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

Increased by:

  • 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
  • Exercise
  • Phosphorus balance

Decreased by:

  • Vitamin D deficiency
  • Gastrointestinal problems
  • Hypochlorhydria (low stomach acid)
  • Stress
  • 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

Calcium Sources

Food Portion Calcium (mgs.)
Swiss cheese 2 oz. 530
Jack cheese2 oz.420
Cheddar cheese2 oz400
Other cheeses2 oz.300–400
Yogurt6 oz.300
Broccoli, cooked2 stalks250
Sardines (w/bones)2 oz.240
Goat milk6 oz.240
Cow’s milk6 oz.225
Collard greens, cooked6 oz.225
Turnip greens, cooked6 oz.220
Almonds3 oz.210
Brazil nuts3 oz.160
Soybeans, cooked6 oz.150
Molasses, blackstrap1 Tbl.130
Corn tortillas (4, w/lime)2 oz.125
Carob flour2 oz.110
Tofu3 oz.110
Dried figs3 oz.100
Dried apricots3 oz.80
Parsley1½ oz.80
Kelp¼ oz.80
Sunflower seeds2 oz.80
Sesame seeds2 oz.75

contain 20-30 times as much phosphorus as calcium. Molasses is fairly high in calcium, while some fruits, such as citrus, figs, raisins, and dried apricots, have modest amounts.

When the diet is high in phosphorus, we can lose extra calcium through the urine, resulting in calcium being pulled out of the bones. Phosphorus is very plentiful in meat foods and is of particular concern in soda pops that have added phosphoric acid (phosphate). This phosphorus-calcium imbalance may lead to kidney stones and other calcification problems, as well as increased atherosclerotic plaque. This issue is fairly complex and is under investigation. It is currently felt that the best calcium-phosphorus ratio in the diet is about 1:1.

Sunlight increases the manufacture of vitamin D in the body, and is like having an extra calcium source because vitamin D improves absorption of any available dietary calcium. Calcium supplements could be taken in the first couple of hours after sunbathing to improve utilization.

Dolomite and bonemeal (ground cattle bones) are good sources of calcium and magnesium. In recent years, however, both of these natural sources have been found to be contaminated with lead and other heavy metal toxicants. It is probably wise not to take these supplements in large amounts or over prolonged periods of time unless they are tested for contamination. Calcium found in “hard” water may also be an important source for maintaining body levels.

Functions: Calcium has some very important life-supporting functions; the best known is the development and maintenance of bones and teeth. Our need for calcium is critical during the growth years of infancy and childhood, but it is also important lifelong to keep our bones healthy. Exercise, vitamin D, and many other nutrients, such as phosphorus and magnesium, are also needed to maintain our skeleton. Bones are primarily calcium phosphate and a protein matrix. Tooth enamel is the hardest substance in the body, made up of 99 percent minerals, primarily calcium.

Bones are not only our most basic physical support structure, they are the main reservoir for calcium. Most minerals are in a state of dynamic activity and function, and even the calcium in bones is being added to and removed depending on the calcium balance in the body. The bones provide calcium to the blood and other tissues when we are not getting sufficient amounts from our diet. Vitamin D, parathyroid hormone, and calcitonin are responsible for maintaining this balance.

Circulating calcium also performs many other vital functions. Ionized (Ca++) calcium is needed for muscle contraction, as in muscular activity and in regulating the heartbeat. Heart function is mediated by several minerals: calcium stimulates contraction, magnesium supports the relaxation phase, and sodium and potassium are also important in generating the electrical impulse. Exercise can improve the circulation of calcium as well as that of all the other nutrients and thereby help the tone and function of the muscles, heart, and nervous system.

With regard to the nervous system, calcium is important in nerve transmission. Calcium ions influence nerve and cell membranes and the release of neurotransmitters. Calcium activates some enzyme systems, such as choline acetylase, which helps generate acetylcholine, an important neurotransmitter. Norepinephrine and serotonin are also affected by calcium. Calcium is said to be calming to the nerves, as higher concentrations tend to decrease nerve irritability.

