An essential mineral nutrient and the most abundant mineral in the body. Calcium represents approximately 2 percent of the total body weight; about 98 percent of this is found in the bones and teeth. The small amount of calcium in body fluids and cells plays an important role in nerve transmission, muscle contraction, heart rhythm, hormone production, wound healing, immunity, blood coagulation, maintaining normal blood pressure, and STOMACH ACID production. Calcium promotes blood clotting through the activation of the fibrous protein FIBRIN, the building block of clots. It lowers blood pressure in patients with spontaneous HYPERTENSION (not caused by KIDNEY disease) because it relaxes blood vessels, and it may also diminish the symptoms of PREMENSTRUAL SYNDROME (PMS).
High intake of saturated fat tends to raise LOW-DENSITY LIPOPROTEIN (LDL) cholesterol (the less desirable form) and to increase the risk of colorec-tal CANCER. On the other hand, calcium binds saturated fats, preventing their uptake by the intestine; consequently, calcium-rich diets may reduce LDL cholesterol. A high calcium intake also seems to reduce the risk of colon cancer.
If blood levels of calcium decrease in response to low calcium consumption, the body pulls calcium out of bones to use elsewhere. Thus, bones are dynamic tissues, constantly releasing calcium and reabsorbing it to maintain their strength. The level of calcium in the blood is carefully regulated by hormones. Parathyroid hormone from the parathyroid gland stimulates bone-degrading cells to break down bone tissue to release calcium and phosphate into the bloodstream (a process called bone resorp-tion). Parathyroid hormone also stimulates calcium absorption from the intestines by activating VITAMIN D, and stimulates calcium reabsorption from the kidney filtrate back into blood. This effect is counterbalanced by calcitonin, released from the thyroid gland when blood calcium levels are high. Calci-tonin triggers bone-building cells (osteoblasts) to take up calcium from blood to lay down new bone.
During growth spurts, more calcium is absorbed than lost. Therefore, adequate calcium intake in childhood and adolescence is critical for bone building. In addition, ZINC, manganese, fluoride, copper, boron, MAGNESIUM, calcium, and vitamin D, together with EXERCISE, minimize bone loss after the age of 35. Calcium absorption requires the hormone calcitriol, formed from vitamin D.
According to the U.S. Department of Agriculture (USDA), most Americans do not consume adequate amounts of calcium. The lack of calcium in the diet of children and adolescents is especially alarming because 90 percent of an adult’s bone mass is established by the age of 19. Only 14 percent of girls and 36 percent of boys age 12 to 19 in the United States consume enough calcium daily to meet current requirements. Those who do not are at increased risk of developing osteoporosis and other bone diseases.
Symptoms of prolonged calcium deficiency include insomnia, heart palpitations, and muscle spasms, as well as arm and leg numbness. Chronic
low calcium intake can lead to easily fractured bones due to bone thinning (OSTEOPOROSIS), and possibly hypertension. Severe deficiency symptoms are rare: convulsions, dementia, osteomalacia, rickets (bent bones and stunted growth in children), and periodontal disease.
In addition to age and heredity, many lifestyle and dietary factors increase the risk of developing calcium-related problems: age; heredity; chronic emotional STRESS; lack of exercise; dieting; excessive CAFFEINE, SODIUM, phosphorus (as found in processed foods and soft drinks), or dietary FIBER; high-fat foods; possibly high protein diets; low vitamin D intake; long term use of corticosteroids; and cigarette smoking. Condition like INFLAMMATORY BOWEL SYNDROME, low stomach acidity, LAC-TASE deficiency, kidney failure, and diabetes increase the need for calcium, while mineral oil (laxative), lithium, and some DIURETICS (water pills) block calcium uptake.
