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Nutrition & Product Information
Calcium Counseling Resource
Calcium and Chronic Disease Prevention
Increasing scientific evidence indicates that an adequate calcium intake reduces the risk of several major chronic diseases including osteoporosis, hypertension, colon cancer, and possibly other disorders. These disorders are responsible for considerable morbidity and mortality in many patients, as well as rising national health expenditures.
OSTEOPOROSIS
Osteoporosis (porous bones) is a skeletal disease in which bones become so fragile they spontaneously break as a result of a minor fall or even from everyday activities such as bending over to pick up a newspaper. Decreased bone mass and microarchitectural damage to bone tissue cause the bones to become fragile (1,2,3).
The rate of osteoporosis has reached epidemic proportions in the United States and is responsible for considerable morbidity, mortality, and economic costs (2). Osteoporosis affects as many as 28 million Americans, mostly women and mature adults (2). Ten million Americans already have osteoporosis and 18 million more are at high risk due to low bone mass (2). This disease leads to 1.5 million fractures a year. As many as 20% of patients with hip fractures will die in the year after the fracture due to complications resulting from surgery, or from being bedridden and unable to move around (2). Osteoporotic fractures incur an estimated direct cost of $10 to $15 billion a year in the U.S. (2). Although every bone in the body can become fragile in osteoporotic patients, fractures of the spine, hip, and wrist are the most common (2).
One in three women and one in eight men older than 50 years of age will experience a vertebral (spinal) fracture. The prevalence of osteoporosis increases with age. By the year 2040, about 25% of the U.S. population will be 60 years of age and older (4). Based on this projection, hip fractures are estimated to increase from about 275,000 a year in 1990 to more than 800,000 a year in 2040. However, osteoporosis is preventable and no longer considered a natural part of aging.
Clinical Features
Often termed a silent disease, osteoporosis usually goes unnoticed until back pain or a spontaneous fracture occurs. The back pain is caused by the collapse of one or more vertebrae. Collapse of several vertebrae leads to a loss of height and disfigurement commonly known as dowager's hump.
Diagnosis
Because there often are no symptoms prior to most osteoporotic fractures, relatively few people are medically diagnosed in time for effective therapy. Consequently, many people experience unnecessary fracture-related pain, expense, disability, and decreased quality of life.
The first step in diagnosing osteoporosis is to assess risk according to one's lifestyle and medical history. Based on this information, a physical examination and an occasional bone densitometry (bone mass measurement) may follow. Bone mineral density is the main determinant of an individual's risk of osteoporotic fracture (2). If low bone mass is detected, additional tests may be performed to rule out other diseases that can cause bone loss, such as osteomalacia (a vitamin D deficiency) or hyperparathyroidism. According to the NIH Consensus Panel on Osteoporosis, an individualized approach to screening for osteoporosis is recommended (2).
Bone mass measurement can be useful in deciding whether to begin a prevention or treatment program (2). Bone densitometry is an accurate, safe, painless x-ray technique that compares an individual's bone density at specific sites in the skeleton (spine, hip, forearm, wrist) with the norm for a specific age, gender, and race. Several types of bone densitometry are available: single absorptiometry, quantitative computed tomography, dual energy x-ray absorptiometry (DXA), and ultrasound densitometry (2). Recent studies using quantitative ultrasound testing of the heel have been demonstrated to effectively predict hip and nonvertebral fractures (2). To improve the ability to predict the chance of fractures, risk factors should be considered in conjunction with bone mineral density testing (2). Routinely used x-rays are not sensitive enough to detect osteoporosis until a large amount of bone tissue has been lost. Because these types of bone densitometry tests differ in accuracy, reproducibility, and cost, patients should discuss which test is best for them with their physician.
