive time, calcium absorption and bioavailability. Fiber increases dietary bulk and speeds its transit time through the digestive tract, which are good things, but this also reduces calcium absorption.30^,34 Concerned customers might consider adding a bit more calcium to their diet to compensate for decreased absorption from a healthy, high-fiber diet. Fat, on the other hand, slows transit time, which lengthens the duration of intestinal contact. This has its drawbacks, but the increased contact time boosts calcium absorption. The balance of fiber in relation to fat is significant, accounting for 13.8 percent of the variation in calcium absorption among healthy individuals.34

Protein and sodium negatively affect calcium balance. High-protein diets and high-sodium intakes leach calcium from bones and increase calcium excretion, while low-protein diets interfere with intestinal calcium absorption.3^,10^,35^,36^,37

Severe weight loss reduces calcium intake and retention and promotes bone loss.3 Dieters, strict vegetarians or individuals not getting enough dietary calcium can help ensure proper calcium balance and preserve bone mass with calcium supplementation.37

Some medications hinder calcium absorption and retention. Diuretics increase urinary calcium excretion. Laxatives speed nutrients through the intestines, reducing absorption time.² Conversely, calcium can interfere with other medications, such as levothyroxine (a thyroid medication),³⁸ alendronate (for osteoporosis) and tetracycline (an antibiotic).5 Customers should discuss drug-nutrient interactions with a pharmacist.

Conclusion
Calcium carbonate, calcium citrate and calcium citrate malate are excellent supplement choices ensuring adequate calcium intake. They contain high amounts of elemental calcium, dissolve easily and are well absorbed. Although calcium carbonate has the most elemental calcium, it needs acidity for absorption and should be taken with foods. It is best for people who eat frequently or have heartburn. Calcium citrate and calcium citrate malate are better for people with low stomach acid and poor absorption and for those who do not eat much. Although these forms have less elemental calcium, because they include an acid they are more soluble and can be taken without foods.

Monique N. Gilbert is a health advocate, freelance writer and author of Virtues of Soy: A Practical Health Guide and Cookbook (Universal Publishers, 2001). She can be reached at [email protected].

References
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26. Pfeifer M, et al. Effects of a short-term vitamin D(3) and calcium supplementation on blood pressure and parathyroid hormone levels in elderly women. J Clin Endocrinol Metab 2001 Apr;86(4):1633-7.
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28. Curhan GC, et al. Comparison of dietary calcium with supplemental calcium and other nutrients as factors affecting the risk for kidney stones in women. Ann Intern Med 1997 Apr 1;126(7):497-504.
29. Curhan GC, et al. A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med 1993 Mar 25;328(12):833-8.
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35. Kerstetter JE, et al. Changes in bone turnover in young women consuming different levels of dietary protein. J Clin Endocrinol Metab 1999 Mar;84(3):1052-5.
36. Nordin BC. Calcium requirement is a sliding scale. Am J Clin Nutr 2000 Jun;71(6):1381-3.
37. Weaver CM, et al. Choices for achieving adequate dietary calcium with a vegetarian diet. Am J Clin Nutr 1999 Sep;70(3 Suppl):S543-8.
38. Schneyer CR. Calcium carbonate and reduction of levothyroxine efficacy. JAMA 1998 Mar 11;279(10):750.

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