CALCIUM, NUTRITION, & BONE HEALTH

The health and strength of our bones rely on a balanced diet and a steady stream of nutrients, most importantly, calcium and Vitamin D.

Calcium is a mineral that people need to build and maintain strong bones and teeth. It is also very important for other physical functions, such as muscle control and blood circulation.

Calcium is not made in the body — it must be absorbed from the foods we eat. To effectively absorb calcium from food, our bodies need Vitamin D.

If we do not have enough calcium in our diets to keep our bodies functioning, calcium is removed from where it is stored in our bones. Over time, this causes our bones to grow weaker and may lead to osteoporosis — a disorder in which bones become very fragile.

Postmenopausal women are most vulnerable to osteoporosis. Although loss of estrogen is the primary reason for this, poor lifelong calcium and Vitamin D intake, as well as lack of exercise, play a role in the development of osteoporosis.

Note that men also are at risk for osteoporosis — typically later in life than women — and it is important for them to keep track of calcium intakes, as well.

Calcium

Calcium needs vary with age. The Food and Nutrition Board (FNB) of the Institute of Medicine of the National Academies provides guidelines on the amount of calcium needed each day.

Recommended Daily Allowance in Milligrams (mg)

Women and Men 9 to 18 years: 1,300 mg
Women and Men 19 to 50 years: 1,000 mg
Women 51 to 70 years: 1,200 mg
Men 51 to 70 years: 1,000 mg
Women and men > 70 years: 1,200 mg
Pregnant or nursing women 14 to 18 years: 1,300 mg
Pregnant or nursing women 19 to 50 years: 1,000 mg

Reprinted and adapted with permission from Tables S-1 and S-2, Dietary Reference Intakes for Calcium and Vitamin D, 2011 by the National Academy of Sciences, Courtesy of the National Academies Press, Washington, D.C.

Dietary Sources of Calcium

People can get the recommended daily amount of calcium by eating a healthy diet that includes a variety of calcium-rich foods. Milk, yogurt, cheese, and other dairy products are the biggest food sources of calcium. Other high-calcium foods include:

• Kale, broccoli, Chinese cabbage (bok choy) and other green leafy vegetables
• Sardines, salmon, and other soft-bone fish
• Tofu
• Breads, pastas and grains
• Calcium-fortified cereals, juices, and other beverages.

Good sources of calcium include milk, cheeses, leafy green vegetables, and almonds.

A more complete listing of calcium-rich foods is included at the end of this article.

Some foods make it harder for the body to absorb calcium. In particular, sodas and carbonated beverages should be avoided, not just for bone health but for many nutritional reasons, including preventing obesity. Sodas decrease calcium absorption in the intestines and contain empty calories. Milk, calcium-fortified juices, and water are better beverage alternatives for all age groups.

Calcium Supplements

Although adequate calcium can be obtained through your diet, it is difficult for many people, particularly for those who avoid dairy products. People who are lactose-intolerant or vegetarians who do not eat dairy products have a harder time getting enough calcium from foods.

It is also hard to get enough calcium from the diet during certain times of our lives, such as in adolescence when our bodies require more calcium to build strong bones for life. Postmenopausal women and men older than age 70 also require more calcium to slow down bone loss.

Doctors recommend calcium supplements to those who do not get enough calcium from the foods they eat. Although calcium is sometimes found in multivitamins, it is typically not in significant amounts. Many people need to take separate calcium supplements to ensure they reach the Recommended Dietary Allowance for their life stage.

Not all the calcium consumed — whether through food or supplement — is actually absorbed in the intestines. Research shows that calcium is absorbed most efficiently when it is taken in doses less than 500 mg. Because many calcium supplements come in 500 mg doses, people who require 1,000 mg of supplementation each day should take their doses at separate times. Newer daily slow release formulations of calcium citrate that supply 1200 mg have recently become available.

Most calcium supplements also contain Vitamin D, which helps the body absorb calcium.

Vitamin D

Without Vitamin D, our bodies cannot effectively absorb calcium, which is essential to good bone health.

Children who lack Vitamin D develop the condition called rickets, which causes bone weakness, bowed legs, and other skeletal deformities, such as stooped posture. Adults with very low Vitamin D can develop a condition called osteomalacia (soft bone). Like rickets, osteomalacia can also cause bone pain and deformities of long bones.