The National Osteoporosis Foundation recommends bone density tests under the following situations:
- To make treatment decisions for women withlow estrogen levels
- For individuals with vertebral deformitiesor radiographic osteopenia
- For patients receiving long-term glucocorticoidmedication (glucocorticoids encourage bone loss)
- For patients with primary asymptomatic hyperparathyroidism
- To monitor response to treatment of establishedosteoporosis
RISK FACTORS
Risk of developing osteoporosis is influenced by a variety of genetic and environmental factors (5). We cannot control such risk factors as genetics, family history, gender, hormonal status, race/ethnic heritage, age, and body frame/weight. On the other hand, we can control lifestyle risk factors, including diet, physical activity, cigarette smoking, alcohol intake, and use of medications to help protect against developing osteoporosis.
Genetics
Having a close relative with osteoporosis or an osteoporotic fracture increases the likelihood of developing this disease. Twin and parent-offspring studies indicate that genetics influences bone mass (2). Researchers have identified a major genetic component for bone mass that is linked to polymorphism in the vitamin D receptor gene (6,7). Individuals with a certain variant of the gene have reduced calcium absorption, which can increase susceptibility to developing osteoporosis. This genetic variation may increase the risk of osteoporosis, but it can be counteracted by increasing calcium intake (8). When women with the genetic variant consumed 1,500 mg calcium/day, they absorbed just as much calcium as women with a normal gene. But when calcium intake was low (less than 300 mg/day), the women with the variant absorbed significantly less calcium than women with a normal gene (8).
Gender
Women are four times more likely than men to develop osteoporosis (2). White postmenopausal women experience nearly three-quarters of the hip fractures and have the highest fracture incidence (2). The reduction in estrogen (which helps bones retain calcium) at menopause (around age 50) is a major reason why women are more susceptible to osteoporosis than men. Other reasons include women's lower bone mass because of their generally smaller size, and their tendency to consume less dietary calcium than men. Men generally attain a higher bone density before age-related loss starts and they tend to have more muscle than women, which puts a greater mechanical stress on their bones (5). In addition, the male hormone testosterone, which provides some protection against bone loss, is not reduced until age 60 to 70 years, thereby providing men with extra years of bone protection. Women also live longer than men. Since a certain amount of bone loss is a normal consequence of aging, longevity increases the likelihood of developing osteoporosis.
Race/Ethnic Heritage
Caucasian women, particularly those of northern Europe ancestry, are at higher risk of osteoporosis than Hispanic or African Americans. However, women of other racial and ethnic groups are also affected (2).
Age
Both men and women may begin to lose bone in their 30s (2,9). The older a person becomes, the greater the risk of osteoporosis.
Thin, Small-Boned Frame
Small-framed adults are at greater risk of osteoporosis than larger- boned individuals. Some body fatness, especially in older women, may provide some protection against osteoporosis (10,11,12). This does not mean that adults should abandon efforts to maintain a healthy weight. What it does mean is that it is important to consider body weight when assessing risk for osteoporosis. Weight loss and disordered eating (e.g., anorexia nervosa) can lead to low bone mass and increased risk of developing fractures (13,14,15,16). Low body weight, estrogen deficiency, low calcium intake, and overall poor nutrition contribute to the bone loss in patients with anorexia nervosa (2).
Diet
Diet, specifically calcium intakes below the AIs throughout life, may increase the risk of osteoporosis (2,17). Also, an inadequate vitamin D status contributes to low bone mass. Ensuring an adequate calcium and vitamin D status throughout life reduces osteoporotic fracture risk (2,17). Other dietary factors like high intakes of sodium, protein, and phosphorus may increase the risk of osteoporosis by impacting calcium status. However, if calcium intake is adequate, these other dietary factors have relatively little effect on osteoporosis risk (2,17).
Other nutrients important to bone health include phosphorus, zinc, manganese, copper, vitamin C, and vitamin K (4).
Physical activity throughout life, from childhood to later adult years, benefits bone health (2,18).
