You have more calcium in your body than any other mineral. Calcium has many important jobs. The body stores more than 99 percent of its calcium in the bones and teeth to help make and keep them strong. The rest is throughout the body in blood, muscle and the fluid between cells. Your body needs calcium to help muscles and blood vessels contract and expand, to secrete hormones and enzymes and to send messages through the nervous system.

It is important to get plenty of calcium in the foods you eat. Foods rich in calcium include

• Dairy products such as milk, cheese, and yogurt
• Leafy, green vegetables
• Fish with soft bones that you eat, such as canned sardines and salmon
• Calcium-enriched foods such as breakfast cereals, fruit juices, soy and rice drinks, and tofu. Check the product labels.

The exact amount of calcium you need depends on your age and other factors. Growing children and teenagers need more calcium than young adults. Older women need plenty of calcium to prevent osteoporosis. People who do not eat enough high-calcium foods should take a calcium supplement.

Calcium is a mineral your body needs to build and maintain strong bones and to carry out many important functions. Calcium is the most abundant mineral in the body.

Almost all calcium in the body is stored in bones and teeth, giving them structure and hardness.

Your body needs calcium for muscles to move and for nerves to carry messages between your brain and every part of your body. Calcium also helps blood vessels move blood throughout your body and helps release hormones that affect many functions in your body. Vitamin D helps your body absorb calcium.

Calcium is not only the most abundant mineral in bone, it is also the most abundant mineral in the human body. Calcium ions are needed not only for bone mineralization but for tooth health, regulation of the heart rate and strength of contraction, blood coagulation, contraction of smooth and skeletal muscle cells, and regulation of nerve impulse conduction. The normal level of calcium in the blood is about 10 mg/dL. When the body cannot maintain this level, a person will experience hypo- or hypercalcemia.

Hypocalcemia , a condition characterized by abnormally low levels of calcium, can have an adverse effect on a number of different body systems including circulation, muscles, nerves, and bone. Without adequate calcium, blood has difficulty coagulating, the heart may skip beats or stop beating altogether, muscles may have difficulty contracting, nerves may have difficulty functioning, and bones may become brittle. The causes of hypocalcemia can range from hormonal imbalances to an improper diet. Treatments vary according to the cause, but prognoses are generally good.

Conversely, in hypercalcemia , a condition characterized by abnormally high levels of calcium, the nervous system is underactive, which results in lethargy, sluggish reflexes, constipation and loss of appetite, confusion, and in severe cases, coma.

Obviously, calcium homeostasis is critical. The skeletal, endocrine, and digestive systems play a role in this, but the kidneys do, too. These body systems work together to maintain a normal calcium level in the blood.

Calcium is a chemical element that cannot be produced by any biological processes. The only way it can enter the body is through the diet. The bones act as a storage site for calcium: The body deposits calcium in the bones when blood levels get too high, and it releases calcium when blood levels drop too low. This process is regulated by PTH, vitamin D, and calcitonin.

Cells of the parathyroid gland have plasma membrane receptors for calcium. When calcium is not binding to these receptors, the cells release PTH, which stimulates osteoclast proliferation and resorption of bone by osteoclasts. This demineralization process releases calcium into the blood. PTH promotes reabsorption of calcium from the urine by the kidneys, so that the calcium returns to the blood. Finally, PTH stimulates the synthesis of vitamin D, which in turn, stimulates calcium absorption from any digested food in the small intestine.

When all these processes return blood calcium levels to normal, there is enough calcium to bind with the receptors on the surface of the cells of the parathyroid glands, and this cycle of events is turned off (image).

When blood levels of calcium get too high, the thyroid gland is stimulated to release calcitonin (image), which inhibits osteoclast activity and stimulates calcium uptake by the bones, but also decreases reabsorption of calcium by the kidneys. All of these actions lower blood levels of calcium. When blood calcium levels return to normal, the thyroid gland stops secreting calcitonin.

Review

Calcium homeostasis, i.e., maintaining a blood calcium level of about 10 mg/dL, is critical for normal body functions. Hypocalcemia can result in problems with blood coagulation, muscle contraction, nerve functioning, and bone strength. Hypercalcemia can result in lethargy, sluggish reflexes, constipation and loss of appetite, confusion, and coma. Calcium homeostasis is controlled by PTH, vitamin D, and calcitonin and the interactions of the skeletal, endocrine, digestive, and urinary systems.

The foods we eat contain a variety of vitamins, minerals, and other important nutrients that help keep our bodies healthy. We need to get enough of two nutrients in particular, calcium and vitamin D, to help keep our bones strong.