Vitamin D Recommended Dietary Allowance

The FNB recommends 400 International Units (IU) of Vitamin D for infants during the first year of life. The RDA for everyone from age 1 through 70 years is 600 IU. Recent research, however, supports that the body needs at least 1000 IU per day for good bone health, starting at age 5 years.

Many foods contain some Vitamin D, however, few contain enough to meet the daily recommended levels for optimal bone health.

The American Academy of Pediatrics recommends that all children take Vitamin D supplements.

In the 19th and early 20th centuries, children were routinely given cod liver oil for a range of medicinal purposes. When cod liver oil was tied to the prevention and treatment of rickets, Vitamin D was discovered. Soon after, Vitamin D was added to milk — one glass of milk contains about 100 IU of Vitamin D. As a result, parents stopped using cod liver oil. Because today's children do not drink as much milk as in the past, it is difficult for them to get enough Vitamin D from milk. In addition, other dairy products are not typically supplemented with Vitamin D. Getting enough Vitamin D from what we eat is very difficult.

Although our bodies can make Vitamin D in our skin when it is exposed to good sunlight, it is very important to protect our skin by using sunscreen when we are outdoors. This blocks the excessive UV radiation that can cause skin cancer. Sunscreen does, however, also block our skin's ability to make Vitamin D. This is why doctors often recommend Vitamin D supplements for both adults and children. The American Academy of Pediatrics recommends that all children — from infancy through adolescence — take Vitamin D supplements.

Safe Levels of Calcium and Vitamin D

Too much calcium and/or Vitamin D can be harmful and cause serious side effects. In addition to establishing RDA guidelines, the FNB has established Tolerable Upper Intake Levels (ULs). These represent the highest levels of calcium and Vitamin D that can be consumed by the average individual and still be safe.

These ULs are important guidelines for people who may require different dosages of these supplements. For example, people who live in areas with little sun, those with darker skin, and people who are obese may need more Vitamin D than the recommended daily amount.

Note that ULs are not levels that people should try to reach — they are the safe limits based on current research. When intake goes beyond the ULs listed below, the risk for serious side effects increases.

Upper Safe Limit for Calcium Intake

Life Stage: Upper Safe Limit

Birth to 6 months: 1,000 mg
Infants 7-12 months: 1,500 mg
Children 1-8 years: 2,500 mg
Children 9-18 years: 3,000 mg
Adults 19-50 years: 2,500 mg
Adults 51 years and older: 2,000 mg
Pregnant and breastfeeding teens: 3,000 mg
Pregnant and breastfeeding adults: 2,500 mg

Upper Safe Limit for Vitamin D Intake

0-6 months: 1,000 IU1,000 IU  
7-12 months: 1,500 IU1,500 IU  
1-3 years: 2,500 IU2,500 IU  
4-8 years: 3,000 IU3,000 IU  
≥9 years: 4,000 IU4,000 IU4,000 IU4,000 IU

Reprinted and adapted with permission from Tables S-1 and S-2, Dietary Reference Intakes for Calcium and Vitamin D, 2011 by the National Academy of Sciences, Courtesy of the National Academies Press, Washington, D.C.

More foods in the U.S. are being fortified with calcium and Vitamin D, and awareness of the importance of these nutrients for bone health is growing. In recent years, the media has reported on the potential health benefits of taking high levels of Vitamin D, such as in the areas of cancer prevention, diabetes management, and heart health. As a result, it is becoming more likely that people may consume unsafe quantities of these nutrients.

The FNB conducted an extensive review of the medical literature and found enough evidence of bone health benefits to support raising the UL levels on Vitamin D in adults from 2000 IU to 4000 IU. What was also determined, however, is that very high levels of Vitamin D (above 10,000 IUs per day) can cause kidney damage and dangerously high serum calcium levels. Too much calcium from dietary supplements can also cause adverse health effects, including kidney stones, higher risks for heart problems, and possibly increased risk for prostate cancer.

Calcium and Vitamin D are essential for good bone health, but must be consumed safely. If you are not sure what intake levels are right for you and your health needs, be sure to talk to your doctor.