Physical Activity
Regular weight-bearing physical activity benefits bone health from childhood through later years (2,18,19,20). In particular, strength training, such as lifting weights, increases bone density (2,19). In later years, regular physical activity helps to improve balance and neuromuscular function, which reduces risk of falls (2). Because the effects of weight-bearing exercise on bones are site-specific, it is best to participate in a variety of activities. Also, because the benefits of exercise on bones last only as long as activity continues, physical activity should become a part of everyone's lifestyle. A meta-analysis of intervention trials indicates the positive effect of physical activity on bone mineral density may only occur when calcium intakes are greater than 1,000 mg/day (21). Both adequate calcium intake and regular weight-bearing physical activity benefit bones (21,22,23,24).
Cigarette Smoking
Smokers generally have lower bone densities and are more likely to suffer osteoporotic fractures than nonsmokers (2,25,26,27). Cigarette smoking may affect bone mass by reducing calcium absorption, suppressing the production of estrogen, altering the way the body handles estrogen, lowering body weight, and/or directly affecting bone (26).
Alcohol Intake
The effect of alcohol intake on bone mass is inconsistent (2). However, alcoholism is a risk factor for osteoporosis (3). This may be due in part to alcohol's interference with calcium absorption and its damaging effects on bone-forming cells. Plus, if liver functions are affected, vitamin D activity may be compromised. Excessive alcohol intakes may also contribute to poor nutrition and trouble with balance, increasing the risk of falls, and consequently breaks (28).
Diseases/Medications
A number of diseases or medical disorders such as hypogonadism, hyperthyroidism, severe liver disease, intestinal malabsorption, and anorexia nervosa can cause bone loss and lead to secondary osteoporosis (2). Lack of physical activity or mechanical stress on bones as a result of illness, injury, or surgery can contribute to bone loss. For this reason, a calcium-rich diet is recommended as part of every post-illness recovery.
Prolonged use of a variety of medications is another concern (3). Glucocorticoids (used to treat asthma, rheumatoid arthritis, and certain cancers), excess thyroid hormones, some anti-seizure medications, and aluminum-containing antacids cause bone loss and increase risk of osteoporosis (2,3). If glucocorticoids are used long term, bisphosphonate drugs may be needed to counteract bone loss.
CALCIUM AND OSTEOPOROSIS
The key to preventing osteoporosis is to:
- Maximize genetically determined peak bone massreached by about age 30 or earlier.
- Reduce the rate of bone loss in later years.
While no single nutrient can prevent osteoporosis, consuming adequate calcium throughout life plays a critical role in attaining peak bone mass and preventing and treating osteoporosis (2,17).
Osteoporosis experts agree that an optimal intake of calcium throughout life, from early childhood and adolescence through the postmenopausal and later adult years, reduces the risk of osteoporosis (1,2,3,17,29). Research indicates that it is never too early or too late to improve bone health and reduce the risk of osteoporosis (17).
A recent analysis of 139 papers relating to calcium intake and bone health published since 1975 provides convincing evidence of the beneficial role of calcium and calcium-rich foods throughout life in skeletal health (17). In all but two of 52 investigator-controlled, calcium intervention trials, increasing calcium intake positively affected bone gain during growth, reduced bone loss with aging, and/or resulted in fewer osteoporotic fractures. The six studies that used dairy sources of calcium were all positive (17). Similar beneficial effects of calcium were demonstrated in approximately three-quarters of 86 observational studies, most of which used food sources of calcium (17).
It is important to appreciate that osteoporosis is a multifactorial disease and many factors influence bone health. An increase in calcium intake may be ineffective in increasing bone mass if calcium is not the limiting factor (5,17).
For example:
- Calcium status is influenced by calcium intake,absorption of calcium, and/or excretion of calcium. In general, about30% of calcium is absorbed, although absorption can be much lower orhigher. Some individuals may need to consume higher amounts of calciumbecause of their low absorption of this mineral. Dietary and nondietaryfactors influence the bioavailability of calcium (2).
- In some studies, the effect of calcium on bonemass may be overshadowed by the reduction in estrogen at menopause (2,5).In the early menopausal years, bone loss is specifically related tothe reduction in estrogen production. While it is important to ensureadequate calcium intake throughout menopause, calcium intake has a greatereffect on bone mass three to six years after the onset of menopause(1).