What does calcium do for our bones?

Calcium is a mineral. Our bodies contain many different minerals, such as iron and magnesium, but the most abundant mineral in the body is calcium.

Also, our bones are partly made of calcium salts (mixtures of calcium with other minerals), especially calcium phosphate. Calcium phosphate hardens and strengthens bone.

Our bodies cannot make calcium. We need to get calcium from foods and drinks, and supplements when needed, to build strong bones and teeth and to keep them healthy. Healthy bones are less likely to break if we fall.

What happens to our bones without enough calcium?

Calcium does more than build strong bone. It also helps our muscles, heart, and nerves work properly.

If we do not get enough calcium in our diets, our body takes the calcium we need from our bones. Over time, this process makes bones weaker and raises the risk of osteoporosis. Osteoporosis is a disease that makes bones weak and brittle, and people with osteoporosis have a higher risk of fractures (broken bones).

How much calcium do we need?

The amount of calcium each person needs depends on their age and sex. Table 1 lists how much calcium people need every day to keep their bones strong and healthy.

Table 1

Source: National Institutes of Health Office of Dietary Supplements

What foods have calcium?

The best food sources of calcium are milk and other dairy products, like cheese and yogurt. Other food sources of calcium include:

• Fish (such as salmon and tuna).
• Leafy green vegetables (such as kale and turnip greens).
• Tofu (set with calcium).
• Small fish with bones (including sardines and canned salmon).

Some foods and drinks are fortified with calcium. A food is fortified when the manufacturer adds vitamins or minerals that the food does not naturally have. Examples of foods fortified with calcium are certain brands of juice, soymilk, oatmeal, and breakfast cereal.

Some supplements contain calcium. Talk to your health care provider if you are thinking about taking a calcium supplement.

What does vitamin D do for our bones?

Vitamin D promotes bone health in these ways:

• Helps absorb the calcium we get from food.
• Along with calcium, helps protects older adults from osteoporosis.
• Promotes healthy functioning of our muscles and immune system. We need strong muscles to help us balance and reduce the risk of falling and breaking bones.

What happens to our bones without enough vitamin D?

When children do not get enough vitamin D, they can develop rickets. With rickets, the bones are soft, weak, deformed, and painful.

In adults, too little vitamin D over time can cause:

• Osteomalacia, a painful condition that makes bones and muscles weak and more likely to bend and break.
• Osteoporosis.

How much vitamin D do we need?

Table 2 lists how much vitamin D people need every day to keep their bones healthy.

Table 2

Source: National Institutes of Health Office of Dietary Supplements

In the United States, most people do not consume enough vitamin D to meet recommended intakes. Some vitamin D can be made in the body with sun exposure. Still, many people have low vitamin D status.

What foods have vitamin D?

Not many foods have vitamin D. Some kinds of fish can provide some of the vitamin D we need each day. Examples are:

• Trout.
• Salmon.
• Tuna.
• Mackerel.

Most of the vitamin D that Americans get from food comes from fortified foods. Most milk and some yogurts in the United States are fortified with vitamin D, but most other dairy products do not have added vitamin D.

Other foods that may be fortified (check the label to be sure) include:

• Ready-to-eat breakfast cereals.
• Orange juice.

Some supplements contain vitamin D. Talk to your health care provider if you are thinking about taking a vitamin D supplement.

The amount of calcium you need each day depends on your age. Average daily recommended amounts are listed below in milligrams (mg):

Many people don't get recommended amounts of calcium from the foods they eat, including:

• Boys aged 9 to 13 years,
• Girls aged 9 to 18 years,
• Women older than 50 years,
• Men older than 70 years.

When total intakes from both food and supplements are considered, many people—particularly adolescent girls—still fall short of getting enough calcium, while some older women likely get more than the upper limit. See our Health Professional Fact Sheet on Calcium for more details.

Certain groups of people are more likely than others to have trouble getting enough calcium:

• Postmenopausal women because they experience greater bone loss and do not absorb calcium as well. Sufficient calcium intake from food, and supplements if needed, can slow the rate of bone loss.
• Women of childbearing age whose menstrual periods stop (amenorrhea) because they exercise heavily, eat too little, or both. They need sufficient calcium to cope with the resulting decreased calcium absorption, increased calcium losses in the urine, and slowdown in the formation of new bone.
• People with lactose intolerance cannot digest this natural sugar found in milk and experience symptoms like bloating, gas, and diarrhea when they drink more than small amounts at a time. They usually can eat other calcium-rich dairy products that are low in lactose, such as yogurt and many cheeses, and drink lactose-reduced or lactose-free milk.
• Vegans (vegetarians who eat no animal products) and ovo-vegetarians (vegetarians who eat eggs but no dairy products), because they avoid the dairy products that are a major source of calcium in other people's diets.

