Calcium is a mineral that people obtain from their diets and that is crucial to building and maintaining strong bones and teeth. In addition, calcium is involved in many other body processes, including muscle and blood vessel contraction, regulating the heartbeat and conducting nerve impulses to send messages through the nervous system.

Calcium is the most common mineral found in the body. About 99 percent of calcium is found in the bones and teeth. Smaller amounts are also found in the bloodstream and soft tissues.

The body needs calcium but does not make it, so obtaining appropriate amounts of calcium from the diet is crucial. Milk and dairy products offer the biggest sources of calcium. Other sources include some types of canned fish, green leafy vegetables, and fortified breakfast cereal, cereal bars and orange juice.

A calcium deficiency occurs when people do not have adequate stores of calcium. This can result from inadequate calcium intake, decreased calcium absorption and increased calcium excretion. When the body does not have enough calcium, it draws calcium from the bones, which can cause the bones to weaken, raising the risk of fracture. Calcium deficiency over an extended period of time also increases the risk of loss of bone density known as osteoporosis.

Several factors affect how calcium is absorbed from the digestive tract into the body’s circulation. These include the amount of calcium consumed during a single meal, a person’s age, and the presence of other vitamins (especially vitamin D) and substances in the body.

Calcium is the most common mineral found in the body. It is necessary to build and maintain strong bones and teeth and aids other body processes. On average, calcium makes up about 3 pounds (1.36 kilograms) of the body weight of a person who weighs 130 pounds (59 kilograms).  

Almost all calcium in the body is stored in a person’s bones. This mineral helps bones to remain strong and stiff enough to carry the weight of the body and to protect internal organs. Calcium in the bones also serves as a storehouse for the mineral when it is needed elsewhere, such as in the bloodstream and soft tissues. Calcium helps many body processes, including muscle and blood vessel contraction, blood clotting, enzyme and hormone secretion and helps the central nervous system to function.

Consuming appropriate levels of calcium is crucial because the body does not make its own calcium. In addition, the body needs a constant supply of calcium to keep bones growing and strong. Bone remodeling is an ongoing process in which bone is broken down (resorbed) and calcium is deposited to replace the bone material that is lost (bone formation). When the amount of calcium in the bloodstream is low, calcium from bone is broken down and is used in the bloodstream. This can deplete the calcium in bones, a process that may take many years.

During childhood, people undergo a greater amount of bone formation and less bone breakdown. Formation and breakdown are relatively equal during early and middle adulthood. After age 35, more bone is lost than is replaced. Bone loss especially accelerates in women following menopause. People can continue to build bone mass until their mid-30s. After that, they can only slow the bone loss that is a natural part of aging.

By consuming adequate levels of calcium early in life, people can build up a maximum amount of bone that will help protect them later in life when their bones inevitably begin to lose more calcium than they replace. Weight-bearing exercise (e.g., walking, running) throughout the lifespan helps to maximize bone strength and bone density (the amount of bone tissue in a certain volume of bone).

Children and adolescents especially should try to build peak bone mass. This can help reduce the risk of fractures and osteoporosis later in life. By the time a child’s growth spurt ends in the later teen years, 90 percent of adult bone mass already has been established, although bone mass continues to gradually build before peaking in a person’s 30s. Children who do not consume adequate levels of calcium cannot make up for this missed opportunity later in life. Fewer than one in 10 girls and only one in four boys ages 9 to 13 are at or above their adequate intake of calcium, according to the National Institutes of Health (NIH).

Although people cannot continue to build new bone after their 30s, adequate calcium intake and engaging in weight-bearing exercises can help them to hold on to the bone mass that they already have developed. Experts also urge people to get enough vitamin D in their diets because this vitamin is necessary to absorb calcium. Milk that is fortified with vitamin D is a good source of this nutrient.  Other sources include vitamin D fortified yogurt and vitamin D fortified ready-to-eat breakfast cereals.

The Institute of Medicine (IOM) of the National Academy of Sciences has issued the following daily calcium intake guidelines, expressed in milligrams (mg):

Experts generally recommend that a person over the age of one year not consume more than 2,500 milligrams of calcium per day. Excessively high intakes of calcium can result in hypercalcemia (elevated levels of calcium in the blood), impaired kidney function and decreased absorption of other minerals. However, contrary to earlier belief, increased calcium consumption does not appear to cause increased formation of kidney stones.

Calcium’s major role is to help bones grow and to keep them strong. Numerous studies show that low rates of calcium intake are highly correlated with low bone mass and bone fracture rates. Calcium is also necessary to help baby teeth and adult teeth develop fully. After teeth have developed, calcium may help protect them from decay. About 99 percent of all the calcium in the body is found in the bones and teeth.

