What's up with calcium supplements?

Originally Published: September 5, 2003
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Dear Alice,

What is the difference between and the pros and cons of the three types of calcium: oyster shell, calcium carbonate, calcium citrate? Does one stand out for osteoporosis?

Curious about calcium

Dear Curious about calcium,

Calcium is only one component necessary to build and/or maintain healthy bones; you also need vitamins C, D, and K; the minerals boron, potassium, and magnesium; and, adequate protein and resistance exercise — all the more reason to eat a variety of foods and move your body.

However, many people need to supplement their eating plan to get enough calcium for good health and osteoporosis prevention. Calcium supplements may help a person reach her or his daily need for this important mineral, but dietary sources of calcium provide many other nutrients needed for bone health and blood pressure regulation — two key functions of calcium.

Daily calcium needs are listed as follows:

Age (years) Adequate Intake [AI] of elemental calcium (mg/day)
1 - 3 500
4 - 8 800
9 - 18 1,300
19 - 30 1,000
31 - 50 1,000
51 - 70 1,200
> 70 1,200

Calcium supplements contain calcium as part of a compound: calcium carbonate and calcium citrate are the most popular supplement types. Others also exist, such as calcium gluconate, calcium citrate malate, calcium phosphate, calcium lactate, and calcium from dolomite (an extract from limestone and marble that also contains magnesium). Oyster shell is a form of calcium carbonate that has been known occasionally to contain small amounts of lead, a toxin. Reports of lead in dolomite or bone meal (another form of supplemental calcium — just as oyster shell is one) have also surfaced in the past. Calcium supplementation from sources other than oyster shell, dolomite, and bone meal would be safer to take.

The form of calcium a person takes depends on his or her individual needs. You may have heard or read that one form is more easily absorbed than another. As long as you take in enough elemental calcium to meet your needs, the type is not that important. Look at the label to see how much elemental calcium is supplied by each tablet. If the label doesn't state the amount of elemental calcium, you can figure it to be:

  • 40 percent for calcium carbonate
  • 21 percent for calcium citrate
  • 13 percent for calcium lactate
  • 9 percent for calcium gluconate

This means that if you take 1,000 mg of calcium carbonate, you will get 40 percent elemental calcium, or 400 mg. Calcium carbonate tablets typically hold more elemental calcium than calcium citrate, so one can take fewer calcium carbonate than calcium citrate pills to get the same amount of elemental calcium. Many antacids are merely calcium carbonate. This is a convenient way for some to take their calcium to meet their daily needs, since they are easy to carry around and are chewable.

If someone prefers to take a supplement that can be swallowed rather than chewed, s/he may want to consider its absorbability and purity. Labels that contain the letters USP indicate that the product meets the purity and dissolution standards established by the U.S. Pharmacopoeia. This is a voluntary process, and many fine products do not have this distinction on their labels. If USP is not listed on a calcium label, The National Osteoporosis Foundation recommends placing a tablet in a glass of warm water for 30 minutes, stirring occasionally. If it hasn't mostly dissolved in that time, chances are it won't get absorbed by the stomach.

Calcium, in any form, is better absorbed when taken in smaller doses, 500 mg or less of elemental calcium from a supplement at one time. If you need to supplement with more calcium, split the dose into two or three, and take them four hours apart throughout the day.

When to take calcium supplements depends on the type. Calcium carbonate needs to be taken with meals, as the acidity of the stomach is greater when food is being digested. This acidic environment allows for its absorption. Calcium citrate, however, is absorbed more efficiently than calcium carbonate on an empty stomach, so it needs to be taken between meals.

Another factor to consider is how well one form of calcium supplementation is tolerated over another. It's recommended to begin by taking no more than 500 mg of elemental calcium per day to see how it affects you, if at all, and then to increase after a week or so to meet your calcium needs. If the kind you are taking causes gas or constipation, and you should know after one week, try another type.

Avoid taking calcium supplements at the same time as any medication that needs to be taken on an empty stomach. Also don't take them at the same time as tetracycline (an antibiotic), iron supplements, thyroid hormones, or corticosteroids, because calcium binds to these substances, interfering with their effectiveness and also its own absorption.

One form of calcium won't prevent osteoporosis better than another — it's more important to take in a sufficient amount throughout a lifetime, along with the other important nutrients mentioned earlier. The real time to prevent osteoporosis is when bones are forming, during childhood and the teen years. After this time, maintenance of the bone mass that's been built is key. Since our bones are constantly taking in and releasing calcium (known as absorbing and resorbing calcium), we need to protect ourselves by preventing net losses that result in softening of the bones. We can do this by taking in adequate calcium, participating in regular resistance exercise, and eating a balanced diet. Stay away from excessive sodium, protein, smoking, and caffeine — the "bone robbers" of calcium.