Calcium plays an important role in the cells as well; it is necessary in cell division. Calcium is needed to activate prothrombin, which helps convert fibrinogen to fibrin and is essential to blood coagulation.

Uses: Calcium is one of the minerals most commonly prescribed by medical doctors (potassium is the other main one) because calcium deficiency is common and causes bone weakness through loss of bone calcium in the disease called osteoporosis. Osteoporosis is more common in the elderly population and occurs four times as often in women as in men. It can also occur at younger ages with chronic dietary insufficiency of calcium or with early menopause. Good evidence shows that there is a relationship between decreased calcium intake and osteoporosis; lack of exercise also increases bone loss. Moderate daily exercise as well as supplementing calcium and vitamin D results in reduced to restore a positive calcium balance. The best way to combat osteoporosis is to prevent it with regular exercise, a calcium-rich diet not too high in phosphorus, calcium supplements, and for menopausal women considering estrogen therapy. Exercise can actually stimulate bone renewal by improving bone uptake of calcium and other minerals.

Osteoporosis (the term literally means “porous bones”) is actually a loss of bone mass as the result of the loss of both minerals and protein; this differs slightly from osteomalacia, the bone problem seen in adults with vitamin D deficiency, which involves a softening of bones due to mineral loss alone. Rickets is the childhood equivalent of osteomalacia and is also caused by vitamin D deficiency. Extra calcium can help alleviate these problems somewhat, but the body needs supplemental vitamin D to get appreciable levels of calcium into the blood, tissues, and bones.

Calcium is the primary substance used in the prevention and treatment of osteoporosis, though estrogen used in menopausal hormone replacement therapy can reduce the likelihood of this disease in women. Since osteoporosis is found mainly in menopausal and postmenopausal woman, calcium is commonly seen as a treatment for problems of menopause. It does, in fact, reduce a number of the potential symptoms. Calcium not only helps the bones, especially when supplemented with magnesium and vitamin D, but may also reduce the headaches, irritability, insomnia, and depression sometimes associated with menopause. It is likely that a high percentage-as much as 70-90 percent-of bone fractures in people over 60 years of age are due to osteoporosis. These fractures are often more serious than an average fracture, because demineralized bones shatter when they break and take longer to heal. Actually, because osteoporotic fractures usually occur in the elderly and are so disabling, about one in six people dies within three months after sustaining them. Therefore, by helping to retard osteoporosis, calcium can prevent some fractures.

Osteoporosis is most common in elderly white women with a history of borderline calcium intake. Calcium is often drained from the bones during pregnancy and nursing and becomes hard to replace in later years, especially with reduced consumption of milk products and a lower calcium intake in general. Calcium supplementation can be helpful in reducing the leg cramps of pregnancy and the fatigue and depression after delivery. Children’s leg cramps are usually reduced by giving them calcium and magnesium. Calcium supplements tend to stimulate retention of calcium and decrease urinary excretion.

Calcium is often helpful for menstrual problems, particularly menstrual cramps, irritability or apprehension, and muscle cramps that occur around menstruation. The recently recognized premenstrual syndrome is often helped in part with additional calcium, though magnesium supplementation may be even more important. In some cases, however, reducing calcium intake can be helpful. Generally, muscle cramps or leg and foot cramps can be helped by calcium and vitamin D. Also, some cases of hyperkinesis in children, when associated with calcium deficiency, may be helped by supplementation.

Other problems related to bone health affect the mouth, jawbone, and teeth. In some cases, calcium may be helpful for problems of loose teeth, gingivitis (gum inflammation), and periodontal disease. Usually 1,000 mg. of calcium supplemented in the diet along with a dietary intake of phosphorus ranging from 1,000-2,000 mg. is suggested.

