Calcium is an essential mineral that plays an important role in bone formation, muscle contraction, and blood clotting. The best way to get calcium is from the foods you eat, but people who can't get enough calcium from food may need to take calcium supplements.

Calcium in foods and supplements occurs in compounds, or substances that contain more than one ingredient. Available supplements include:

• calcium carbonate
• calcium citrate
• calcium phosphate
• calcium lactate
• calcium gluconate.

The calcium in these compounds is called elemental calcium. When reading labels, be sure to check the amount of elemental calcium, rather than the amount of the entire compound.

Not all the calcium you eat is absorbed by your body. How much your body absorbs depends on several factors, including the type of calcium compound you use. Calcium carbonate requires an acidic environment in order to be absorbed. Stomach acid increases in the presence of food, so calcium carbonate should be taken with a meal. Calcium citrate does not require extra stomach acid to be dissolved, and should be taken on an empty stomach.

Your body absorbs calcium less efficiently as your intake increases, therefore it is best to take calcium in small doses throughout the day. Taking more than 500 milligrams at one time is not recommended.

Another factor in absorption of calcium from supplement tablets is whether the tablet itself dissolves in the body. To ensure you are taking a supplement that will dissolve, take one that has USP written on the label. This indicates that it meets the U.S. Pharmacopeia's standards for dissolution, or breakdown, of a supplement.

The best calcium supplements are calcium citrate and calcium carbonate, because they are easily available, contain large amounts of elemental calcium, and dissolve well in the body. Calcium phosphate, calcium lactate, and calcium gluconate are not as good because they have small percentages of elemental calcium in each tablet, so you have to take many tablets to get enough calcium. Taking more than 2,500 milligrams of elemental calcium per day is not recommended.