Calcium, the most abundant mineral in the human body, has several important functions. More than 99% of total body calcium is stored in the bones and teeth where it functions to support their structure. The remaining 1% is found throughout the body in blood, muscle, and the fluid between cells. Calcium is needed for muscle contraction, blood vessel contraction and expansion, the secretion of hormones and enzymes, and sending messages through the nervous system. A constant level of calcium is maintained in body fluid and tissues so that these vital body processes function efficiently.
Bone undergoes continuous remodeling, with constant resorption (breakdown of bone) and deposition of calcium into newly deposited bone (bone formation). The balance between bone resorption and deposition changes as people age. During childhood there is a higher amount of bone formation and less breakdown. In early and middle adulthood, these processes are relatively equal. In aging adults, particularly among postmenopausal women, bone breakdown exceeds its formation, resulting in bone loss, which increases the risk for osteoporosis (a disorder characterized by porous, weak bones).
What is the recommended intake for calcium?
Recommendations for calcium are provided in the Dietary Reference Intakes (DRIs) developed by the Institute of Medicine (IOM) of the National Academy of Sciences. Dietary Reference Intake (DRI) is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people. Three important types of reference values included in the DRIs are Recommended Dietary Allowances (RDA), Adequate Intakes (AI), and Tolerable Upper Intake Levels (UL). The RDA recommends the average daily intake that is sufficient to meet the nutrient requirements of nearly all (97-98%) healthy individuals in each age and gender group. An AI is set when there is insufficient scientific data available to establish a RDA. AIs meet or exceed the amount needed to maintain a nutritional state of adequacy in nearly all members of a specific age and gender group. The UL, on the other hand, is the maximum daily intake unlikely to result in adverse effects. It is listed in the section "Is there health risk of too much calcium?" of this fact sheet.
For calcium, the recommended intake is listed as an Adequate Intake (AI), which is a recommended average intake level based on observed or experimentally determined levels. Table 1 contains the current recommendations for calcium for infants, children and adults.
Table 1: Recommended Adequate Intake by the IOM for Calcium
*mg=milligrams
There is a widespread concern that Americans are not meeting the recommended intake for calcium. According to the Continuing Survey of Food Intakes of Individuals (CSFII 1994-96), the following percentage of Americans are not meeting their recommended intake for calcium: 44% boys and 58% girls ages 6-11 64% boys and 87% girls ages 12-19 55% men and 78% of women ages 20+
What foods provide calcium?
In the United States (U.S.), milk, yogurt and cheese are the major contributors of calcium in the typical diet. The inadequate intake of dairy foods may explain why some Americans are deficient in calcium since dairy foods are the major source of calcium in the diet. The U.S. Department of Agriculture's Food Guide Pyramid recommends that individuals two years and older eat 2-3 servings of dairy products per day. A serving is equal to: 1 cup (8 fl oz) of milk 8 oz of yogurt 1.5 oz of natural cheese (such as Cheddar) 2.0 oz of processed cheese (such as American)
A variety of non-fat and reduced fat dairy products that contain the same amount of calcium as regular dairy products are available in the U.S. today for individuals concerned about saturated fat content from regular dairy products.
Although dairy products are the main source of calcium in the U.S. diet, other foods also contribute to overall calcium intake. Individuals with lactose intolerance (those who experience symptoms such as bloating and diarrhea because they cannot completely digest the milk sugar lactose) and those who are vegan (people who consume no animal products) tend to avoid or completely eliminate dairy products from their diets. Thus, it is important for these individuals to meet their calcium needs with alternative calcium sources if they choose to avoid or eliminate dairy products from their diet. Foods such as Chinese cabbage, kale and broccoli are other alternative calcium sources. Although most grains are not high in calcium (unless fortified), they do contribute calcium to the diet because they are consumed frequently. Additionally, there are several calcium-fortified food sources presently available, including fruit juices, fruit drinks, tofu and cereals. Figure 1 compares portion sizes of various foods that provide the amount of calcium in one cup of milk. This figure takes into account that calcium absorption varies among foods. Certain plant-based foods such as some vegetables contain substances which can reduce calcium absorption. Thus, you may have to eat several servings of certain foods such as spinach to obtain the same amount of calcium in one cup of milk, which is not only calcium-rich but also contains calcium in an easily absorbable form. Table 2 contains additional listings of food sources of calcium.
Figure 1: Calcium Content of 8 fl oz of Milk Compared to Other Food Sources of Calcium
Helping hints for meeting your calcium needs
As the 2000 Dietary Guidelines for Americans states, "Different foods contain different nutrients and other healthful substances. No single food can supply all the nutrients in the amounts you need". For more information about building a healthful diet, refer to the Dietary Guidelines for Americans http://www.usda.gov/cnpp/DietGd.pdf and the US Department of Agriculture's Food Guide Pyramid http://www.nal.usda.gov/fnic/Fpyr/pyramid.html.
