You have more calcium in your body than any other mineral. Approximately 99 percent is in bones and teeth. The other 1 percent is needed for blood vessels, nerves and muscles to work properly. It's also needed for the secretion of hormones, including insulin. Calcium is so necessary for life itself that your body will take it from bones if your calcium intake is too low to maintain normal levels in the blood. If that happens, bones become weak and brittle.
Bone Formation
Bone is living tissue formed through a process called ossification. When you were a fetus, cartilage was the primary component of your skeleton, but as you grew in the womb, cells called osteoblasts and osteoclasts replaced the cartilage. Osteoclasts break down old bone, while osteoblasts make new bone by taking minerals, including calcium, from the blood and depositing them in bone tissue. Ossification was partially complete by birth, but your bones continued to lengthen through adolescence. After adolescence, osteoblasts continue to make new bone to maintain thickness and strength. Until around age 40, the rate of bone breakdown and rebuilding is about equal; after age 40, the rate of new bone development slows down.
Vitamin D and Other Nutrients
Ingesting calcium is not enough to ensure that blood levels remain high enough to keep your body from robbing your bones of calcium. Other nutrients are needed, including vitamin D. Your body synthesizes vitamin D from sunlight, and it is present in fortified foods and fatty fish, such as salmon. Your body converts vitamin D into its active form, calcitriol, which increases calcium absorption in the small intestine. Other minerals and vitamins are necessary for calcium absorption and optimal functioning of enzymes that help form bone. These include iron, copper, boron, magnesium, manganese, phosphorus, potassium, zinc and vitamins C and K. All are present in fruits, vegetables, whole grains, nuts and seeds. Another important mineral, fluoride, is present in fluoridated water and seafood.
Hormones
Hormones are important to bone formation and calcium absorption. Parathyroid hormone, secreted by the parathyroid glands, stimulates bone breakdown when blood levels of calcium fall too low. If more-than-enough calcium is in the blood, the thyroid gland secretes calcitonin, which stimulates osteoblasts to make bone tissue to store the excess calcium. Estrogen, secreted by the ovaries, also stimulates osteoblasts to form bone. The falling levels of estrogen after menopause increase a woman's risk of developing the low bone mass of osteoporosis.
Exercise
Exercise that puts stress on bones, such as high-impact exercise and weights, stimulates osteoblasts to build bone, while lack of stress on bones speeds breakdown. If you're young and athletic, you can build more bone mass to last you when you're older. When you're older, you cannot necessarily replace all bone mass that is lost, but you can slow the rate and increase muscle strength to help prevent falls and resultant fractures. You also need sufficient vitamin D and calcium -- 1,000 mg or more -- for exercise to have an effect.
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