Your body contains roughly two pounds of calcium, about 99 percent of it locked into bone. Think of that calcium as the mortar that cements and solidifies the components that give bone its substance and strength. The rest of your calcium is dissolved in your blood and the fluid inside and outside cells. That dissolved calcium helps conduct nerve impulses, regulates your heartbeat, and controls other cell functions.

Like an obsessive remodeler, your body constantly builds up bone and tears it down. Early in life, building up dominates. Throughout midlife, the two processes generally balance out. Later on, though, demolition may outpace construction and lead to weak or broken bones.

Many factors influence bone remodeling. Putting a bone under repeated stress, like the stress of lifting a weight or carrying a body at a trot, triggers growth. Lack of stress, like sitting all day, leads to degeneration. Sex hormones such as estrogen and testosterone stimulate bone-building activity.

The chaotic rush of these hormones during puberty sets off an adolescent’s growth spurt. Their loss later in life—a gradual ebbing away in men, a more abrupt cessation in women—nudges the balance toward bone loss, a shift that can be sudden and dramatic in women.

Recommended intake: The current recommended daily intakes for adults are 1,000 milligrams a day for women up to age fifty and 1,200 milligrams a day after that; and 1,000 milligrams a day for men up to age seventy and 1,200 milligrams a day after that. Given the inconsistent and sometimes misleading evidence on calcium and bone health, this is probably more than enough. You certainly need some calcium each day—it’s a good idea to get at least 500 milligrams—but 1,200 milligrams is probably more than you need, especially for men.

Good food sources: contrary to popular belief, dairy products aren’t the only or the best, way to get plenty of calcium. Other good food sources of calcium include sardines, tofu, canned salmon, turnip greens and kale, and fortified soymilk or orange juice. If you feel that you aren’t getting enough calcium in your diet and want to get more, try a calcium supplement. They contain no calories and no saturated fat and are far cheaper than several daily servings of dairy products. Chewable calcium-based antacids such as Tums are a cheap and efficient way to get calcium. A calcium supplement that also includes vitamin D is even better.

Safety: A high level of calcium in the blood (hypercalcemia) can cause problems ranging from kidney stones to hardening of the arteries. Although this can happen as a result of very high calcium intake, it is usually caused by overactivity of the parathyroid glands or cancer. Consuming too much calcium can cause constipation and may interfere with the absorption of iron and zinc. according to current evidence links higher intake of calcium from supplements with increased risk of kidney stones and prostate cancer. Calcium from foods, though, may reduce the risk of kidney stones by binding oxalate, a compound found in rhubarb, beets, spinach, nuts, tea, and a variety of chocolate and soy products, that has been linked to the formation of kidney stones.

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