Calcium is the most abundant mineral in the human body and one of the most important. The calcium topic is huge and complex; let me try to make it concise and simple. This mineral constitutes about 1.5-2.0 percent of our body weight. Almost all (98 percent) of our approximately three pounds of calcium is contained in our bones, about 1 percent in our teeth, and the rest in the other tissues and the circulation.
Calcium and magnesium are the "earth alkali" minerals found in the earth's crust, usually as salts that are fairly insoluble. (The word calcium comes from the Latin calc, meaning "lime," as in limestone, a calcium carbonate substance.) Dolomite, a calcium-magnesium earth mineral combination that is a little more soluble and usable by the body than some other forms, is a commonly used calcium supplement.
Many other nutrients, vitamin D, and certain hormones are important to calcium absorption, function, and metabolism. Phosphorus as well as calcium is needed for normal bones, as are magnesium, silicon, strontium, boron possibly, and the protein matrix-all part of our bone structure. The ratio of calcium to phosphorus in our bones is about 2.5:1; the best proportions of these minerals in our diet for proper metabolism are currently under question.
Calcium works with magnesium in its functions in the blood, nerves, muscles, and tissues, particularly in regulating heart and muscle contraction and nerve conduction. Vitamin D is needed for much calcium (and phosphorus) to be absorbed from the digestive tract. Along with parathyroid hormone and calcitonin secreted by the thyroid, vitamin D helps maintain normal blood calcium levels.
Maintaining a balanced blood calcium level is essential to life, especially for cardiac function. A normal blood calcium level is about 10 mg. percent-that is, about 10 mg. per 100 milliliters (ml.) of blood. Of that, approximately 5.5 mg. are in ionic form as Ca++, about 4 mg. are bound to carrier proteins, and about 0.5 mg. is combined with phosphate or citrate. If there is not enough calcium in the diet to maintain sufficient amounts of calcium in the blood, the parathyroid glands will be activated to release more parathyroid hormone (PTH), which will then draw calcium out of the bones as well as increase intestinal absorption of available calcium. So even though most of our body's calcium is in the bones, the blood and cellular concentrations of this mineral are maintained first. This is why, with nearly 30 percent of people in this country eating calcium-deficient diets, osteoporosis (a loss of bone substance) is so prevalent.
Elderly people usually have less calcium in their diets than others do, and calcium deficiency particularly affects postmenopausal women. But low dietary calcium is only one factor in the huge and complex topic of calcium bio-availability from foods, calcium absorption, and osteoporosis. Many factors are involved in making calcium available for its many essential functions. Vitamin D is, of course, most essential to calcium absorption, though this may be less necessary when the calcium chelates, such as calcium aspartate or calcium citrate, are used. Some clinical studies need to be done to see which calcium supplements are readily transported into the body and how vitamin D may affect them. Many doctors do not consider this important absorption issue and prescribe an oyster shell or a dolomite or bone-meal source as a calcium supplement. Frequently, calcium lactate or calcium carbonate (such as Tums) pills-which are more alkaline and slowly absorbed-are prescribed without suggesting additional vitamin D and magnesium, so important for calcium balance and metabolism. A woman who needs more calcium could be taking an extra gram a day without really getting much of it into her blood or bones.
In general, calcium absorption becomes less efficient as we age. During infancy and childhood, 50-70 percent of the calcium ingested may be absorbed, whereas an adult might use only 30-50 percent of dietary calcium in his or her body. It is likely this is based on natural body needs. Various factors can improve our calcium absorption. Besides vitamin D, vitamins A and C can help support normal membrane transport of calcium. Protein intake helps absorption of calcium, but too much protein may reduce it. Some dietary fat may also help absorption, but high fat may reduce it. Lactose helps calcium absorption, and because of this as well as the protein-fat combination, the calcium content of milk is a reliable source of easily assimilated calcium. For other reasons, though, milk is not an ideal food for many people, especially the homogenized variety fortified with synthetic vitamin D, making milk a less-than-perfect (and definitely not the only) source of calcium. Nonfat milk does not improve calcium absorption and, in fact, may decrease it.
