Table of Contents
Calcium (Ca) is the most abundant mineral in the body. About 99% of calcium in the body is in bones and teeth. The remaining 1% is in blood and soft tissue. Calcium in body fluids is an electrolyte with a charge of + 2. Humans must meet their need for calcium through diet.
Calcium is essential for:
- building and maintaining strong bones and teeth.
- muscle contraction.
- blood vessel contraction and relaxation.
- nerve impulse transmission.
- regulating fluid balance in the body.
(Illustration by GGS Information Services/Thomson Gale.)
Most calcium in the body is stored in bones and teeth. Here it combines with phosphate to form strong, stable crystals. The remaining 1% is dissolved in body fluids and much of it forms Ca 2+ ions. In the body, these electrically charged particles are called electrolytes. Calcium and other electrolytes are not distributed evenly throughout the body. Dissolved calcium is found mainly in the fluid outside cells (extracellular fluid). Metabolic events cause the movement of calcium across cell membranes result in muscle contraction, nerve impulse transmission, and various chemical reactions. The cell then uses energy to restore the balance of calcium between the inside and outside of the cell membrane, so that the event can be repeated.
Normal calcium requirements
The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for many vitamins and minerals. The DRIs consist of three sets of numbers. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97-98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.
The IOM has not set RDAs for calcium, but instead it has set AI levels for all age groups based on observed and experimental information. However, many studies show that Americans across almost all age groups are not meeting these AI levels. One large study, the Continuing Survey of Food Intakes of Individuals, found that in children ages 6-11 44% of boys and 58% of girls were getting lower than recommended amounts of calcium. In adolescents ages 12-19, the rate of deficiency was higher—64% for boys and a huge 87% for girls. Adults over age 20 did not do much better with 55% of men and 78% of women taking in less calcium than recommended. IAs and ULs for calcium are measured in milligrams (mg). The following list gives the recommended AL and UL levels of calcium for each age group.
- children birth-6 months: AI 210 mg; UL not established;.
- children 7-12 months: AI 270 mg; UL not established.
- children 1-3 years: AI 500 mg; UL 2,500 mg
- children 4-8 years: AI 800 mg; UL 2,500 mg
- children 9-13 years: RDA 9 mg; UL 2,500 mg
- adolescents 14-18 years: IA 1,300 mg; UL 2,500 mg
- adults age 19-50: RDA 1,000 mg; UL 2,500 mg
- adults over age 50: RDA 1,200 mg; UL 2,500 mg
- pregnant women 18 years and younger: RDA 1,300 mg; UL 2,500 mg
- pregnant women over age 18: RDA 1,000 mg; UL 2,500 mg
- breastfeeding women 18 years and younger: RDA 1,300 mg; UL 2,500 mg
- breastfeeding women over age 18: RDA 1,000 mg;2,500 mg
Sources of calcium
In the United States, dairy products—milk, yogurt, and cheese—are the main sources of dietary calcium. Low-fat dairy products, such as skim milk or reduced-fat cheese, contain about the same amount of calcium as whole milk products. Other sources of calcium include canned fish with bones, dark green leafy vegetables, and tofu made with calcium sulfate. Other types of tofu do not contain significant amounts of calcium. Processed foods such as orange juice, breakfast cereal, instant breakfast drinks, and bread are often fortified with calcium. This will be indicated on the label.
The following list gives the approximate manganese content for some common foods:
- milk, any type, 1 cup (8 ounces): 300 mg
- yogurt, plain, 8 ounces: 415 mg
- yogurt with fruit, 8 ounces: 245-384 mg
- cheddar cheese, 1.5 ounces: 305 mg
- mozzarella cheese, 1.5 ounces: 372 mg
- sardines with bones, canned in oil, 3 ounces: 324
- salmon with bones, canned, 3 ounces: 181 mg
- tofu, firm, made with calcium sulfate, 1/2 cup: 204 mg
- pinto or red beans, cooked, 1/2 cup: 43 mg
- white beans, cooked, 1/2 cup: 113 mg
- bok choy, 1/2 cup cooked: 61 mg
- spinach, cooked, 1/2 cup: 120 mg
- bread, whole wheat, 1 slice 20 mg
- orange juice, fortified, 6 ounces: 200–260 mg
- instant breakfast drink, powder prepared with water, 105–250 mg
- breakfast cereal, fortified, 1 cup: 100–1,000 mg
Although experts recommend that people meet as many of their vitamins and minerals needs through diet as possible, it is difficult for many people to get enough calcium from food alone. This is especially true for vegans, who eat no dairy products, adolescent girls who are very calorie conscious and tend to avoid milk and replace it with diet sodas, and people with lactose intolerance who cannot easily digest dairy products. Pregnant women and older individuals may also have a hard time eating enough to meet their calcium needs. People who do not get enough calcium through diet can benefit from taking a dietary supplement containing calcium.
