Calcium is a major mineral that plays several important roles in the body. It’s more abundant in the body than any other mineral: most of it in bones and teeth and a very small amount in the blood and soft tissues.
Here are some of calcium’s key functions:
• Helps form and maintain strong bones and teeth (with vitamin D and phosphorus)
• Helps muscles contract
• Helps blood vessels relax and constrict
• Transmits nerve impulses
• Helps blood clot
• Supports the functions of proteins (including enzymes and hormones)
• Helps regulate blood pressure
Although calcium is found in high amounts in a variety of foods including dairy products, fish with bones, and leafy green vegetables, the amount you absorb depends on several factors. Absorption is higher when you consume less and when your needs are higher (as in infancy and pregnancy). Absorption is lower when your vitamin D intake is low. Substances such as phytates (found in nuts, seeds, and grains) or oxalates (found in spinach) can also lower your absorption of calcium, as can consuming too much wheat bran. Women might absorb less calcium after menopause because of low estrogen levels. High intakes of supplemental phosphorus or magnesium can also inhibit the absorption of calcium from foods.
Calcium is found naturally in a variety of foods. The most absorbable calcium is found in dairy foods such as milk, yogurt, and cheese. Other good or excellent sources of highly absorbable calcium are canned fish with bones; vegetables such as kohlrabi, Brussels sprouts, kale, and broccoli; and tofu and soy milk make with calcium.
Calcium is also added to some foods and beverages like orange juice, ready-to-eat cereals, breads, and yogurt products. The amount of calcium that can be absorbed from such products varies considerably.
Too little dietary calcium can contribute to bone or tooth loss and muscle cramps. Chronic low intakes can lead to osteoporosis and increase the risk of high blood pressure, colon cancer, and preeclampsia during pregnancy.
Those with kidney failure, parathyroid disorders, or vitamin D deficiency or those who use certain diuretic medications can develop hypocalcemia with symptoms such as muscle spasms or cramps, convulsions, and lethargy (although some symptoms can be due to their illness and not because of the calcium deficiency).
At risk for getting too little dietary calcium include
• Women after menopause who produce less estrogen (a hormone), lose more bone, and absorb less calcium from dietary sources
• Women of childbearing age who do not get their periods because of eating disorders and/or excessive physical activity
• Those with lactose intolerance or lactose maldigestion who consume few or no dairy foods
Excess intakes of calcium, especially from supplements, can cause bloating, constipation, and kidney problems (such as kidney stones). It can also decrease your body’s absorption of the minerals iron, magnesium, and zinc. Although it’s rare, hypercalcemia can be caused by cancer, overproduction of parathyroid hormone (PTH) by the parathyroid gland, or excess vitamin D from supplements. Symptoms can include fatigue, confusion, decreased appetite, constipation, and impaired organ function.
Preeclampsia is a condition that can occur during pregnancy; symptoms include high blood pressure and protein in the urine.
Parathyroid disorders involve parathyroid glands that rest on the thyroid calcium and phosphorus. Hyperparathyroidism is caused by secretion of excess PTH (and elevates blood calcium levels); hypoparathyroidism is caused by secretion of too little PTH (and lowers calcium levels and elevates phosphorus levels). Hypocalcemia is a condition in which blood levels of calcium are lower than normal.
Hypercalcemia is a condition in which blood levels of calcium are higher than normal.