Many factors can affect the amount of calcium absorbed from the digestive tract, including:

• Age. Efficiency of calcium absorption decreases as people age. Recommended calcium intakes are higher for people over age 70.
• Vitamin D intake. This vitamin, present in some foods and produced in the body when skin is exposed to sunlight, increases calcium absorption.
• Other components in food. Both oxalic acid (in some vegetables and beans) and phytic acid (in whole grains) can reduce calcium absorption. People who eat a variety of foods don't have to consider these factors. They are accounted for in the calcium recommended intakes, which take absorption into account.

Many factors can also affect how much calcium the body eliminates in urine, feces, and sweat. These include consumption of alcohol- and caffeine-containing beverages as well as intake of other nutrients (protein, sodium, potassium, and phosphorus). In most people, these factors have little effect on calcium status.

Insufficient intakes of calcium do not produce obvious symptoms in the short term because the body maintains calcium levels in the blood by taking it from bone. Over the long term, intakes of calcium below recommended levels have health consequences, such as causing low bone mass (osteopenia) and increasing the risks of osteoporosis and bone fractures.

Symptoms of serious calcium deficiency include numbness and tingling in the fingers, convulsions, and abnormal heart rhythms that can lead to death if not corrected. These symptoms occur almost always in people with serious health problems or who are undergoing certain medical treatments.

Scientists are studying calcium to understand how it affects health. Here are several examples of what this research has shown:

Bone health and osteoporosis

Bones need plenty of calcium and vitamin D throughout childhood and adolescence to reach their peak strength and calcium content by about age 30. After that, bones slowly lose calcium, but people can help reduce these losses by getting recommended amounts of calcium throughout adulthood and by having a healthy, active lifestyle that includes weight-bearing physical activity (such as walking and running).

Osteoporosis is a disease of the bones in older adults (especially women) in which the bones become porous, fragile, and more prone to fracture. Osteoporosis is a serious public health problem for more than 10 million adults over the age of 50 in the United States. Adequate calcium and vitamin D intakes as well as regular exercise are essential to keep bones healthy throughout life.

Taking calcium and vitamin D supplements reduce the risk of breaking a bone and the risk of falling in frail, elderly adults who live in nursing homes and similar facilities. But it's not clear if the supplements help prevent bone fractures and falls in older people who live at home.

Cancer

Studies have examined whether calcium supplements or diets high in calcium might lower the risks of developing cancer of the colon or rectum or increase the risk of prostate cancer. The research to date provides no clear answers. Given that cancer develops over many years, longer term studies are needed.

Cardiovascular disease

Some studies show that getting enough calcium might decrease the risk of cardiovascular disease and stroke. Other studies find that high amounts of calcium, particularly from supplements, might increase the risk of heart disease. But when all the studies are considered together, scientists have concluded that as long as intakes are not above the upper limit, calcium from food or supplements will not increase or decrease the risk of having a heart attack or stroke.

High blood pressure

Some studies have found that getting recommended intakes of calcium can reduce the risk of developing high blood pressure (hypertension). One large study in particular found that eating a diet high in fat-free and low-fat dairy products, vegetables, and fruits lowered blood pressure.

Preeclampsia

Preeclampsia is a serious medical condition in which a pregnant woman develops high blood pressure and kidney problems that cause protein to spill into the urine. It is a leading cause of sickness and death in pregnant women and their newborn babies. For women who get less than about 900 mg of calcium a day, taking calcium supplements during pregnancy (1,000 mg a day or more) reduces the risk of preeclampsia. But most women in the United States who become pregnant get enough calcium from their diets.

Kidney stones

Most kidney stones are rich in calcium oxalate. Some studies have found that higher intakes of calcium from dietary supplements are linked to a greater risk of kidney stones, especially among older adults. But calcium from foods does not appear to cause kidney stones. For most people, other factors (such as not drinking enough fluids) probably have a larger effect on the risk of kidney stones than calcium intake.

Weight loss

Although several studies have shown that getting more calcium helps lower body weight or reduce weight gain over time, most studies have found that calcium—from foods or dietary supplements—has little if any effect on body weight and amount of body fat.

