Calcium and Phosphorus

Calcium and phosphorus are used for building bones and teeth. Over 80 percent of the body's calcium and phosphorus is found in the bones and teeth, where they give rigidity to the structures. Bone is being rebuilt every day, with new bone being formed and old bone being taken apart. There is little turnover of calcium in teeth.

Calcium also circulates in the blood, where a constant level is maintained so it is always available for use. Calcium helps blood to clot, muscles to contract (including the heart muscle), and nerves to transmit impulses. Calcium may help in lowering blood pressure and reducing the risk of colon cancer. In cases of inadequate dietary intake, calcium is taken out of the bones to maintain adequate blood levels.

Like calcium, phosphorus circulates in the blood. Phosphorus is involved in the metabolic release of energy from fat, protein, and carbohydrates. It is also a part of DNA (genetic material) and is therefore needed for growth. Normal body processes produce acids and bases that can cause major blood and body problems, such as coma and death, if not buffered (or neutralized) somehow. In each of the body's cells, phosphorus has a role to buffer both acids and bases. Many enzymes become active when a phosphate group is attached.

The major sources of calcium are milk and milk products. Not all milk products are as rich in calcium as milk. As a matter of fact, butter, cream, and cream cheese contain little calcium. One cup of milk or yogurt or 1-1/2 ounces of cheese each have a little less than one-third of the AI (Adequate Intake) for most adults.

Without milk or milk products in your diet, it may be difficult to get enough calcium. Other good sources of calcium include tofu made with calcium carbonate, calcium-fortified foods such as orange juice, and several greens such as broccoli, collards, kale, mustard greens, and turnip greens. Other greens such as spinach, beet greens, Swiss chard, and parsley are calcium-rich but also contain a binder (oxalic acid) that prevents some calcium from being absorbed. Dried beans and peas, whole-wheat bread, and certain shellfish contain moderate amounts of calcium but are usually not eaten in sufficient quantities to make a significant contribution. Phytic acid, a binder found in wheat bran and whole grains, also prevents some calcium from being absorbed.

• Oxalic acid— An organic acid found in spinach and other leafy green vegetables that can decrease the absorption of certain minerals such as calcium.

• phytic acid— A binder found in wheat bran and whole grains that can decrease the absorption of certain nutrients such as calcium and iron.

About 25 to 30 percent of the calcium you eat is absorbed. The body absorbs more calcium (up to 60 percent) during growth and pregnancy, when additional calcium is needed. Absorption is higher for younger people than for older people. Postmenopausal women, who are at higher risk of developing osteoporosis, often absorb the least calcium unless they are taking estrogen Sufficient vitamin D helps calcium absorption and is added to milk.

Calcium deficiency is much more common in women than men and is a major contributing factor in a disease called osteoporosis, discussed later in this chapter. Calcium can be toxic when large doses of supplements are taken. The Tolerable Upper Intake Level for calcium is 2,500 milligrams per day; amounts above that can contribute to the development of calcium deposits in the kidneys and other organs, kidney failure, and other problems.

Phosphorus is widely distributed in foods and is rarely lacking in the diet. Milk and milk products are excellent sources of phosphorus, as they are for calcium. Other good sources of phosphorus are meat, poultry, fish, eggs, and legumes. Fruits and vegetables are generally low in this mineral. Compounds made with phosphorus are used in processed foods, especially soft drinks (phosphoric acid).

Calcium in Selected Foods 

Food	Calcium Content (milligrams)
Dairy Group
Milk, skim, 8 ounces	302 milligrams
Milk, 2%, 8 ounces	297
Milk, whole, 8 ounces	291
Yogurt, low-fat, 8 ounces	415
Yogurt, low-fat with fruit, 8 ounces	345
Yogurt, frozen, 1 cup	200
Ice cream, vanilla, 1 cup low-fat	176
Cottage cheese, low-fat, 1 cup	155
Swiss cheese, 1 ounces	272
Parmesan, grated, 2 tablespoons	138
Cheddar, 1 ounce	204
Mozzarella, 1 ounce	147
American cheese, 1 ounce	174
Cheese pizza, 1/4 of 14-inch pie	332
Macaroni and cheese, 1/2 cup	181
Meat, Fish, and Alternatives
Sardines, drained, 2 ounces	175
Oysters, cooked, 3 ounces	76
Shrimp, cooked, 3 ounces	33
Tofu, calcium set, 3-1/2 ounces	128
Dried navy beans, cooked, 1 cup	95
Vegetables
Turnip greens, frozen and cooked, 1 cup	249
Kale, frozen and cooked, 1 cup	179
Mustard greens, 1 cup	104
Broccoli, cooked, 1/2 cup	35
Other Foods
Oatmeal, instant, fortified, 1 packet	160
Pancakes, from mix, one 4-inch pancake	30
Wheat bread, 2 slices	40
Orange juice, calcium fortified, 8 ounces	330
Source: U.S. Department of Agriculture Handbook 8, and Home and Garden Bulletin No. 72, and manufacturers.