The following are strategies and tips to help you meet your calcium needs each day: Use low fat or fat free milk instead of water in recipes such as pancakes, mashed potatoes, pudding and instant, hot breakfast cereals. Blend a fruit smoothie made with low fat or fat free yogurt for a great breakfast. Sprinkle grated low fat or fat free cheese on salad, soup or pasta. Choose low fat or fat free milk instead of carbonated soft drinks. Serve raw fruits and vegetables with a low fat or fat free yogurt based dip. Create a vegetable stir-fry and toss in diced calcium-set tofu. Enjoy a parfait with fruit and low fat or fat free yogurt. Complement your diet with calcium-fortified foods such as certain cereals, orange juice and soy beverages.
What affects calcium absorption and excretion?
Calcium absorption refers to the amount of calcium that is absorbed from the digestive tract into our body's circulation. Calcium absorption can be affected by the calcium status of the body, vitamin D status, age, pregnancy and plant substances in the diet. The amount of calcium consumed at one time such as in a meal can also affect absorption. For example, the efficiency of calcium absorption decreases as the amount of calcium consumed at a meal increases.
- Age:Net calcium absorption can be as high as 60% in infants and young children, when the body needs calcium to build strong bones. Absorption slowly decreases to 15-20% in adulthood and even more as one ages. Because calcium absorption declines with age, recommendations for dietary intake of calcium are higher for adults ages 51 and over.
- Vitamin D:Vitamin D helps improve calcium absorption. Your body can obtain vitamin D from food and it can also make vitamin D when your skin is exposed to sunlight. Thus, adequate vitamin D intake from food and sun exposure is essential to bone health. The Office of Dietary Supplement's vitamin D fact sheet provides more information: http://ods.od.nih.gov/factsheets/vitamind.asp.
- Pregnancy:Current calcium recommendations for nonpregnant women are also sufficient for pregnant women because intestinal calcium absorption increases during pregnancy. For this reason, the calcium recommendations established for pregnant women are not different than the recommendations for women who are not pregnant.
Plant substances: Phytic acid and oxalic acid, which are found naturally in some plants, may bind to calcium and prevent it from being absorbed optimally. These substances affect the absorption of calcium from the plant itself not the calcium found in other calcium-containing foods eaten at the same time. Examples of foods high in oxalic acid are spinach, collard greens, sweet potatoes, rhubarb, and beans. Foods high in phytic acid include whole grain bread, beans, seeds, nuts, grains, and soy isolates. Although soybeans are high in phytic acid, the calcium present in soybeans is still partially absorbed. Fiber, particularly from wheat bran, could also prevent calcium absorption because of its content of phytate. However, the effect of fiber on calcium absorption is more of a concern for individuals with low calcium intakes. The average American tends to consume much less fiber per day than the level that would be needed to affect calcium absorption.
Calcium excretion refers to the amount of calcium eliminated from the body in urine, feces and sweat. Calcium excretion can be affected by many factors including dietary sodium, protein, caffeine and potassium.
- Sodium and protein:Typically, dietary sodium and protein increase calcium excretion as the amount of their intake is increased. However, if a high protein, high sodium food also contains calcium, this may help counteract the loss of calcium.
- Potassium:Increasing dietary potassium intake (such as from 7-8 servings of fruits and vegetables per day) in the presence of a high sodium diet (>5100 mg/day, which is more than twice the Tolerable Upper Intake Level of 2300 mg for sodium per day) may help decrease calcium excretion particularly in postmenopausal women.
- Caffeine:Caffeine has a small effect on calcium absorption. It can temporarily increase calcium excretion and may modestly decrease calcium absorption, an effect easily offset by increasing calcium consumption in the diet. One cup of regular brewed coffee causes a loss of only 2-3 mg of calcium easily offset by adding a tablespoon of milk. Moderate caffeine consumption, (1 cup of coffee or 2 cups of tea per day), in young women who have adequate calcium intakes has little to no negative effects on their bones.
Other factors:
- Phosphorus: The effect of dietary phosphorus on calcium is minimal. Some researchers speculate that the detrimental effects of consuming foods high in phosphate such as carbonated soft drinks is due to the replacement of milk with soda rather than the phosphate level itself.
- Alcohol: Alcohol can affect calcium status by reducing the intestinal absorption of calcium. It can also inhibit enzymes in the liver that help convert vitamin D to its active form which in turn reduces calcium absorption. However, the amount of alcohol required to affect calcium absorption is unknown. Evidence is currently conflicting whether moderate alcohol consumption is helpful or harmful to bone.
In summary, a variety of factors that may cause a decrease in calcium absorption and/or increase in calcium excretion may negatively affect bone health.
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