All calcium is not alike. Calcium is found in many foods, but may not always be available to be absorbed by the body.  Calcium supplements vary widely in amounts of usable (ionized) calcium, and also require special timing in order to get the best results.

In the prevention and treatment of osteoporosis, calcium alone in not enough, and must be combined with weight-bearing exercise in order for the bones to accept and hold on to mineral (J Bone Min Res 1988;3:145-9).

One should also take part of the daily calcium intake at bedtime since during sleep the bones are not supporting the weight and are therefore more likely to lose mineral (J Clin Endocrinol Metab 1994;79:730-35).

Calcium is an ion which may compete for absorption in the intestinal tract with other substances.  Therefore, calcium should not be taken within one hour of the following:

The National Institutes of Health organized the Consensus Development Panel on Optimal Calcium Intake in 1994. This panel of experts published a comprehensive review of observations and recommendations based on a vast collection of medical data and clinical experience (J Amer Med Assoc 1994;272:1942-48).  The optimal calcium intake is recommended for each of the following groups:

        Age Group                   Optimal Daily Intake of Calcium (mg.)

        Birth-6 mo.                                                    400
        6 mo - 1 yr.                                                    600

        1-5 yr.                                                           800
        6-10 yr.                                                     800-1200

    Adolescents-Young Adults
        11-24 yr.                                                  1200-1500

        25-65 yr.                                                      1000
        Over 65 yr.                                                  1500

        25-50 yr.                                                      1000
        Over 50 yr. (Postmenopausal)
            On estrogen therapy                                1000
            Not on estrogens                                     1500
        Over 65 yr.                                                  1500
        Pregnant and nursing                               1200-1500


Calcium is contained in molecules that affect its stability and absorption.  These molecules vary in the percentage of calcium relative to total weight, and the body must be able to separate the calcium from the molecules in order to absorb the calcium.   For this reason, calcium carbonate must have an acid environment in the stomach, and absorption is greatly reduced in conditions where stomach acid is reduced, particularly with antacids.  Calcium carbonate absorption is improved when taken with acidic liquids, such as orange juice, or in combination with meals.  However, the protein and mineral content of the food may partially interfere with absorption.  In contrast, calcium citrate, lactate, and gluconate require no acid for absorption.

Here is a list of different common calcium preparations with their relative content of calcium and percentage absorption.  However, one must keep in mind that brand name and generic products are likely to vary widely.

    Preparation                     % Calcium         % Absorption

    Calcium carbonate                        40                                      26
        Oyster shell                              28                                      26

    Tricalcium phosphate                    38                                      25

    Calcium citrate maleate                 21                                      35

    Calcium lactate                             13                                      32

    Calcium gluconate                         9                                        35
    Milk                                            ---                                       33

Ref:  Nutr Rev 1994;52:221-32.

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Copyright, 1998-2007
Kim A. Carmichael, M.D., FACP
Diabetes Center
Washington University School of Medicine
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Last update February 7, 2007