Calcium is often used to reduce heart irregularity; along with magnesium, it helps regulate heart contraction and relaxation. Through increasing contractility, calcium can help in congestive heart failure. Additional calcium may protect us from the toxicity of cadmium, rubidium, or mercury exposure by competing for absorption. Proper calcium intake may reduce the incidence of colon and rectal cancers through forming insoluble soaps with some mild carcinogens produced in the body, including bile acids and free, ionized fatty acids. A good calcium-phosphorus ratio in the diet also reduces the risk of cancer in the large intestine.

Deficiency and toxicity: In general, a high calcium intake for brief periods does not cause any problems, as excesses are usually eliminated in the urine and intestines. With magnesium deficiency, though, high amounts of calcium or vitamin D can lead to calcification of the soft tissues or to kidney stone formation. It is possible that prolonged high amounts of calcium (higher than a 2:1
calcium-phosphorus ratio) and supplemental vitamin D can lead to abnormal calcification of long bones in children or to hypercalcemia (high blood calcium levels) and soft tissue calcification in adults, as well as a decrease in bone strength. Also, if the parathyroid glands are not functioning well, calcium can accumulate and cause problems.

Calcium itself is thought to be one of the concerns in atherosclerosis, forming part of the plaque laid down in the arteries. Guy Abraham, M.D., who is known for his work in premenstrual syndrome, expressed a real concern over routine calcium supplementation in our society as he feels it exacerbates the degenerative process in the blood vessels, kidneys, and other organs and tissues. It is possible that these problems of calcium excess are not specifically related to dietary calcium but rather to calcium’s metabolism in relationship to the endocrine system. More research is clearly needed in this area. This potential toxicity concern makes me realize that it is important to be very aware of calcium metabolism and individual needs and to not just blindly supplement it as is so common recently.

Still, though, calcium deficiency is a more common concern in our culture than is excess calcium. This is especially true for the elderly, for alcoholics, for pregnant women, and for people with gastrointestinal disease. The “standard” American diet does not meet the normal calcium requirements; part of this problem is due to high phosphorus levels in the diet. Phosphorus is found in most foods, but soda pops, diet pops, meats, eggs, and processed foods such as lunch meats and cheese spreads contain especially high amounts. The ideal dietary phosphorus-calcium ratio is about 1:1. The ratio in the average American diet is often greater than 2:1 and sometimes even 4:1 or 5:1. At those levels, excess calcium is removed from bone and eliminated, blood levels are reduced, and there is bone demineralization. A diet high in phosphorus and low in calcium has been shown to cause bone loss and increase tissue calcification.

The skeletal system suffers most from calcium deficiency. Teeth minerals are more stable, though there is a possibility of poor dentition with insufficient calcium. Tooth loss, periodontal disease, and gingivitis can be problems, especially with a high phosphorus intake, particularly from soft drinks. All kinds of bone problems can occur with prolonged calcium deficiency, which causes a decrease in bone mass. Rickets in children, osteomalacia (decreased bone calcium) in adults, and osteoporosis (porous and fragile bones) can occur when calcium is withdrawn from bones faster than it is deposited. Fractures are more common with osteoporosis-almost eight million yearly in the United States are related to this prevalent nutritional deficiency disease. Although there must be loss in bone mass of almost 40 percent before it is visible by X-ray, the problem may be detected earlier through diet history or blood and nutritional tests. A program of regular exercise and calcium intake through diet and supplements, while limiting phosphorus intake, is a good way to prevent bone loss in the first place.

RDAs for Calcium

Birth–6 months360 mg.
6 months–1 year540 mg.
1–10 years800 mg.
11–18 years1,000 mg.
Men and women800 mg.
Pregnant women1,200 mg.
Lactating women1,200 mg.
Postmenopausal women
(not taking estrogen)1,200 mg.