Gastric hydrochloric acid helps calcium absorption. The duodenum is the main location for absorption of calcium because farther down the small intestine the local environment becomes too alkaline. A fast-moving intestinal tract can also reduce calcium absorption. Exercise has been shown to improve absorption, and lack of exercise can lessen it. Stress also can diminish calcium absorption, possibly through its effect on stomach acid levels, digestion, and intestinal motility. Though calcium in the diet improves the absorption of the important vitamin B12, too much of it may interfere with the absorption of the competing minerals magnesium, zinc, iron, and manganese.
Many dietary factors also reduce calcium absorption. Foods that are high in oxalic acid, such as spinach, rhubarb, chard, and chocolate, can interfere with calcium absorption by forming insoluble salts in the gut. Phytic acid, or phytates, found in whole grain foods or foods rich in fiber, may reduce the absorption of calcium and other minerals as well. Protein, fat, and acid foods may help calcium absorption, but high-protein diets may increase calcium elimination through the intestines. Calcium absorption is sensitive and requires energy to transport it into the body. Calcium is often chelated with proteins or amino acids (specifically, glutamic or aspartic acid) to make it more absorbent.
Factors Affecting Calcium Absorption
- Body needs—growth, pregnancy, lactation
- Vitamin D
- Milk lactose
- Acid environment—hydrochloric acid, citric acid, ascorbic acid (vitamin C)
- Protein intake and amino acids such as lysine and glycine
- Fat intake
- Phosphorus balance
- Vitamin D deficiency
- Gastrointestinal problems
- Hypochlorhydria (low stomach acid)
- Lack of exercise
- High fat intake
- High protein intake
- Oxalic acid foods (beet greens, chard, spinach, rhubarb, cocoa)
- Phytic acid foods (whole grains)
- High phosphorus intake
Because of the many complex factors affecting calcium absorption, anywhere from 30-80 percent may end up being excreted. Some may be eliminated in the feces. The kidneys also control calcium blood levels through their filtering and reabsorption functions. Excess salt intake can lead to increased calcium losses in the urine. Sugar intake may reduce the reabsorption of calcium and magnesium and cause more to be eliminated. The table on the previous page summarizes the factors affecting calcium absorption.
Overall, we need good sources of calcium in our diets, good nutritional habits, and a diet that promotes healthy gastrointestinal function. Taking calcium and magnesium at bedtime or between meals, when the stomach may be more acidic, is often helpful for better absorption. Regular exercise, good nutrition, and lots of vegetables are important basics for providing the essential calcium we need and good health in general.
Sources: Calcium is found in many foods but is in high amounts in only a few. Even in light of my previous discussion, milk should be considered a good source of calcium as well as containing protein and fat and having a good balance of magnesium and phosphorus (a balanced calcium-phosphorus ratio is important). The lactose in milk also helps calcium absorption, but about 70 percent of blacks and 6 percent of Caucasians are lactose-intolerant-drinking milk makes them sick. An eight-ounce glass of whole milk contains about 300 mg. of calcium. Most other milk products also give good supplies of calcium-yogurt, most cheeses, and buttermilk, for example.
Many green, leafy vegetables are good sources of calcium, but some contain oxalic acid, so their calcium is not easily absorbed. Spinach, chard, and beet greens are not particularly good sources of calcium, whereas broccoli, cauliflower, and many peas and beans offer better supplies. (Pinto, aduki, and soybeans are good sources of calcium.) Many nuts, particularly almonds, brazil nuts, and hazelnuts, and seeds such as sunflower and sesame contain good amounts of calcium; though their phosphorus content is about double that of their calcium, this is much more of a concern with meats, which often
|Jack cheese||2 oz.||420|
|Cheddar cheese||2 oz||400|
|Other cheeses||2 oz.||300–400|
|Broccoli, cooked||2 stalks||250|
|Sardines (w/bones)||2 oz.||240|
|Goat milk||6 oz.||240|
|Cow’s milk||6 oz.||225|
|Collard greens, cooked||6 oz.||225|
|Turnip greens, cooked||6 oz.||220|
|Brazil nuts||3 oz.||160|
|Soybeans, cooked||6 oz.||150|
|Molasses, blackstrap||1 Tbl.||130|
|Corn tortillas (4, w/lime)||2 oz.||125|
|Carob flour||2 oz.||110|
|Dried figs||3 oz.||100|
|Dried apricots||3 oz.||80|
|Sunflower seeds||2 oz.||80|
|Sesame seeds||2 oz.||75|