If you're going to supplement, you’ve got to be selective in your choice of calcium. For example, if a supplement label says 1000mg of calcium gluconate per tablet, you would probably think that this is a hefty dose of calcium.
Calcium gluconate is only 9% elemental calcium, which means that only 90mg of calcium is available for absorption, a long way from the 1200mg Recommended Dietary Allowance (RDA).
The label needs only to refer to the source of calcium, not the amount available to the body. If you look at other sources used in commercial calcium supplements, most are not much better.
Calcium acetate provides 23% elemental calcium, calcium citrate provides 21% calcium, and calcium lactate provides 14%. For the record, milk is 1% calcium. Calcium carbonate is by far the best source, providing just over 40% elemental calcium. However, this is the amount presented to the body, not the amount absorbed by the body.
Studies show that a mere 32% of the elemental calcium in calcium acetate is absorbed. Only 30% of the calcium in calcium citrate is absorbed and a respectable 27% of the calcium in milk is absorbed.
Once again calcium carbonate comes up trumps with 39% of all elemental calcium being absorbed. So if we crunch the numbers, we’ll see that a calcium supplement containing 1000mg of calcium carbonate (the better source) provides 400mg of elemental calcium, 39% of which is absorbed, that’s only 156mg.
Now you understand how easy it is to not get enough calcium from calcium supplementation.
Before any supplementation is considered, be sure to work through Shar's excellent tutorial on how to assess your current calcium intake. FDN Calcium - are you getting enough?