To absorb calcium, the body also needs vitamin D. Some foods naturally contain small amounts of vitamin D, such as canned salmon with bones and egg yolks. The calcium in supplements is found in combination with another substance, usually carbonate or citrate. Calcium carbonate supplements are usually the best value, as they contain the most elemental calcium (about 40% by weight).
Because calcium carbonate requires stomach acid for absorption, this product is best taken with food. Most people tolerate calcium carbonate well, but some people complain of mild constipation or a feeling of bloating. Some well-known calcium carbonate products include Caltrate, Viactiv Calcium Chews, Os-Cal and Tums. To maximize calcium absorption, do not take more than 500 mg at a time.
You can take a 500 mg supplement in the morning and another in the evening. If you take a supplement that also contains vitamin D, it will help your body absorb calcium more efficiently.