Calcium deficiency is a catch 22. To digest and absorb calcium you need to have strong stomach acid and pepsin to tear apart the bonds it has to other things and be free to absorb in the digestion. The problem comes calcium is needed by the cells to squirt acid and pepsin into the stomach. If you have had digestion problems you will probably have a calcium deficiency. To take a calcium supplement without dealing with the digestion problem you are wasting your money.