Ideally all calcium should come from food and there are many other minerals needed for healthy bone. SO a well balanced healhty diet ( lots of veggies and fruits ) However it is not possible for most of us to get all we need from food.

First, calcium whether from food or supplements should be distrubuted throughout the day. Body cannot absorb more than 400- 500 units at a time... that said 150 supplemental units three times a day along with adequated calcium rich food intake may be suffienct.

I have read that while all calcium is broken down in elemental forms, not all calcium supplementation is digested so it can be broken down. Calcium carbonate requires a good level of stomach acid to break into a usable form. Be wise and shoose a good quality source - pharmacucetical brand that does some purity testing. ALso take Calcuim and D3 at different times if possible. New Chapter has a good quality vegan source of calcium.