For more information on calcium and weight loss, see our consumer fact sheet on Weight Loss.

The Basics: Overview

Your body needs calcium to build strong bones when you are young and to keep bones strong as you get older. Everyone needs calcium, but it's especially important for women and girls. Many people, including most women, don't get enough calcium.

How much calcium do I need every day?

Women:

• If you are age 19 to 50, get 1,000 mg (milligrams) of calcium every day.
• If you are age 51 or older, get 1,200 mg of calcium every day.
• If you are pregnant or lactating, get 1,300 mg of calcium every day.

Men:

• If you are ages 19 to 70, get 1,000 mg of calcium every day.
• If you are age 71 or older, get 1,200 mg of calcium every day.

Kids:

• Kids ages 1 to 3 need 700 mg of calcium every day.
• Kids ages 4 to 8 need 1,000 mg of calcium every day.
• Kids ages 9 to 18 need 1,300 mg of calcium every day.

Calcium can help prevent osteoporosis (weak bones).

Osteoporosis is a disease that makes your bones weak and more likely to break. Some people don’t know they have it until they break a bone.

One in 4 women and 1 in 20 men over the age of 65 in the U.S. have osteoporosis. Calcium helps to keep your bones strong and less likely to break.

The Basics: Foods and Supplements

How can I get enough calcium?

The best way to get enough calcium is to eat foods with calcium every day.

Calcium is in foods like:

• Fat-free or low-fat (1%) milk, yogurt, and cheese
• Soymilk with added calcium, vitamin A, and vitamin D
• Certain vegetables, including soybeans, collard greens, and turnip greens
• Tofu with added calcium
• Orange juice with added calcium
• Breakfast cereal with added calcium

For more ideas, check out this list of foods that are high in calcium.

Getting calcium from foods is best. But if you don’t eat enough foods with calcium, you can take a calcium supplement (pill) every day. You can take a multivitamin with calcium or a pill that has only calcium. Talk to your doctor before you start taking extra calcium.

Take Action: Calcium Sources

Protect your bones – get plenty of calcium every day.

Check food labels.

The Daily Value (DV) on a food label tells you the amount of a nutrient (like calcium) that’s in a serving of the food. Foods that have at least 20% DV of calcium are excellent choices. Foods with at least 10% DV of calcium are good, too.

For example:

• A cup of fat-free milk has about 300 mg of calcium, or 30% DV.
• A cup of orange juice with added calcium has about 350 mg of calcium, or 35% DV.

Learn how to check food labels for calcium information.

Use this calcium shopping list to find foods high in calcium when you are at the grocery store.

If you take a calcium supplement, make it easy to remember.

• Take it at the same time every day. For example, take it when you brush your teeth before bed.
• Leave the pill bottle out where you will see it, like on the kitchen counter or by the bathroom sink.

Take Action: Vitamin D

Get enough vitamin D.

Vitamin D helps your body absorb (take in) calcium. Find out how much vitamin D you need each day.

Your body makes vitamin D when you are out in the sun. You can also get vitamin D from:

• Salmon and tuna
• Milk with added vitamin D
• Some breakfast cereals, yogurt, and juices with added vitamin D
• Vitamin D pills

Talk to your doctor before taking vitamin D pills.

Follow a healthy eating pattern.

Along with eating foods high in calcium, it’s important to follow a healthy eating pattern. That means eating a variety of vegetables, fruits, whole grains, and foods with protein.

Calcium is found in many foods. You can get recommended amounts of calcium by eating a variety of foods, including the following:

• Milk, yogurt, and cheese are the main food sources of calcium for the majority of people in the United States.
• Kale, broccoli, and Chinese cabbage are fine vegetable sources of calcium.
• Fish with soft bones that you eat, such as canned sardines and salmon, are fine animal sources of calcium.
• Most grains (such as breads, pastas, and unfortified cereals), while not rich in calcium, add significant amounts of calcium to the diet because people eat them often or in large amounts.
• Calcium is added to some breakfast cereals, fruit juices, soy and rice beverages, and tofu. To find out whether these foods have calcium, check the product labels.

You may be avoiding dairy products because of lactose intolerance. Or you might have other reasons. But dairy products are a major source of calcium, vitamin D and other nutrients that are important for your body. If you’re avoiding dairy products, you need to take special care to make sure you’re getting enough of these nutrients.

Our heart, muscles and nerves need calcium to work properly. Our bones need it to grow and stay strong. The body also needs vitamin D to absorb calcium. Nutrition surveys have shown that most people in the U.S. aren’t getting the calcium they need. If you’re avoiding milk and dairy products, you may be missing out on important sources of calcium and vitamin D.

