Which one...Calcium carbonate or calcium citrate?

I am confused and can't remember what my surgeon told me...do we take calcium carbonate (which is what I'm taking now in the form of TUMS) or do we take calcium citrate?  What is the difference and why is one preferred over the other?  If we do take calcium citrate, what kind of product contains this?  Thanks so much for your help! Jules

I'm copying this response - the same question was asked a few days ago The information I am sharing comes from a conference I attended last year.  The speaker was  Robert Martindale PhD, MD , Professor of Surgery, Oregon Health & Science Univ. (Some of you may have been his patient - he was previously in Georgia) Normal daily needs are 1000-1200 mg/day. In RNY recs are 1500-2000 mg/day - this assumes a 30-40% malabsorption -Calcium carbonate are not as well absorbed due to decreased acid in stomach and decreased dissolution (breaking apart) in the stomach. -Calcium Citrate with vitamin D is preferred as it is better absorbed - calcium citrate should be taken between meals -Neither calcium should be taken with iron as it decreases iron absorption Then, a study was done looking at the various factors affecting calcium absorption.  It included such variables as vitamin D levels, estrogen therapy, timing of intake, gastric acidity, age, gender, drug standards of dissolution and disintegration, and bioavailability. The results: The lab data slightly favors calcium citrate Cost data favors calcium carbonate clinical result: There was an insignificant level of data to justify one over the other. There are more and more studies out there all the time. Do your research! regards, Danielle Halewijn RD eNutritioncare.com