Calcium also is found in tissues, the blood and other fluids. The calcium found in these parts of the body has several roles, including:

• Aids muscle contraction and blood vessel contraction and expansion
  
  
• Conducts nerve impulses to send messages through the central nervous system
  
  
• Helps regulate heartbeat
  
  
• Stimulates hormone and enzyme secretion
  
  
• Regulates fluid balance by controlling the flow of water into and out of the cells
  
  
• Triggers the formation of blood clots

Recent studies also appear to show that increased consumption of calcium lowers blood pressure. Studies have been inconclusive regarding whether or not increased calcium intake lowers the risk of colorectal and prostate cancer.

One recent study found that mothers who increase their intake of dietary calcium during pregnancy are more likely to give birth to babies with stronger bones and higher birth weights. Another recent study found that women who do not have enough calcium in their diets when breastfeeding risk bone loss around the teeth and gums.

When people do not have adequate stores of calcium, the condition is known as a calcium deficiency. This can result from inadequate calcium intake, decreased calcium absorption and increased calcium excretion.

People must get adequate levels of calcium from their diet every day because the body does not make this mineral. In addition, the body’s calcium stores are constantly being depleted through shed skin, nails, hair, sweat, urine and feces. In addition, calcium can only be absorbed in the digestive tract under certain conditions. A protein produced by vitamin D helps the body absorb calcium. Thus, if the body does not have adequate amounts of vitamin D, it cannot absorb calcium.

When calcium levels in the body fall too low, the body begins to break down bone to obtain calcium so that normal biological functions (e.g., nerve and muscle function) can continue.

Calcium deficiency can cause the bones to weaken, raising the risk of fracture. Long-term calcium deficiency also increases the risk of a thinning of the bones known as osteoporosis. This disease affects 10 million Americans, 80 percent of whom are women, according to the National Institutes of Health (NIH). Another 34 million Americans have low bone mass (osteopenia), a condition that precedes osteoporosis. Although osteoporosis is much more common in women, it also can affect men. In fact, after age 65, the rate of bone loss in men is the same as in women. One-third of men who are at least age 75 have osteoporosis.

Milk and dairy products offer the biggest source of calcium. Milk is a particularly good source of calcium. A single cup (8 fluid ounces) of milk usually contains about 300 milligrams (mg) of calcium. That is about 25 percent of the calcium that children and teenagers need every day. People older than age 2 are urged to choose nonfat or low-fat milk over whole milk, which contains more fat.

Other dairy products that are rich in calcium include cheeses and yogurt. Some people who have an inability to digest the milk sugar lactose (lactose intolerant) may not be able to consume standard dairy products. The enzyme lactase helps people digest lactose and lactase-fortified dairy products are available. Approximately 25 percent of Americans are lactose intolerant, according to the National Institutes of Health (NIH). People who do not eat animal products (vegans) also may not wish to eat traditional dairy products. In such cases, non-dairy calcium foods are available.

Other food sources of calcium include:

• Almonds
  
  
• Fish that is canned (mackerel) or canned with bones (salmon, sardines)
  
  
• Fortified breakfast cereal, cereal bars, orange juice and soy milk
  
  
• Green leafy vegetables such as broccoli, collards, Chinese cabbage, kale, spinach, turnips and mustard greens
  
  
• Some dried beans
  
  
• Some bottled waters
  
  
• Tofu, soy beans and other soy products, such as soy-based beverages and soy yogurt

Vitamin D is also vital for the body to process calcium. Good sources of vitamin D include fortified milk, tuna fish, fortified cereal, liver, egg yolks, fish and fish liver oils. In addition, 15 minutes of exposure to the sun provides a healthy dose of vitamin D. However, people are urged not to expose themselves to the sun without first wearing sunscreen.

Experts generally recommend that people get their calcium from dietary sources, which also provide other essential vitamins and minerals. However, in some cases a physician may recommend that a person take calcium supplements. These are usually taken several times over the course of a day. Most are available in over-the-counter formulations, although there is an injectable calcium supplement available only by prescription.

There are various types of calcium supplements, including those that contain calcium bound to compounds such as carbonate, citrate, lactate or gluconate. The various types of supplements contain different amounts of elemental calcium, which is the amount of calcium in the supplement that the body can absorb. Calcium carbonate usually contains the highest amount of elemental calcium. However, all of these supplements provide absorbable calcium.