One of the major reasons people avoid dairy products is lactose intolerance. Lactose is a natural sugar found in milk and other dairy products. You become lactose intolerant if your body doesn’t have enough lactase—an enzyme produced in the small intestine that you need to digest lactose. Undigested lactose can cause stomach cramps, gas and diarrhea within 30 minutes to 2 hours after eating something with lactose.

Lactose intolerance isn’t common in young children, but many people gradually lose their ability to digest lactose after childhood. That happens more often in some ethnic groups, such as African Americans, Native Americans, Hispanics and Asians. Most people who don’t completely digest lactose have no symptoms and are entirely healthy, but some people do get those uncomfortable symptoms.

Complicating things, some people mistake the symptoms of intestinal disorders, such as irritable bowel syndrome and inflammatory bowel disease, for lactose intolerance.

NIH recently convened a “consensus development” conference to review the scientific evidence and develop objective statements about treating lactose intolerance. The panel concluded that there isn’t enough scientific evidence to answer many questions about lactose intolerance, including how many people have it, whether it causes serious health effects or how these effects should be treated. More research will be needed to answer these questions.

If you think you or your children are lactose intolerant, studies suggest you may not need to completely eliminate milk or dairy products from your diet. There are several strategies you can try to ensure you get the nutrients you need. These include spreading your dairy intake throughout the day, combining it with other foods, taking nutritional supplements and choosing reduced-lactose or non-dairy foods rich in the nutrients found in dairy products.

How much calcium and vitamin D you need depends on your age and other factors. If you’re avoiding dairy products, talk to your doctor to make sure you meet your nutrient requirements. You can also check out the Dietary Guidelines for Americans for recommendations about dietary intake, including dairy or calcium-rich foods.

Getting Enough Calcium

If you’ve been avoiding dairy foods, try the following strategies:

• Try low-fat or fat-free milk in servings of 1 cup or less.
• Have milk with other food, such as breakfast cereal.
• Try other dairy products, such as low-fat or fat-free cheeses
  or yogurt.
• Choose lactose-free milk and milk products.
• Choose other calcium-fortified foods such as orange juice, soy or rice beverages. Some non-dairy foods, such as spinach and broccoli, have calcium, too.
• Use over-the-counter pills or drops that contain lactase, which can eliminate symptoms.
• Talk to your doctor about nutritional supplements.

Calcium is found in many multivitamin-mineral supplements, though the amount varies by product. Dietary supplements that contain only calcium or calcium with other nutrients such as vitamin D are also available. Check the Supplement Facts label to determine the amount of calcium provided.

The two main forms of calcium dietary supplements are carbonate and citrate. Calcium carbonate is inexpensive, but is absorbed best when taken with food. Some over-the-counter antacid products, such as Tums® and Rolaids®, contain calcium carbonate. Each pill or chew provides 200–400 mg of calcium. Calcium citrate, a more expensive form of the supplement, is absorbed well on an empty or a full stomach. In addition, people with low levels of stomach acid (a condition more common in people older than 50) absorb calcium citrate more easily than calcium carbonate. Other forms of calcium in supplements and fortified foods include gluconate, lactate, and phosphate.

Calcium absorption is best when a person consumes no more than 500 mg at one time. So a person who takes 1,000 mg/day of calcium from supplements, for example, should split the dose rather than take it all at once.

Calcium supplements may cause gas, bloating, and constipation in some people. If any of these symptoms occur, try spreading out the calcium dose throughout the day, taking the supplement with meals, or changing the supplement brand or calcium form you take.

Getting too much calcium can cause constipation. It might also interfere with the body's ability to absorb iron and zinc, but this effect is not well established. In adults, too much calcium (from dietary supplements but not food and beverages) might increase the risk of kidney stones. Some studies show that people who consume high amounts of calcium might have increased risks of prostate cancer and heart disease, but more research is needed to understand these possible links.

Most people do not get amounts above the upper limits from food and beverages; excess intakes usually come from the use of calcium supplements. Surveys show that some older women in the United States probably get amounts somewhat above the upper limit since the use of calcium supplements is common among these women.

The daily upper limits for calcium include intakes from all sources—food, beverages, and supplements—and are listed below in milligrams (mg).

