Calcium

Daily Value: 1,000 milligrams

Good Food Sources: Skim milk, nonfat yogurt, cheeses, collard greens, mustard greens, kale, broccoli, canned salmon with bones, sardines with bones, corn tortillas processed with lime, calcium-fortified orange juice

By now, just about everyone knows that getting enough calcium helps prevent diseases such as osteoporosis. Less well known is just how calcium goes about doing this.

When you eat cheese or drink milk, the calcium in these foods is absorbed through your small intestine and into your blood. The amount of calcium in your blood is regulated by a substance called parathyroid hormone. When calcium intake is low, parathyroid hormone signals for bone to be broken down, releasing calcium into the bloodstream. "Diets with adequate calcium intake produce less parathyroid hormone, so we conserve more calcium and more bone," says John Anderson, Ph.D., professor of nutrition in the Schools of Public Health and Medicine at the University of North Carolina at Chapel Hill.

Calcium then combines with phosphorus to help form hard, crystal-like substances that create the latticework undergirding strong bones and teeth. In fact, 99 percent of the calcium in your body is stored in your skeleton. Researchers call this ongoing process of removing old bone and forming new bone remodeling.

You also need a stable level of blood calcium for normal heartbeat, nerve and muscle function and blood clotting. Living cells require calcium to act as a messenger and to help respond to hormones and neurotransmitters.

Even though calcium is vital for bone growth and maintenance in everyone, experts don't advise a one-size-fits-all intake. Here are daily intake levels as set by the Consensus Development Conference of the National Institutes of Health in Bethesda, Maryland.

* Infants, up to age 6 months: 400 milligrams

* Infants, ages 6 to 11 months: 600 milligrams

* Children, ages 1 to 10 years: 800 to 1,200 milligrams

* Adolescents and young adults, ages 11 to 24: 1,200 to 1,500 milligrams

* Men, ages 25 to 65: 1,000 milligrams

* Women, ages 25 to 50: 1,000 milligrams

* Pregnant and nursing women: 1,200 to 1,500 milligrams

* Women at menopause (ages 51 to 65) who are taking estrogen: 1,000 milligrams

* Women at menopause (ages 51 to 65) who are not taking estrogen: 1,500 milligrams

* Men and women over age 65: 1,500 milligrams

Using Calcium Safely

If you want your supplement to be rich in calcium but aren't crazy about eating aluminum or lead, steer clear of those made of natural calcium carbonate, says Richard Wood, Ph.D., associate professor in the School of Nutrition at Tufts University in Medford, Massachusetts, and chief of the mineral bioavailability laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston. "They can be contaminated by these other things that you don't want in your diet," he says. Even some forms of dolomite, a natural calcium-magnesium combination supplement, have been found to contain these metals.

On the other hand, pharmaceutical-grade calcium carbonate should be free of metals. Calcium gluconate, calcium lactate and calcium citrate are also metal-free, but each of these types contains less concentrated forms of calcium. Calcium carbonate is also found in several brands of antacid tablets, and many people use these antacids as sources of calcium. But some antacids also contain aluminum, a metal that can prevent adequate mineralization in bone. So be sure to choose tablets that are aluminum-free, such as Tums or Rolaids.

Calcium is best absorbed when it's taken with food and at a dose not exceeding 500 milligrams. This means that if you're taking supplements exceeding that amount, you should take them in divided doses throughout the day. Also, avoid taking calcium supplements with high-fiber wheat bran cereals, which can reduce absorption by 25 percent.

High calcium intake (more than 2,000 milligrams a day) may cause constipation and kidney stones and inhibit zinc and iron absorption. High blood levels of the mineral cause the body to excrete any excess, which in turn triggers a loss of magnesium.