Other Key Nutrients in Bone Health

Many other nutrients — most found naturally and at sufficient levels in a typical diet — contribute to bone health and growth. They include:

• Phosphorus. A major mineral in the body's bone crystal, phosphorus is found in dairy products and meat. Vitamin D improves phosphorus absorption in the intestine and kidney.
• Magnesium. Primarily found in bone crystals, magnesium improves bone strength. Older adults are more likely to be deficient in magnesium. Calcium supplements that contain magnesium can help.
• Vitamin K. Necessary for bone formation and mineralization, Vitamin K also is important for blood clotting, and may assist in channeling calcium directly to the bone rather than the blood vessels.
• Vitamin C. Collagen is the main protein in bone, and Vitamin C is necessary for collagen synthesis. Vitamin C is present in citrus fruits and tomatoes and in many vegetables.
• Vitamin A. Vitamin A is necessary for cells to differentiate normally and for normal skeletal growth, and also is extremely important for eye health. Vitamin A is available in liver, eggs, butter, green leafy vegetables and carrots. Too little vitamin A is a major cause of blindness worldwide. In contrast, too much vitamin A can cause bone loss and increase the risk of hip fracture. The animal source supplements (retinols) may cause toxicity but plant sources (B carotene) do not. Daily intake of retinols should be less than 10,000 IU.

Dietary Sources of Calcium

Selecting foods high in calcium is one way to help you achieve your targeted daily calcium intake. Here are some major food sources of calcium to assist your meal planning.

Selected Food Sources of Calcium

FoodMilligrams (mg) per servingPercent DV*
Yogurt, plain, low fat, 8 ounces
Orange juice, calcium-fortified, 6 ounces
Yogurt, fruit, low fat, 8 ounces
Mozzarella, part skim, 1.5 ounces
Sardines, canned in oil, with bones, 3 ounces
Cheddar cheese, 1.5 ounces
Milk, nonfat, 8 ounces
Milk, reduced-fat (2% milk fat), 8 ounces
Milk, buttermilk, 8 ounces
Milk, whole (3.25% milk fat), 8 ounces
Tofu, firm, made with calcium sulfate, 1/2 cup***
Salmon, pink, canned, solids with bone, 3 ounces
Cottage cheese, 1% milk fat, 1 cup
Tofu, soft, made with calcium sulfate, 1/2 cup
Instant breakfast drink, various flavors and brands, powder prepared with water, 8 ounces
Frozen yogurt, vanilla, soft serve, 1/2 cup
Ready-to-eat cereal, calcium-fortified, 1 cup
Turnip greens, fresh, boiled, 1/2 cup
Kale, fresh, cooked, 1 cup
Kale, raw, chopped, 1 cup
Ice cream, vanilla, 1/2 cup
Soy beverage, calcium-fortified, 8 ounces
Chinese cabbage (bok choy) raw, shredded, 1 cup
Bread, white, 1 slice
Pudding, chocolate, ready to eat, refrigerated, 4 ounces
Tortilla, corn, ready-to-bake/fry, one 6" diameter
Tortilla, flour, ready-to-bake/fry, one 6" diameter
Sour cream, reduced fat, cultured, 2 tablespoons
Bread, whole-wheat, 1 slice
Broccoli, raw, 1/2 cup
Cheese, cream, regular, 1 tablespoon

* DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration to help consumers compare the nutrient contents among products within the context of a total daily diet. The DV for calcium is 1,000 mg for adults and children aged 4 years and older. Foods providing 20% of more of the DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet. The U.S. Department of Agriculture's Nutrient Database Web site lists the nutrient content of many foods. It also provides a comprehensive list of foods containing calcium.

** Calcium content varies slightly by fat content; the more fat, the less calcium the food contains.

*** Calcium content is for tofu that is processed with a calcium salt. Tofu processed with other salts does not provide significant amounts of calcium.

Source: National Institutes of Health (NIH) (Dietary Supplement Fact Sheet: Calcium): U.S. Department of Agriculture, Agricultural Research Service. 2011. USDA National Nutrient Database for Standard Reference, Release 24. Nutrient Data Laboratory Home Page; Institute of Medicine of the National Academies Dietary Reference Intakes for Calcium and Vitamin D.

HEALTHY BONES AT EVERY AGE

Bone health is important at every age and stage of life. The skeleton is our body's storage bank for calcium — a mineral that is necessary for our bodies to function. Calcium is especially important as a building block of bone tissue, as well as exercise.  