- Additionally, calcium is a threshold nutrient,which means that above a certain level,the effect of calcium on bone mass is reduced. Individuals already meetingtheir calcium needs cannot be expected to derive further bone-preservingbenefits from additional calcium intake (17).
- Other reasons why increased calcium intake doesnot increase bone mass include: differences in the degree of responseto dietary calcium at various skeletal sites; difficulties in accuratelymeasuring bone mass; and difficulties estimating calcium intake.
Even though osteoporosis is a multi-factorial disease, research supports calcium's critical role in risk reduction (2,17). For instance, differences in calcium nutrition early in life may be responsible for a 5% to 10% difference in peak bone mass and a 50% difference in the incidence of hip fractures in later years (30).
According to a recent NIH Consensus Panel on Osteoporosis, "the bone mass attained early in life is perhaps the most important determinant of life-long skeletal health" (2). Studies in children ages 6 to 10 have demonstrated that increasing calcium intake to 1,200 to 1,500 mg/day increases bone mass (31,32). Similarly in adolescents and young adults, calcium intakes of 1,200 to 1,600 mg/day or higher increase bone density at specific skeletal sites (32,33,34,35,36,37). In young adults, bone density is increased by consuming calcium in amounts of 1,200 to 1,500 mg/day (38,39,40).
After peak bone mass is achieved by age 30 or earlier, bone mass is fairly stable, as long as hormonal status is maintained. To maintain bone mass in adult women 25 to 50 years of age, a calcium intake of approximately 1,000 mg/day or higher is necessary (41). Increasing calcium intake to over 1,500 mg/day, mostly from dairy foods, has been demonstrated to increase vertebral bone mineral density in women ages 30 to 42 (42). Consuming food sources of calcium during childhood and adolescence appears to have prolonged bone health benefits (43,44). Higher bone densities have been found in premenopausal women who reported drinking more than 21/2 cups of milk/day during their childhood and early teenage years than in those who reported low milk intakes (1 cup or less/day) (43). Similarly, a recent cross-sectional study in young women aged 18 to 31 years found that higher calcium (and milk) intakes during the teenage years were associated with greater bone mineral measures at several skeletal sites during the development of peak bone mass (44).
Unfortunately, calcium intakes of many children and adolescents fall short of dietary recommendations (2,45). Restriction of dairy products, a generally low intake of fruits and vegetables, and a high intake of low calcium beverages such as sodas contribute to low calcium intakes among children and teens (2,45). The American Academy of Pediatrics, Committee on Nutrition, in a recent policy statement on calcium requirements of infants, children, and adolescents, encourages pediatricians to recommend milk, cheese, yogurt, and other calcium-rich foods for children (45).
Menopause and Bone Loss
During menopause, accelerated bone loss is due mostly to the sharp decline in estrogen levels (2,5). Estrogen replacement therapy is the most effective means to slow bone loss, especially in the first six to eight years after menopause (1). Adequate calcium intake, while less effective than estrogen in slowing bone loss, nevertheless is important at this time. In women who were between three and six years past menopause, increasing calcium intake to 1,700 mg/day for three years slowed bone loss, although the combination of calcium and estrogen therapy was more effective than calcium alone (46). A high calcium intake may reduce the dosage of estrogen necessary to prevent bone loss in postmenopausal women (47,48,49). A meta-analysis found that increasing calcium intake in conjunction with estrogen increased bone mass in the spine, hip, and arm about three times more than with estrogen alone (48).
In postmenopausal women, bone loss is minimized by calcium intakes of 1,000 to 1,500 mg/day (50,51,52,53,54). Particularly noteworthy is the finding of significantly fewer fractures in New Zealand women who consumed about 1,700 mg calcium/day compared to women who consumed 700 mg calcium/day (53).