Calcium dietary supplements can interact or interfere with certain medicines that you take, and some medicines can lower or raise calcium levels in the body. Here are some examples:

• Calcium can reduce the absorption of these drugs when taken together:
  • Bisphosphonates (to treat osteoporosis)
  • Antibiotics of the fluoroquinolone and tetracycline families
  • Levothyroxine (to treat low thyroid activity)
  • Phenytoin (an anticonvulsant)
  • Tiludronate disodium (to treat Paget's disease).
• Diuretics differ in their effects. Thiazide-type diuretics (such as Diuril® and Lozol®) reduce calcium excretion by the kidneys which in turn can raise blood calcium levels too high. But loop diuretics (such as Lasix® and Bumex®) increase calcium excretion and thereby lower blood calcium levels.
• Antacids containing aluminum or magnesium increase calcium loss in the urine.
• Mineral oil and stimulant laxatives reduce calcium absorption.
• Glucocorticoids (such as prednisone) can cause calcium depletion and eventually osteoporosis when people use them for months at a time.

Tell your doctor, pharmacist, and other healthcare providers about any dietary supplements and medicines you take. They can tell you if those dietary supplements might interact or interfere with your prescription or over-the-counter medicines or if the medicines might interfere with how your body absorbs, uses, or breaks down nutrients.

What is calcium?

Calcium is an essential dietary mineral commonly found in milk, yogurt, cheese, and dark green vegetables. It also is found in certain grains, legumes (including peas, beans, lentils, and peanuts), and nuts.

Calcium is a major component of bones and teeth. It also is required for the clotting of blood to stop bleeding and for normal functioning of the nerves, muscles, and heart.

How much calcium is needed for good health?

Calcium is an important part of a healthy diet; however, the recommended intake differs according to age. As can be seen in the following table, the highest recommended intake is for children and adolescents between the ages of 9 and 18, when bones are growing rapidly.

For adults (including women who are pregnant or breastfeeding) and for children age 1 or older, the safe upper limit of calcium intake is 2.5 grams (or 2500 mg) per day.

Too much calcium in the diet and from dietary supplements can lead to unwanted side effects.

The U.S. Department of Agriculture’s 1994–1996 Continuing Survey of Food Intakes by Individuals showed that the average daily calcium intakes in the United States for males and females over age 9 were 925 mg and 657 mg, respectively, or less than the recommended intake.

How much calcium is in foods and calcium supplements?

Calcium is found in many foods. Foods high in calcium include dairy products, dark green vegetables, some soy products, fish, nuts, and legumes. The following table shows how much calcium is contained in some common foods.

Packaged foods are required to have a Nutrition Facts label (3). On foods that contain calcium, this label lists how much calcium there is in each serving of the packaged food. However, the Nutrition Facts labels on packaged foods do not list the calcium content in mg. They only provide the Percent Daily Value (%DV), which is the amount one serving of a food item contributes to the total amount of calcium you need each day. The %DV for calcium is based on a recommended Daily Value of 1000 mg per day. Therefore, a food with 20%DV or more contributes a fair amount of a person’s daily total, whereas a food with 5%DV or less contributes only a little. As an example, 1 cup of milk provides 300 mg of calcium and 30%DV.

Calcium supplements most often contain either calcium carbonate or calcium citrate, which are calcium salts. Sometimes, they contain both compounds. Calcium carbonate and calcium citrate have different amounts of elemental calcium, which is the actual amount of usable calcium in a supplement. Specifically, calcium carbonate has about 40 percent elemental calcium, meaning that 500 mg of calcium carbonate actually contains 200 mg of elemental calcium or 20%DV. In contrast, calcium citrate has approximately 21 percent elemental calcium. Therefore, nearly twice as much calcium citrate is needed to obtain the equivalent amount of elemental calcium as in calcium carbonate. Calcium supplements may also contain other calcium salts, but the body may not be able to use the calcium in these compounds. As with food labels, you should look at the Nutrition Facts label on a supplement to determine how much calcium it contains.

Is it safe to take calcium supplements?

For most people, it is safe to eat foods containing calcium and to take calcium supplements that together do not exceed the tolerable upper intake level of 2.5 grams of calcium per day. This upper level for daily calcium intake in adults is the highest level that likely will not pose risks of unwanted side effects in the general population. The upper level of 2.5 grams a day is an average recommendation for all healthy people who are older than a year, regardless of gender.