We must get calcium from the foods we eat. If we do not have enough calcium in our diets to keep our bodies functioning, calcium is removed from where it is stored in our bones. Over time, this causes our bones to grow weaker.

Loss of bone strength can lead to osteoporosis — a disorder in which bones become very fragile and more likely to break. Older adults with osteoporosis are most vulnerable to breaks in the wrist, hip, and spine. These fractures can seriously limit mobility and independence.

Fortunately, there are many things we can do at every age to keep our bones strong and healthy - the first being exercise. Exercise equals forces being put upon the bones, and while joints may swell slightly out of protest, forces increase osteoclast activity.  Osteoclasts are the basic starter-cell to bone production.  The more forces implemented upon the bones, the more dense the bone.  This can help to prevent osteoporosis or slow it's symptoms later in life. Here at Champion Performance and Physical Therapy, we can give you an exercise program to gradually increase your daily activity, and therefore, increase your bone density and strengthen the muscles around the bones.

Peak Bone Mass

Our maximum bone size and strength is called peak bone mass. Genes play a large role in how much peak bone we have. For example, the actual size and structure of a person's skeleton is determined by genetic factors.

Between the ages of 10 and 20 we can greatly increase our peak bone mass with a calcium-rich diet and regular weightbearing exercise.

© Thinkstock, 2012

Although peak bone mass is largely determined by our genes, there are lifestyle factors — such as diet and exercise — that can influence whether we reach our full bone mass potential.

There is a limited time that we can influence our peak bone mass. The best time to build bone density is during years of rapid growth. Childhood, adolescence, and early adulthood are the times when we can significantly increase our peak bone mass through diet and exercise. Not surprisingly, we can also make choices that decrease peak bone mass, such as smoking, poor nutrition, inactivity, and excessive alcohol intake.

Most people will reach their peak bone mass between the ages of 25 and 30. By the time we reach age 40, however, we slowly begin to lose bone mass. We can, however, take steps to avoid severe bone loss over time. For most of us, bone loss can be significantly slowed through proper nutrition and regular exercise.

Although everyone will lose bone with age, people who developed a higher peak bone mass when young are better protected against osteoporosis and related fractures later in life.

Some people, however, are at higher risk for bone loss and osteoporosis because of problems with the way their bodies remodel bone. A healthy diet and exercise can help, but bone will still be lost at a faster rate. The good news is that in recent years, medications have been developed to treat this metabolic problem. In severe cases, bone loss may even be reversed with newer, bone-forming medications.

Gender and Peak Bone Mass

Men have a higher peak bone mass than women. Men accumulate more skeletal mass than women do during growth, and their bone width and size is greater. Because women have smaller bones with a thinner cortex and smaller diameter, they are more vulnerable to developing osteoporosis. Although men have a higher peak bone mass, they also are at risk for osteoporosis, especially after age 70 when bone loss and fracture risk increases significantly.

This chart shows bone mass in women as it relates to age.

Reproduced from J Bernstein, ed: Musculoskeletal Medicine. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2003.

Bone Health at Every Stage

There are things we can do at every stage of life to ensure good bone health. Especially important is making sure we get enough calcium and Vitamin D. The sections below provide guidelines from the Food and Nutrition Board (FNB) at the Institute of Medicine of the National Academies on calcium and Vitamin D daily intake at every age for the general public.

Please note that some people may require different dosages of these supplements. For example, people who live in areas with little sun, those with darker skin, and people who are obese may need more Vitamin D than the recommended daily amount. The upper safe limit for people older than 9 years of daily Vitamin D is 4000 IU, but talk to your doctor about the best dose for you. Also, be aware that taking calcium and Vitamin D at higher than recommended levels may cause adverse side effects.

For a complete overview of calcium and Vitamin D Recommended Dietary Allowances (RDAs), as well as Tolerable Upper Intake Levels (ULs), refer to "Calcium, Nutrition, and Bone Health"

• Birth to Age 9

Calcium is an essential mineral for babies and young children to ensure they are able to grow strong bones and teeth. Because our bodies need Vitamin D to absorb calcium from our diets, getting enough Vitamin D goes hand-in-hand with getting enough calcium. Young children who do not get enough Vitamin D are at risk for rickets, a disease that can cause bone weakness, bowed legs, and other skeletal deformities.

