Long-term calcium deficiency can lead to osteopenia, which is a loss of bone density. Osteopenia may progress into osteoporosis, a health condition where bones become weak and brittle. Most adults need from 1,000 to 1,200 milligrams of calcium every day. That need can be met when you eat a balanced diet that includes dairy products (or dark green vegetables and calcium-fortified foods).
You probably won't feel any actual symptoms of calcium deficiency, unless you have hypocalcemia (low blood calcium), which is usually due to health conditions or certain medications and treatments. The symptoms of hypocalcemia include muscle cramps, lethargy, numbness and tingling in the fingers, and problems with heart rhythm. These can all be signs of other health conditions too, so if you have them, you need to see your health care provider.
Calcium deficiency can occur when you don't eat enough foods that contain calcium or if you eat too much protein and sodium-rich foods, which cause your body to excrete calcium. If you're low in vitamin D, you may have trouble absorbing calcium from the foods you eat, especially as you get older and your ability to absorb calcium naturally decreases.
You can increase your calcium intake by eating more dairy products, green vegetables, or by consuming bony fish, such as salmon or sardines. You can also take calcium supplements, which often contain vitamin D.
It's possible that taking large amounts of calcium supplements (over 2,500 to 3,000 milligrams per day) can cause constipation, interfere with absorption of other minerals, or increase your risk of kidney stones. It is unlikely that taking too much calcium will cause hypercalcemia -- that's typically due to health conditions that affect the parathyroid glands.
Calcium may interfere with some medications, so if you're on any medication, it's best to speak with a health care provider before taking large doses dietary supplements.
Dietary Supplement Fact Sheet: Calcium. Office of Dietary Supplements. National Institutes of Health. Accessed May 17, 2011. http://ods.od.nih.gov/factsheets/calcium/.