Consuming too much calcium—in excess of 5 grams a day, or 3 grams a day in people with existing kidney problems—can lead to several harmful side effects. Most of these side effects result from people taking too many calcium supplements. Rare harmful side effects from excess calcium include kidney stones, hypercalcemia (too much calcium in the blood), and kidney failure. In addition, excessive consumption of milk (which is high in calcium) and some types of antacids, especially antacids containing calcium carbonate or sodium bicarbonate (baking soda), over a long period of time can cause milk-alkali syndrome, a condition that can also lead to calcium deposits in the kidneys and other tissues and to kidney failure.

Is there evidence that calcium may help reduce the risk of colorectal cancer?

The results of epidemiologic studies regarding the relationship between calcium intake and colorectal cancer risk have not always been consistent.

In the American Cancer Society’s Cancer Prevention Study II Nutrition Cohort, the diet, medical history, and lifestyle of more than 120,000 men and women were analyzed. Men and women who had the highest intakes of calcium through both their diet and supplement use had a modestly reduced risk of colorectal cancer compared with those who had the lowest calcium intakes. However, the benefit from calcium appeared to plateau, or level off, at an intake of approximately 1200 mg per day. When calcium from the diet was analyzed by itself, no reduction in colorectal cancer risk was found. However, the use of calcium supplements in any amount was associated with reduced risk. This association was strongest (a 31 percent reduction in risk) for people who took calcium supplements of 500 mg per day or more.

A stronger relationship between calcium intake and colorectal cancer risk was found when participants of the Nurses’ Health Study and the Health Professionals Follow-up Study were combined in an analysis that included more than 135,000 men and women. Individuals who had a calcium intake of more than 700 mg per day had a 35 percent to 45 percent reduced risk of cancer of the distal (lower) part of the colon than those who had a calcium intake of 500 mg or less per day. No association was found between calcium intake and risk of cancer of the proximal (middle and upper) part of the colon. Another large study of Finnish men showed a similar relationship between higher calcium intake and reduced risk of colorectal cancer. This study, however, did not evaluate proximal and distal colorectal cancers separately.

In a study that included more than 61,000 Swedish women, colorectal cancer risk was approximately 28 percent lower among individuals who had the highest calcium intakes (approximately 800–1000 mg per day) compared with those with the lowest calcium intakes (approximately 400–500 mg per day). Data from this study also suggested that the benefit associated with calcium was limited to the distal colon. In a study that involved more than 34,000 postmenopausal Iowa women, high intakes of calcium (approximately 1280 mg per day or more) compared with lower calcium intakes (approximately 800 mg per day or less) from both the diet and supplements were associated with a 41 percent reduction in risk of rectal cancer. Reduced risks of rectal cancer were also observed for dietary calcium alone and supplemental calcium alone, but these associations were not statistically significant.

In an analysis involving more than 293,000 men and 198,000 women in the National Institutes of Health-American Association of Retired Persons (NIH-AARP) Diet and Health Study, high intakes of total calcium, dietary calcium, and supplemental calcium were associated with an approximately 20 percent lower risk of colorectal cancer among men and an approximately 30 percent lower risk of colorectal cancer among women.

Findings from two large randomized, placebo-controlled clinical trials, the Calcium PolypPrevention Study and the European Cancer Prevention Organisation InterventionStudy showed that daily supplementation with 1200 to 2000 mg elemental calcium was associated with a reduced risk of recurrence of colorectal polyps known as adenomas in both men and women. Adenomas are thought to be the precursors of most colorectal cancers. In these trials, individuals who previously had one or more large adenomas removed during colonoscopy were randomly assigned to receive calcium supplementation or a placebo, and the rates of polyp recurrence and other factors were compared between the groups.

The Calcium Polyp Prevention Study involved 930 participants who were randomly assigned to receive 3 grams of calcium carbonate (1200 mg elemental calcium) daily for 4 years or a placebo and then receive follow-up colonoscopies approximately 9 months later and again 3 years after that. Compared with those in the placebo group, the individuals assigned to take calcium had about a 20 percent lower risk of adenoma recurrence.

The European Cancer Prevention Organisation Intervention Study involved 665 participants who were randomly assigned to one of three treatment groups: 2 grams of elemental calcium daily (from calcium gluconolactate and calcium carbonate), 3 grams of fiber supplementation daily, or a placebo. The results showed that calcium supplementation was associated with a modest reduction in the risk of adenoma recurrence, but this finding was not statistically significant.

The results of another clinical trial conducted as part of the Women’s Health Initiative showed that supplementation with 1000 mg elemental calcium (from calcium carbonate) per day for an average duration of 7 years was not associated with a reduced risk of colorectal cancer. The calcium supplements in this trial also contained vitamin D (400 international units [IU]). During the trial, 128 cases of invasive colorectal cancer were diagnosed in the supplementation group and 126 cases were diagnosed in the placebo group.