Studies in adults ages 65 and older reveal that an adequate calcium intake reduces age-related bone loss and risk of fractures (55,56,57,58,59,60,61,62). When women 69 to 106 years of age (mean age of 84) increased their calcium intake from 500 mg to 1,700 mg/day and consumed 800 IU vitamin D a day for 18 months, hip bone density increased, the rate of hip fractures was reduced by 41%, and nonvertebral fractures declined by 30% (55). After 36 months, overall risk of hip fractures was reduced by 29% and risk of nonvertebral fractures was lowered by 24% (56). A meta-analysis of seven calcium supplementation and 30 epidemiological trials found that increasing calcium intake by 1,000 mg/day decreased hip fracture risk by 24% (60).
Intake of milk and milk products has been demonstrated to protect against osteoporosis by increasing bone retention and reducing fractures (17).
In a study involving over 5,500 women 50 years of age and older from six countries in southern Europe, an adequate calcium intake from milk decreased the number of hip fractures by 35% (59).
Food sources of calcium, such as milk and other dairy foods, are especially beneficial for bones (62). When older adults aged 55 to 85 years increased their usual low dairy intake (<1.5 an="" and="" blood="" bone="" by="" calcium="" conclude="" cost-efficient="" decreased.="" finding="" for="" hormone="" improve="" increasing="" intake="" is="" led="" levels="" loss="" milk="" nutrition.="" parathyroid="" researchers="" that="" the="" this="" to="" way="">
To optimize bone health, the National Academy of Sciences (AIs) (63) recommends 800 mg calcium/day for 4- to 8-year-olds, 1,300 mg/day for 9- to 18-year-olds, 1,000 mg/day for 31- to 50-year-olds, and 1,200 mg/day thereafter. These recommendations differ quantitatively from the National Institutes of Health (NIH) (1) optimal calcium recommendations, which are supported by the American Medical Association (AMA) (29). Research indicates that calcium intakes of at least the amounts currently recommended by the National Academy of Sciences (AIs) are necessary to protect against osteoporosis.
OSTEOPOROSIS PREVENTION
Medical experts agree that optimal calcium intake during childhood, adolescence, and early adulthood can maximize genetically determined peak bone mass and slow bone loss that occurs with aging (2,17). Important measures to reduce the risk of osteoporosis include:
- Consume three or more servings of milk and milkproducts a day. Without them, it is difficult to meet calcium requirements(17,63). Also, milk and milk products such as cheese and yogurt containother nutrients such as vitamin D, protein, magnesium, and trace minerals,which are important for bone health (64).
- Obtain an adequate amount of vitamin D. Foradults who have limited exposure to sunlight, recommended vitamin Dintakes are 400 IU/day for 51- to 70-year-olds and 600 IU/day for thoseover 70 years to protect their bones (63). One cup of milk contains100 IU of vitamin D.
- Increase physical activity. Although the idealexercise program to reduce the risk of osteoporosis is unknown, bothweight-bearing activities such as walking or aerobics and strength exercisesprotect bones (2,18).
- Maintain a healthy body weight.
- Avoid smoking and excessive alcohol intake.
- Consider estrogen replacement therapy, if appropriate.
TREATMENT OF OSTEOPOROSIS
The goal of osteoporosis treatment is to alleviate patients' symptoms, restore their functional independence, and reduce the risk of further bone loss and fractures. All patients should consume intakes of calcium and vitamin D meeting at least the amounts recommended by the National Academy of Sciences (AIs) (63) to reduce bone loss (62). In fact, "calcium is the specific nutrient most important for attaining peak bone mass and for preventing and treating osteoporosis," according to a recent NIH Panel on Osteoporosis (2). Calcium intakes of up to 2,500 mg/day and vitamin D intakes up to 2,000 IU/day are the upper intake levels considered to be safe for most individuals (63).
