Calcium supplements: What kind of calcium is in that pill?

Calcium-rich foods give you calcium paired with natural organic acids, a combination that your body easily digests and absorbs.
The form of calcium most commonly found in supplements, however, is calcium carbonate, the kind of calcium that occurs naturally in limestone and oyster shells.
Calcium carbonate is a versatile compound. Not only does it build strong bones and teeth, but it also neutralizes stomach acid and relieves heartburn. Calcium carbonate antacids can be used as calcium supplements. They’re nutritionally sound and generally cost less than products designed solely as nutritional supplements. Some calcium supplements contain compounds that mix calcium with an organic acid. Calcium lactate is calcium plus lactic acid, the combination that occurs naturally in milk. Calcium citrate is calcium plus citric acid, an acid found in fruits. These compounds are easier to digest, but they’re sometimes more expensive than calcium carbonate products. Calcium carbonate is nearly half calcium, a very high percentage. But unless your stomach is very acidic, it’s hard for your digestive system to break the compound open and get at the elemental calcium (the kind of calcium your body can use). You can increase your absorption of calcium from calcium carbonate by taking the tablets with meals.
Because different calcium compounds yield different amounts of elemental calcium, the label lists both the calcium compound and the amount of elemental calcium provided, like this:
Calcium carbonate, 500 milligrams, providing 200 milligrams elemental calcium. Whenever you see the word calcium alone, it stands for elemental calcium.
The human body absorbs calcium most efficiently in amounts of 500 milligrams or less. You get more calcium from one 500-milligram calcium tablet twice a day than one 1,000-milligram tablet. If the 1,000-milligram tablets are a better buy, break them in half.
Warning: Not all antacids double as dietary supplements. Antacids containing magnesium or aluminum compounds are safe for neutralizing stomach acid, but they won’t work as supplements.
In fact, just the opposite is true. Taking magnesium antacids reduces your absorption of calcium, and taking aluminum antacids reduces your absorption of phosphorus. Because manufacturers sometimes change the ingredients in their products without notice, you always need to read the product label before assuming that an antacid can double as a calcium supplement