Calcium deficiency in the blood can cause a wide range of other symptoms, such as toxemia of pregnancy, anxiety, hyperkinesis, otosclerosis, and alcoholism. One theory about multiple sclerosis correlates it with calcium and vitamin D deficiency in puberty. Mild calcium deficiency can cause nerve sensitivity, paresthesias, muscle twitching, brittle nails, irritability, palpitations, insomnia, confusion, or a feeling of chronic depression. As it progresses, leg and foot or other muscle cramps, heart palpitations, numbness, tingling, and, finally, tetany, the sustained contraction of some muscles causing severe pain, may all occur. Evidence shows that drinking soft water, which is high in sodium and low in calcium can lead to increases in cardiovascular disease. Hard water supplies extra calcium and magnesium, which may protect the heart.

Requirements: Since absorption of calcium is so variable, it is difficult to determine the right amount of calcium for all people. Many factors regarding absorption come into play. And the body can adapt to lower levels of calcium, even as low as 200 mg. per day, and still maintain calcium balance, though this adjustment usually needs to be started in childhood. In most Western cultures, with average absorption rates ranging from 30-50 percent, even the 800 mg. RDA may not be enough to prevent osteoporosis and other calcium deficiency problems. Possibly half of the population is getting less than the RDA, and many people are consuming a diet that supplies less than two-thirds of the RDA for calcium. An additional concern is that absorption usually decreases with age and with excessive use of antacids. But, luckily, humans are adaptable. Lower intake may lead to greater absorption efficiency, and higher intake usually leads to more elimination in the urine and feces. And the body may naturally guide us to calcium foods that we can use.

The RDAs for calcium, shown in the table, are based on an absorption rate of approximately 40 percent and average daily losses of an estimated 320 mg.

A more liberal (or, really, conservative) suggestion is 1,000 mg. (1 gram) daily in adults. In pregnancy and during nursing, 1.5 grams per day of calcium are suggested, especially in the last two months of pregnancy when over half of the baby’s calcium needs are supplied. The calcium intake suggested for postmenopausal women has recently been changed to 1.5 grams per day with some additional magnesium and vitamin D because of higher elimination and decreased absorption in these women.

People with high-protein, high-fat, or high-phosphorus diets need even more calcium. When we increase calcium, we should also increase our magnesium intake, keeping it at about one-half the calcium supply. Magnesium helps calcium stay more soluble, and thereby may reduce the risk of kidney stone formation and other calcifications. For phosphorus, an intake of about 800-1,000 mg. is recommended when the calcium intake is 1,000-1,200 mg.

Calcium is not absorbed well in an alkaline environment because it is less soluble. It is best taken between meals or in the absence of foods when the stomach is more acidic. Taking calcium with vitamin D and extra hydrochloric acid also increases absorption. Supplements of calcium or of calcium and magnesium are often taken at night before bed to help absorption and to prevent the extra loss of body calcium that can occur during the night. And calcium with magnesium is a good evening tranquilizer.

Other ideas for maximizing use of dietary calcium are spreading out calcium intake in balanced portions throughout the day; consuming protein, vitamin D, and vitamin C foods or supplements; adding more calcium-rich foods to the diet, especially in place of junk foods or phosphorus-rich soda pops; and taking supplemental calcium as part of a total mineral balance with magnesium, zinc, and manganese, for example. Recently, the trace mineral boron has been shown to help in calcium utilization and bone health.

The form in which calcium is supplied is also very important. Bonemeal and dolomite are good natural calcium and magnesium sources. They do not contain vitamin D but are still reasonably absorbable, though less so than other forms. There is some concern over lead and other toxic metals contaminating both dolomite and bonemeal. The form that I most highly recommend is aspartate or citrate salts of calcium, which are probably the most absorbable. Calcium aspartates are between 50-90 percent absorbable, which will likely place us in a positive calcium balance-exactly where we wish to be. Chelated calcium with amino acids are also easily absorbed. Calcium gluconate is the next choice, followed by calcium carbonate and lactate, which are also absorbable sources.

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Written by Elson M. Haas MD

Explore Wellness in 2021