First year. According to the FNB, infants, age birth to 6 months, need 200 milligrams (mg) of calcium each day, and infants, ages 7 to 12 months, 260 mg. During this first year, both breast milk and infant formula provide sufficient calcium.

The FNB daily recommendation of Vitamin D for infants birth to 12 months is 400 International Units (IU). Although Vitamin D can be found in breast milk and infant formula, it is not in sufficient amounts. The American Academy of Pediatrics now recommends that babies take daily Vitamin D supplement drops, unless they are drinking 32 oz. of infant formula each day.

Ages 1 to 3 years. The amount of calcium and Vitamin D that a young child needs increases with age.

The FNB recommended dietary allowance (RDA) for children ages 1 to 3 are 700 mg of calcium and 600 IU of Vitamin D. Milk is one of the best sources of calcium for children — plus a cup of milk is fortified with 100 IU of Vitamin D. Doctors recommend whole milk for children between the ages of 1 and 2 years. Lowfat and skim milk are good options after age 2.

The American Academy of Pediatrics recommends that all children take Vitamin D supplements.

© Thinkstock, 2012

Because very few foods contain substantial levels of Vitamin D, the American Academy of Pediatrics recommends that all children — from infancy through adolescence — take Vitamin D supplements.

Ages 4 to 8 years. Children ages 4 through 8 need 1,000 mg of calcium each day, or the equivalent of about two cups of yogurt and one glass of milk.

The FNB recommends 600 IU of Vitamin D for everyone from age 1 through 70 years. Recent research, however, supports that the body needs at least 1000 IU per day for good bone health, starting at age 5 years. Taking a Vitamin D supplement is the most effective way for your child to get 1000 IU of Vitamin D every day.

• Between 10 and 20 Years of Age

This is the stage of life when peak bone mass is established.

Puberty. Puberty is a very important time in the development of the skeleton and peak bone mass. Half of total body calcium stores in women and up to 2/3 of calcium stores in men are made during puberty. At the end of puberty, men have about 50% more body calcium than women.

Adolescents grow rapidly and need 1,300 mg of calcium each day for the best possiblle development of the skeleton.

© Thinkstock, 2012

On average, girls begin puberty at age 10 and start having menstrual periods about age 12. Having a regular period is important to girls' and women's bone health because it indicates that sufficient estrogen is being produced. Estrogen is a hormone that improves calcium absorption in the kidneys and intestines.

The average girl grows the fastest in height between the ages of 11 and 12 years, and stops growing between the ages of 14 and 15 years. About 95% of a young woman's peak bone mass is present by age 20, and some overall gains in mass often continue until age 30.

The average boy has his fastest rate of growth in height between ages 13 and 14, and stops growing between ages 17 and 18. Peak bone mass occurs 9 to 12 months after the peak rate in height growth.

Early or late onset of puberty affects peak bone mass. Boys with late puberty generally have less bone mass for life than those who start puberty at the typical time, about age 11 1/2. Obesity delays the start of puberty in boys and, therefore, may have a negative effect on peak bone mass.

Obesity in girls, however, accelerates the onset of puberty. The effect that obesity and early puberty have on the peak bone mass is variable in girls.

Nutrional requirements. Many adolescents and young adults do not get enough calcium. Both boys and girls age 10 to 20 years need at least 1,300 mg of calcium each day, the equivalent of:

• One cup of orange juice with added calcium
• Two cups of milk
• One cup of yogurt

Other dairy products, green leafy vegetables, fish, and tofu are also good sources of calcium.

A Vitamin D supplement is necessary to ensure the calcium that adolescents do take in is absorbed in the intestines. Sodas and carbonated beverages should be avoided for many nutritional reasons, including for bone health and to prevent obesity. Sodas decrease calcium absorption in the intestines and contain empty calories. Milk, calcium-fortified juices, and water are better beverage alternatives for all age groups.

Adolescent pregnancy. Like other adolescents, young women who are pregnant and/or breastfeeding between the ages of 14 and 18 should have 1300 mg of calcium each day. The RDA for Vitamin D remains 600 IU, although as mentioned above, recent research supports a daily dosage of 1000 IU for better bone health.

