Calcium Absorption-An Update
Chester J. Zelasko, Ph.D. | October 2, 2001

When it comes to absorption of food supplements, nothing seems to stimulate more debate than calcium. The issue revolves around which form of calcium is better absorbed: calcium carbonate or calcium citrate. In the past two years, four studies have received a lot of attention (1-4). One research group finds that calcium citrate is better absorbed than calcium carbonate (1-2), while another research group finds that there are no differences in the absorption of these two different forms of calcium (3-4). Why the confusion?

Let's look at the similarities and the differences in the studies. Both laboratories used commercially available preparations, gave the supplements as directed on the label, and used similar groups of subjects--post-menopausal women. Two of the studies (one from each lab) compared a single dose of the supplements and two (again, one from each lab) used a research design that tested all groups under all conditions--something researchers called a crossover design. They also examined similar outcome variables: changes in blood calcium levels, changes in parathyroid hormone, and the amount of calcium excreted in the urine. The primary difference appears to be in the length of time the data were collected. In the studies that demonstrated that calcium citrate was better absorbed, the data collection lasted 4-6 hours. In the studies that showed no difference in absorption between the two forms of calcium, data were collected for 24 hours.

The differences in the absorption of the two forms of calcium may be as simple as one being absorbed faster than the other. Because calcium supplements are taken primarily to insure that bones remain strong, fast versus slow absorption is probably irrelevant, although that hypothesis hasn't been tested. For all practical purposes, there appears to be no difference in absorption between the two forms of calcium.

Based on prior research, calcium carbonate requires stomach acid to aid in absorption, so it's probably best taken with meals. On the other hand, taking your calcium-magnesium supplement before bed helps soothe and relax your muscles and nervous system. What there's no doubt about is this: take your calcium supplement with magnesium to boost effectiveness.

This will not end the controversy over which calcium supplement is better to take, but the goal of the Newsletters are to inform you of current research so you can reach your own conclusions. Based on the current data available, calcium carbonate and calcium citrate are both acceptable sources for calcium supplementation.


  1. Heller HJ, et al. Pharmacokinetics of calcium absorption from two commercial calcium supplements. J Clin Pharmacol 39(11):1151-4. 1999.
  2. Heller HJ, et al. Pharmacokinetic and pharmacodynamic comparison of two calcium supplements in postmenopausal women. J Clin Pharmacol 40(11):1237-44. 2000.
  3. Heaney RP, et al. Absorption of calcium as carbonate and citrate salts, with some observations on method. Osteoporos Int 9(1):19-23,1999.
  4. Heaney RP, et al. Absorbability and cost effectiveness in calcium supplementation. J Am Coll Nutr 20(3):239-46. 2001.
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