Ross CA, Taylor CL, Yaktine AL, Del Valle HB, eds; Committee to Review Dietary Reference Intakes for Vitamin D and Calcium; Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academy Press; 2011. Accessed at http://www.nap.edu/catalog.php?record_id=13050 on 31 May 2012.
Different forms of calcium:
Calcium contained naturally in food sources: Calcium contained naturally in food sources is believed to be the best source of calcium intake because of good absorption and the greater number of other nutrients contained in these same foods.
Calcium carbonate:
Calcium carbonate is the most common form of calcium and least expensive available which contains about 40% elemental calcium. It is better absorbed with food and an acidic environment. A good time to take this form of calcium is at the time of an acidic meal such as tomato products, citrus or foods with vinegar. The daily intake of calcium should be based upon how much elemental calcium the supplement contains and not the amount of calcium carbonate.