The NRDC has also petitioned the FDA to reduce its allowable limit to 0.5 micrograms per 1,000 milligrams, and it appears that the problem is now under control. Tums, by the way, has consistently tested lowest in lead, while dolomite and bone-meal calcium have been more likely to be high in lead. Don't pay extra for pills that make these meaningless claims. Most foods contain phosphorous, but this should not interfere with calcium absorption. It's okay to take calcium in the form of calcium phosphate.
Oxalate is a substance that binds calcium.
The aluminum in some antacids can also decrease calcium absorption. Some kinds of fiber affect absorption-for example, wheat bran may reduce it somewhat. If you eat a wheat bran cereal for breakfast, you might wait and take your supplement at lunch. It probably is, though it has not been thoroughly tested. Chewables are probably best absorbed.