Several antiresorptive drugs, including estrogen replacement therapy for women and testosterone for men, calcitonin (a hormone produced in the thyroid gland and given to patients by injection or as a nasal spray), and bisphosphonates are used to arrest bone loss in osteoporotic patients (2,3). In 1996, a bisphosphonate drug, alendronate (trade name Fosamax®) was approved for the treatment of osteoporosis. This drug blocks bone loss and may restore some lost bone (65). Raloxifene (a selective estrogen receptor modulator or SERM) is the first of the so-called "designer estrogens" which provide the beneficial effects of estrogens without their potential disadvantages to be approved for treating and preventing osteoporosis (2,3). In large clinical trials, this drug has been demonstrated to substantially reduce the risks of vertebral fractures (2,3). The beneficial effects of various drug therapies for osteoporosis have been achieved when used in conjunction with adequate intakes of calcium and vitamin D (2). Treatment of osteoporosis may also include physical therapy to improve functioning and reduce disability. Measures to decrease osteoporotic patients' risks of falls should be included as part of the treatment for this disease. Because most treatments do not restore lost bone or reverse the loss in height or collapsed vertebrae characteristic of osteoporosis, prevention of osteoporosis is critical.
HYPERTENSION
One in four Americans, or approximately 50 million adults, has hypertension (66). Hypertension is defined as a blood pressure equal to or greater than 140 mm mercury (Hg) systolic (contracting) and/or 90 mm Hg diastolic (resting). This disease is a major risk factor for heart disease and stroke.
The cause of most hypertension is unknown. However, both genetic and lifestyle factors such as diet, alcohol intake, body weight, exercise, stress, and cigarette smoking influence risk of this disease (66). For years, dietary sodium has been the nutritional factor most often associated with blood pressure control (66). However, the link between sodium intake and blood pressure may be related to "salt sensitivity." Individuals differ in their blood pressure response to dietary sodium. About 15% to 45% of people with normal blood pressure and about 30% to 60% of hypertensives are "salt sensitive" (67). In "salt-sensitive" individuals, blood pressure increases following intake of excess sodium and decreases with a reduction in sodium intake. At present, there is no diagnostic test to determine who is and is not "salt sensitive." However, certain individuals - African Americans, obese persons, individuals with kidney diseases, those with a family history of hypertension, and older adults - are more likely to be "salt sensitive" than others (66).
Calcium and Blood Pressure
Since the early 1980s, scientific research has shown that calcium plays an important role in regulating blood pressure. Findings from numerous population studies, experimental animal investigations, and clinical trials indicate that increasing calcium intake lowers blood pressure or reduces the risk of hypertension (68,69,70,71,72,73,74,75). In population studies, people with high blood pressure have lower intakes of calcium than people with normal blood pressure, and vice versa. In humans and experimental animals with hypertension, abnormalities in calcium metabolism have been detected. And in clinical trials of calcium supplementation, blood pressure is reduced in many individuals, particularly those at high risk of hypertension and/or who have inadequate calcium intakes.
Calcium's blood pressure-lowering effect has been demonstrated in untreated human hypertensives, normotensives, women with pregnancy-related hypertension, and children (70,71). However, like sodium, the benefit of increasing calcium intake on blood pressure regulation is greater for some individuals than for others (76). Increasing dietary calcium reduces blood pressure more in individuals whose calcium intake is low, such as hypertensive persons, African Americans, older adults, and/or individuals such as pregnant women whose needs for calcium are high (76,77).
According to a meta-analysis that included 33 randomized, controlled studies involving 2,412 individuals, increasing calcium intake by 1,000 mg to 2,000 mg a day was associated with a small, but statistically significant, reduction in systolic blood pressure (68). The researchers suggest that the modest findings may obscure the larger impact of calcium intake on blood pressure in subgroups, such as calcium-deficient individuals and/or individuals with a high biological need for calcium (68). In fact, this was found to be the case in a subsequent meta-analysis of calcium and blood pressure in pregnancy (69).
Based on an analysis of 14 randomized, controlled trials involving 2,459 pregnant women, the researchers found that increasing calcium intake led to a substantial reduction in both systolic and diastolic blood pressures and preeclampsia (69).