In 2007, the World Cancer Research Fund/American Institute for Cancer Research (WCRF/AICR) published the most authoritative review of existing evidence relating food, nutrition, and physical activity to cancer risk. The report concluded that calcium probably has a protective effect against colorectal cancer.

Is there evidence that calcium can help reduce the risk of other cancers?

The results of some studies suggest that a high calcium intake may decrease the risk of one or more types of cancer, whereas other studies suggest that a high calcium intake may actually increase the risk of prostate cancer.

In a randomized trial that included nearly 1,200 healthy, postmenopausal Nebraska women, individuals were randomly assigned to receive daily calcium supplementation alone (300–600 mg elemental calcium), calcium supplementation (300–600 mg elemental calcium) combined with vitamin D supplementation (1000 IU), or a placebo for 4 years. The incidence of all cancers combined was approximately 60 percent lower for women who took the calcium plus vitamin D supplements compared with women who took the placebo. A lower risk of all cancers combined was also observed for women who took calcium supplements alone, but this finding was not statistically significant. The numbers of individual types of cancer diagnosed during this study were too low to be able to draw reliable conclusions about cancer-specific protective effects.

The results of some but not all studies suggest that a high intake of calcium may increase the risk of prostate cancer. For example, the European Prospective Investigation into Cancer and Nutrition analyzed the intakes of animal foods (meat, poultry, fish, dairy products, etc.), protein, and calcium in relation to prostate cancer risk among more than 142,000 men and found that a high intake of protein or calcium from dairy products was associated with an increased risk of prostate cancer (19). Calcium from nondairy sources, however, was not associated with increased risk. In addition, a prospective analysis of dairy product and calcium intakes among more than 29,000 men participating in the National Cancer Institute’s (NCI) Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial showed increased risks for prostate cancer associated with high dietary intakes of calcium and dairy products, particularly low-fat dairy products. Calcium from supplements was not associated with increased prostate cancer risk. In contrast, results from the NIH-AARP Diet and Health Study showed no increased risk of prostate cancer associated with total calcium, dietary calcium, or supplemental calcium intakes.

Other studies have suggested that intakes of low-fat milk, lactose, and calcium from dairy products may reduce the risk of ovarian cancer, but this risk reduction has not been found in all studies.

An analysis from the Nurses’ Health Study that included more than 3,000 women found that higher calcium intakes (more than 800 mg per day) from dairy products—particularly low-fat or nonfat milk, yogurt, and cheese—compared with lower calcium intakes (200 mg or less per day) from dairy products was associated with a reduced risk of breast cancer among premenopausal but not postmenopausal women. Calcium from nondairy sources was not associated with a reduction in risk. Another analysis that involved more than 30,000 women in the Women’s Health Study found a reduced risk of breast cancer associated with higher (1366 mg per day or more) versus lower (less than 617 mg per day) total intakes of calcium among premenopausal but not postmenopausal women. In this study, higher versus lower calcium intakes from the diet, from supplements, and from total dairy products were not associated with reduced risk.

How might calcium help prevent cancer?

Although the exact mechanism by which calcium may help reduce the risk of colorectal cancer is unclear, researchers know that, at the biochemical level, calcium binds to bileacids and fatty acids in the gastrointestinal tract to form insoluble complexes known as calcium soaps. This reduces the ability of the acids (or their metabolites) to damage cells in the lining of the colon and stimulate cell proliferation to repair the damage. Calcium may also act directly to reduce cell proliferation in the lining of the colon or cause proliferatingcolon cells to undergo differentiation, which, in turn, leads to a reduction in cell proliferation. Calcium also may improve signaling within cells and cause cancer cells to differentiate and/or die.

How does the body absorb calcium from foods and supplements?

Calcium is absorbed passively (no cellular energy required) in the intestines by diffusing through the spaces between cells. It is also absorbed actively (cellular energy required) through intestinal cells by binding to a transport protein known as calbindin. The production of calbindin is dependent on vitamin D.

Does NCI recommend the use of calcium supplements to prevent colorectal cancer?

No. Although substantial evidence suggests that calcium may provide some protection against colorectal cancer, the evidence of potential benefit from calcium supplements is limited and inconsistent. Therefore, NCI does not recommend the use of calcium supplements to reduce the risk of colorectal or any other type of cancer.

Note: The information in this fact sheet is not to be used as the basis for making health claims about calcium-containing products.