Typically, the supplement lists the amount of elemental calcium on the label. However, some brands list only the total weight of each tablet. This amount also includes the weight of any substance to which the calcium is bound. People can still determine the amount of calcium available in such tablets by looking at the nutrition label. The percentage of the daily value (DV) listed is based on 1,000 mg of elemental calcium. Therefore, every 10 percent of DV corresponds to 100 mg of elemental calcium.

In addition, people are urged to choose brands of supplements that contain the abbreviation USP (U.S. pharmacopeia). This indicates that the supplements are of a quality that meets the voluntary standards established by the USP. Calcium supplements that contain bone meal, unrefined oyster shell, coral calcium or dolomite should be avoided because they may contain toxic substances such as lead, mercury or arsenic.

To check the absorbability of a calcium supplement, drop it into a small glass of warm water for 30 minutes, stirring it occasionally. If the supplement has not dissolved in that time, it likely will not dissolve well in the stomach. More expensive supplements called chelated calcium appear to have no advantage over other, less expensive types of calcium supplements.

Some medications may interact poorly with calcium supplements. Such drugs include:

• Aluminum or magnesium containing antacids
• Antibiotics such as tetracycline and fluoroquinolone
• Anticonvulsants such as phenytoin
• Digoxin (heart medication)
• Diuretics (blood pressure drugs) such as thiazide
• Glucocorticoids  (anti-inflammatory drug)
• Levothyroxine (drug used to treat thyroid condition)
• Mineral oil or stimulant laxatives

One recent study found that calcium supplementation during childhood has little effect on reducing fractures. This supports the notion that children should instead receive most of their calcium through dietary sources.

Several factors influence how calcium is absorbed from the digestive tract into the body’s circulation. The higher the amount of calcium consumed in a single meal, the less efficient calcium absorption is likely to be. For this reason, calcium intake is better spread throughout the course of the day rather than ingested in a single sitting.

A person’s age can also have an impact on calcium absorption. Net calcium absorption may be as high as 60 percent in infants and young children, and as low as 15 percent to 20 percent or less in adulthood. For this reason, recommendations for dietary intake of calcium are higher for adults ages 51 and older.

Vitamins and other substances also can affect calcium absorption. Vitamin D is important for calcium absorption. Substances such as phytic acid (found in whole grain bread, beans, seeds, nuts, grains and soy) and oxalic acid (found in spinach, collard greens, sweet potatoes, rhubarb and beans) may bind to calcium and prevent full absorption. For example, milk contains more readily absorbable calcium than spinach, and it would take 8 cups of cooked spinach to equal the calcium in one 8-ounce cup of milk.

Calcium is also excreted from the body through shed skin, nails and hair, urine, feces and sweat. In some people, an increased presence of dietary sodium, protein and potassium increases calcium excretion. Caffeine can also cause a minor increase in calcium excretion.

Certain steps can be taken to boost a person’s calcium intake. These steps include:

• Add low-fat or fat-free milk to coffee. Or, drink low-fat latte or cappuccino.
  
  
• Choose calcium-fortified cereals, juices and soy beverages.
  
  
• Drink low-fat or fat-free milk instead of soda.
  
  
• Make fruit smoothies with low-fat or fat-free yogurt.
  
  
• Snack on calcium-rich treats such as low-fat or fat-free frozen yogurt, ice milk, cheese and pudding.
  
  
• Eat canned fish with edible bones (e.g., salmon, sardines).
  
  
• Sprinkle grated low-fat or fat-free cheese into salads and soups and on pasta.
  
  
• Substitute low-fat or fat-free milk instead of water when making pancakes, mashed potatoes, pudding and instant, hot breakfast cereals.
  
  
• Try goat’s milk. Goat's milk and cow's milk contain about the same amount of calcium.
  
  
• Use diced calcium-set tofu in stir-fried vegetables.
  
  
• Add low fat powdered milk to soups and drinks.

Preparing questions in advance can help patients have more meaningful discussions regarding their conditions. Patients may wish to ask their doctor or registered dietitian the following questions related to calcium:

1. Where can I find out more about planning a diet that contains adequate intake levels of calcium?
   
   
2. How can I tell if my calcium intake is too low?
   
   
3. What are the signs that might indicate that my calcium intake is too high?
   
   
4. Am I a good candidate for calcium supplements?
   
   
5. What are my options if I take a medication known to interfere with supplements?
   
   
6. I am vegan. What steps should I take to make sure I obtain enough calcium?
   
   
7. Can boosting calcium intake really help me now despite decades of low calcium intake?
   
   
8. What foods should I consume that might help boost my absorption of calcium?
   
   
9. Should I avoid certain foods that might impair my body’s ability to absorb calcium?
   
   
10. What are good sources of vitamin D?