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