Calcium question

I have never understood why we cannot absorb the carbonate calcium.  Can anyone explain?

I have heard numerous times that it is because we RNY'rs do not have enough gastric acid in our pouches to break it down. Calcium is very dense and citrate calcium supposedly digests better.

Calcium citrate disolves in water.  Calcium carbonate does take the stomach acid and for a RNY it would be a waste of your money.  They won't brake down for you very well and you won't get the benefit of the calcium.  Sometimes your body just passes them whole if they don't break down.

Thank you for the responses. 

carbonate causes kidney stones also..

to remember the difference calcium citrate and calcium carbonate
think carbonate..carbonation..soda..bad for us
citrate ..citrus ..fruit..good for us..( well not high sugar fruit and not early on)
But you get the drift..this is the only way I could remember at the drug store...