Exercise. Weightbearing exercise during the teen years is essential to reach maximum bone strength. Examples of weightbearing exercise include walking and running, as well as team sports like soccer and basketball.

Young women who exercise excessively can lose enough weight to cause hormonal changes that stop menstrual periods (amenorrhea). This loss of estrogen can cause bone loss at a time when young women should be adding to their peak bone mass. It is important to see a doctor if there have been any menstrual cycle changes or interruptions.

• Between 20 and 30 Years of Age

Although your body is no longer forming new bone as readily as before, your bones will reach their peak strength during these years. It is important to get adequate calcium and exercise to help achieve peak bone density. Both men and women need at least 1,000 mg of calcium each day, and probably a Vitamin D supplement because it is difficult to get 1000 IU of Vitamin D even from a healthy diet.

To promote good bone health, adults need at least 30 minutes of weightbearing activity (such as a brisk walk), 4 or more days per week. Muscle-strengthening activities at least two days of the week are also recommended.

Pregnancy and breastfeeding. The calcium requirement for pregnant or breastfeeding women is the same for any adult in this age range: 1,000 mg. Of course, getting the recommended calcium and Vitamin D daily allowance is especially important when you are pregnant or breastfeeding. Without it, a growing baby in the womb may pull calcium out of the mother's bones in order to build its own skeleton. A nursing baby also requires calcium, and a mother can lose bone mass during the time she breastfeeds. In most cases, once breastfeeding ends, a healthy diet and exercise will help a mother regain any bone lost.

• Between 30 and 50 Years of Age

After you reach your peak bone mass, you will begin to gradually lose bone. All through your life, your body is continually removing old bone and replacing it with fresh bone. This process is called remodeling. Up until about age 40, all the bone removed is replaced. After age 40, however, less bone is replaced. At this stage in life, getting enough exercise and calcium (1,000 mg) and Vitamin D (1,000 IU) every day, are crucial to minimizing bone loss. Exercise is also important for maintaining your muscle mass, which preserves and strengthens surrounding bone and helps prevent falls.

• Older Than 50 Years of Age

The daily calcium recommendation for men over 50 remains the same at 1,000 mg. Many women over 50 are entering or have gone through menopause, and the FNB recommends that women over 50 increase their daily calcium intake to 1,200 mg.

Menopause. Most women enter menopause between the ages of 42 and 55. As the levels of estrogen drop dramatically, women undergo rapid bone loss. In fact, in the 10 years after menopause, women can lose 40% of their spongy, inner bone and 10% of their hard, outer bone. This reduces bone strength and increases a woman's risk for fracture. It also helps explain why osteoporosis is much more common in women than in men (who do not experience this acute loss of hormone in middle age).

In the past, estrogen replacement therapy was frequently used to protect aging women from bone loss. Research has since shown that there are significant risks in taking estrogen long-term after menopause. These include increased risk of serious blood clots, stroke, heart attack, breast and ovarian cancers, gall bladder disease, and possibly dementia.

If your period becomes irregular, or, if you develop signs of menopause, such as hot flashes, talk with your doctor. You also may want to ask about bone density testing. This is a safe, painless, x-ray technique that compares your current bone density with the peak bone mass someone your same gender and ethnicity should have reached at 20 to 25 years of age.

A duel-energy x-ray absorptiometry (DXA) scan is the "gold standard" of bone mineral density testing.

Reproduced with permission from JF Sarwark, ed: Essentials of Musculoskeletal Care, ed 4. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2010

Age 70 and above. Both men and women should get 1,200 mg of calcium each day. The RDA for Vitamin D at this age is 800 IU.

After they reach the age of 70, men are more likely to experience low bone mass and fractures. Men over age 70 should discuss bone density testing with a doctor.

Fall prevention becomes especially important for people over age 70. Falls are the leading cause of injury to elderly people in the United States. Falls cause many seniors to lose their independence, requiring a change in living arrangements, such as moving to a nursing home or assisted living facility. Fortunately, many falls can be prevented, and having strong bones can help prevent fractures.

No matter your age, adequate calcium intake and exercise can limit bone loss and increase bone and muscle strength.

Source: National Institutes of Health (NIH) (Dietary Supplement Fact Sheet: Calcium) ; Institute of Medicine of the National Academies (Dietary Reference Intakes for Calcium and Vitamin D).