Bones are always in a state of change called remodeling. Remodeling breaks down existing bone and deposits new bone in its place. Milk and milk products are the best sources of calcium. Few other foods provide such a concentrated source of calcium that is as readily available for absorption as do milk and milk products. Milk's calcium absorption is about 30 percent.
Some foods reduce calcium absorption and actually increase the loss of calcium in the urine.
The lower absorption of calcium from foods is usually linked to the presence of fiber, phytate or oxalate. Spinach, rhubarb, beet greens and red beans all contain oxalates, which are a salt of a crystalline organic acid also used as a bleach or rust remover. In spinach, only 5 percent of the calcium is absorbed because oxalate is present. Other factors in calcium absorption are estrogen levels, pregnancy or lactation, caffeine consumption, smoking, alcohol intake, some diseases and some medications. Supplements should be supervised by a health professional and are recommended only for those individuals who for whatever reason are unable to eat foods rich in calcium.