Calcium: Is Absorption An Issue?
Chester J. Zelasko, Ph.D. | April 3, 2001

Most people know that calcium is the primary mineral component of bone, but many are uncertain about the best form of calcium to use as a food supplement. The magazine and Internet advertisements contribute to this confusion with ads that talk about one form of calcium being better absorbed than other forms. It's difficult to know how to separate the facts from the hype, because there are so many conflicting opinions on the matter and every position is backed by the latest scientific studies. So what should you believe?

Calcium is poorly absorbed. Whether it's from food or from a supplement, only about one-third of the calcium you take in will be absorbed. The two most popular forms of calcium supplements are calcium carbonate and calcium citrate. There have been numerous studies on both. Some studies show that calcium citrate may be better absorbed than calcium carbonate, but the results of the studies vary (1-2). Calcium carbonate averages about 31% absorption, while calcium citrate averages about 35%. Although those statistics are absolutely true, they're relatively meaningless—unless you're selling calcium citrate supplements.

Calcium citrate is a larger molecule than calcium carbonate. A person would need to take larger tablets or take more of them to get the same amount of calcium as in calcium carbonate. Calcium citrate is also more expensive than calcium carbonate. All things considered, it's a draw. What's important is that both forms of calcium appear to help prevent osteoporosis when combined with a regular exercise program.

While either form will work for you, here are a few key points to consider when choosing calcium supplements.

  • The calcium supplement should readily dissolve. You can test it by dropping the tablet into two ounces of vinegar, a very weak acid. It should be 2/3 dissolved in 30 minutes. If it isn't, it won't dissolve in the acid in your stomach.
  • Calcium is absorbed better with magnesium. Select a calcium supplement that contains twice as much calcium as magnesium.
  • Purchase your supplements from a company that uses Good Manufacturing Practices. That increases the probability that the calcium content listed on the label is accurate and that the manufacturing process limits the inclusion of undesirable minerals.
When it comes to calcium, forget the hype about which form is better and just be sure to use a good quality product and exercise regularly. Your bones will thank you for it.


  1. Heaney RP, Dowell MS, Barger-Lux MJ Absorption of calcium as carbonate and citrate salts, with some observations on method. Osteoporos Int 9(1):19-23, 1999.
  2. Heller HJ, et. al. Pharmacokinetic and pharmacodynamic comparison of two calcium supplements in postmenopausal women. Clin Pharmacol 40(11):1237-44. 2000.
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