Calcium supplements are available over–the–counter. The most common supplements supply calcium in the form of calcium carbonate or calcium citrate. Calcium carbonate is usually the most economical calcium supplement. People who are taking medications to reduce stomach acid may more easily absorb calcium citrate. Some supplements combine calcium and vitamin D because vitamin D helps the body absorb calcium. No calcium supplement contains enough calcium meet the entire daily adequate intake, because the pill would be too large to swallow. In addition, the body absorbs calcium best in doses of 500 mg or less. People who need more than 500 mg of supplemental calcium should divide the dose in half to be taken morning and evening.
Calcium deficiency, called hypocalcemia, can occur because of inadequate calcium intake, excess calcium excretion by the kidney (usually caused by kidney damage), the inability to adequately absorb calcium, or because of interactions between calcium and some prescription drugs. People at highest risk of calcium deficiency are teenagers, women past the age of menopause, individuals who are lactose intolerant, vegans, and people with kidney (renal) damage.
Calcium deficiency rarely shows up in blood tests because calcium is withdrawn from the bones to maintain blood levels of calcium. The bones then become less dense, weaker, and more likely to break. This condition is called osteoporosis and it is most noticeable in the elderly who have a high rate of broken bones resulting from falls. Osteoporosis is a part of aging, but eating a healthy diet high in calcium, getting adequate vitamin D, and doing weight–bearing exercises regularly can delay its onset. Severe calcium deficiency, is usually caused by a medical condition rather than inadequate calcium intake. It causes symptoms such as muscle cramps, tingling in the fingers, lethargy, convulsions, heart rhythm abnormalities, and death. These symptoms can also be caused by many other diseases.
Calcium excess is called hypercalcemia. It usually results from poor kidney function (renal failure) or from a malignant cancer tumor It can also be caused by very large supplemental doses of vitamin D. Very rarely is hypercalcemia caused by too much calcium from food or dietary supplements. High levels of calcium interfere with the absorption of other minerals such as iron, zinc, magnesium, and phosphorous. People with hypercalcemia usually have multiple medical problems and are under the supervision of a physician.
People of all ages, races, and gender need to be alert to getting enough calcium in their diet. Building strong, dense bones begins in childhood and adolescence, even though the results cannot be seen until old age. People mentioned above as being at especially high risk of low dietary calcium intake should investigate taking a calcium supplement.
Absorption of calcium is affected by several conditions.
- Age. Infants absorb as much as 60% of the calcium in their digestive system. This decreases to 15–20% in adulthood, and even less in old age.
- Pregnancy. Pregnancy increases the efficiency of calcium absorption in the intestine to meet the demands of fetal development.
- Vitamin D The presence of vitamin D improves calcium absorption. Vitamin D deficiency can worsen calcium deficiency.
- Plant products. Phytic found in beans and oxalic acid found in spinach and leafy greens decrease the amount of calcium absorbed from those foods, but does not affect the absorption of calcium from other foods present at the same time in the intestine. Fiber such as wheat bran also reduced calcium absorption.
Prescription medications can also affect or be affected by the absorption of calcium. These include:
- tetracycline antibiotics
- thiazide–type diuretics
- mineral oil
- stimulant laxatives
People taking these drugs should check with their healthcare provider or pharmacist about potential adjustments in their medications or calcium intake.
No complications are expected when healthy people take calcium in amounts equal to the AI level and less than the UL level. Some people experience gas, nausea, and abdominal discomfort from calcium supplements. Taking the supplement with meals, taking smaller doses spread out over the day, or changing the type of supplement usually solves this problem. Complications of excess calcium and calcium deficiency are discussed above.
Building strong bones starts in childhood, and parents should be aware of how much calcium their child needs compared to how much he or she is getting. As children get older they tend to replace milk in their diet with juice, bottled water, and especially carbonated soft drinks. This leads to large calcium deficiencies during adolescence. Parents should monitor their child’s diet and encourage calcium supplements if they cannot induce their adolescents to eat more dairy products and other calcium–rich food.
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Tish Davidson, A.M.