Dietary Sources of Calcium
The DIETARY REFERENCE INTAKE for children between ages 4 and 8 is 800 mg; for children from 9 to 13, 1,300 mg; for adolescents between 14 and 18, 1,300 mg; for adults between 19 and 50, 1,000 mg; and for adults over 50, 1,200 mg. For calcium, the lowest observed adverse effect level is 2.5 g for adults. Milk products like yogurt and CHEESE represent rich calcium sources. They need not be high in fat. Low-fat dairy products like skim or low-fat milk and low-fat YOGURT contain about 300 mg calcium per cup. SARDINES and canned SALMON with cooked bones and high in calcium; plant sources include green leafy vegetables, COLLARD greens, CHARD, beet tops, BOK CHOY, spinach, and BROCCOLI, as well as various seeds and SOYBEANS. The calcium in spinach is less easily absorbed. Two very good plant sources are TOFU, prepared with calcium to curdle soybean protein, and corn tortillas, prepared with lime. The following are examples of low-fat, high-calcium food:
1% fat cottage cheese (half cup) 70 mg calcium
non-fat yogurt (half cup) 225
skim milk (1 cup) 300
cooked greens (1 cup) 100
cooked collard greens (1 cup) 280
cooked soybeans (1 cup) 450
tofu (1 ounce) 130
corn tortilla (1 ounce) 300
sardines (3 ounces) 370
Calcium Fortification
Calcium is added to foods and beverages. The food industry has responded to consumer fears of OSTEOPOROSIS (age-related thinning of bones) by adding calcium to a variety of foods and diet drinks, including some brands of orange juice, BREAKFAST cereals, whole milk, yogurt, cheese, sliced cheese, cottage cheese, white flour, bread, and cocoa. Fortified or enriched foods can supply 25 percent to 100 percent of the calcium RDA per serving. Individuals prone to kidney stones might have problems with excessive calcium, and excessive calcium from any source can cause milk-alkali syndrome, which damages the kidneys. A very high calcium intake can block the uptake of MANGANESE, another essential mineral.
Calcium Supplements
The advantages of obtaining calcium from food are twofold. First, calcium is better absorbed, and second, it is almost impossible to overdose on calcium from food. However, the typical U.S. diet provides only 450 to 550 mg of calcium daily, and individuals who avoid dairy products may encounter difficulty in obtaining adequate calcium from foods alone. Certain groups are more likely to develop calcium deficiencies: dieters, smokers, women past menopause or who have had hysterectomies, and those who drink several cups of coffee or several alcoholic beverages daily. For those who have a marginal calcium intake, calcium supplementation with vitamin D is a responsible alternative.
Most types of calcium supplements are effective, and calcium carbonate is inexpensive. Orange juice can aid calcium uptake from calcium carbonate. It is generally believed that chelated calcium (calcium citrate, lactate, gluconate, orotate) may be more easily absorbed than calcium carbonate when stomach acid production is low, although this view has been challenged. Calcium tablets need to disintegrate in water for calcium absorption to occur. The best way to take calcium supplements is to combine them with vitamin D. Look for calcium
supplements that are “essentially lead free” to minimize possible contamination with small amounts of lead.
Calcium supplementation can reduce depression, water retention, and pain related to premenstrual syndrome (PMS). Calcium supplementation reduces the risk of osteoporosis in postmenopausal women. Continuous supplementation with calcium after menopause can improve bone mass by 10 percent and reduce the risk of bone fractures by 50 percent. Moreover, drugs used to treat osteoporosis are most effective when calcium intake is adequate.
There are several precautions to be aware of in using calcium supplements. Excessive calcium supplementation (3,000 to 8,000 mg per day) increases the risk of ZINC and MAGNESIUM deficiencies. Calcium supplements taken with meals may block the uptake of other minerals like COPPER, IRON, and zinc. Overdosing with calcium supplements also increases the risk of kidney stones in susceptible people. Excessive calcium supplements can lead to vomiting, high blood pressure, DEPRESSION, excessive urination, muscle wasting, and CONSTIPATION.
NIH Consensus Development Panel On Optimal Calcium Intake, “Optimal Calcium Intake,” Journal of the American Medical Association, 272, no. 24 (December 1994): 1,942–1,948.