Increasing pregnant women's calcium intake by 1,500 to 2,000 mg/day reduced the incidence of pregnancy-induced high blood pressure by 70% and preeclampsia by 62% (69).
Pregnancy-induced high blood pressure occurs in 10% to 20% of pregnancies. Further, consuming optimal amounts of calcium reduced the likelihood of developing preeclampsia (69). Preeclampsia is a potentially life-threatening disorder of late pregnancy that occurs in 2% to 8% of all pregnancies. The disorder is characterized by high blood pressure, fluid retention, and protein in the urine. Increasing pregnant women's calcium intake may also lower blood pressure in their children (78).
A calcium intake of 1,000 to 1,200 mg/day is sufficient to lower blood pressure in individuals who are responsive (71). This amount of calcium is consistent with current dietary calcium recommendations (1,63) or amounts equivalent to three to four servings from the Milk Group a day. Unfortunately, many Americans do not consume adequate amounts of calcium (79). This is especially true for hypertensives and individuals in groups at high risk of hypertension, such as adults ages 65 and older, African Americans, and pregnant women (70). Also, in patients on low-sodium diets, calcium intakes may be low (80). For these groups in particular, increasing dietary calcium may reduce blood pressure and its associated adverse health and economic consequences.
Researchers acknowledge that the beneficial effects of calcium on blood pressure might be even greater if calcium is derived from food, such as milk and milk products, instead of supplements (75). The findings that milk has a better antihypertensive effect than calcium alone (81) support this speculation. In addition to calcium, milk and milk products contain potassium and magnesium, each of which has been demonstrated to reduce blood pressure (70,71,76).
A recently published review of scientific evidence regarding the role of dairy products in blood pressure regulation led the authors to conclude that "two decades of intense clinical investigation have yielded consistent, reproducible evidence …that consumption of dairy products at the level currently recommended by expert panels optimizes blood pressure regulation in humans " (71).
Meeting calcium recommendations through milk and milk products increases the overall nutrient quality of the diet (82,83,84). A four-year osteoporosis prevention study found that women who consumed fat free milk significantly improved not only their intake of calcium, but also many other key nutrients including potassium, magnesium, phosphorus, riboflavin, thiamin, and zinc. In contrast, the women who took calcium supplements only increased their intake of calcium and sodium (83). Likewise, in a study of older adults whose typical intake of dairy products was low (<1.5 as="" by="" consumption="" fluid="" for="" free="" increased="" increasing="" intake="" many="" milk="" nutrients="" of="" or="" other="" significantly="" their="" three="" weeks="" well="">
Clearly, the impact of individual nutrients on blood pressure is complicated by the complexity of dietary interactions. To gain increased understanding of the combined effects of nutrients that occur together in foods, the National Institutes of Health sponsored a multi-center feeding trial called the "Dietary Approaches to Stop Hypertension" (DASH) (85,86). Over 450 adults with normal or mild elevations in blood pressure consumed one of three diets for eight weeks: a control diet low in fruits, vegetables, and dairy products and a typical fat content; a diet rich in fruits and vegetables; or a "combination" low fat diet rich in fruits, vegetables, and lowfat dairy foods. All diets were equal in sodium intake (approximately 3 g/day) and all participants maintained body weight. The combination diet had the greatest blood pressure-lowering effect. A diet rich in low fat dairy foods (almost 3 servings/day) and fruits and vegetables (8 to 10 servings/day) significantly and quickly (within two weeks) lowered blood pressure. Fruits and vegetables provide potassium, magnesium, and fiber; dairy products provide calcium, potassium, and magnesium.
Researchers estimate that if Americans follow the DASH diet, coronary heart disease could be reduced by 15% and stroke by 27% (85). The DASH study is unique in that it examines the effect of dietary patterns on blood pressure, as opposed to individual nutrients on blood pressure. Support for the DASH diet is provided by several health professional organizations including The American Dietetic Association (86), and the Nutrition Committee of the American Heart Association (87), among others mentioned by Miller et al. (71). The National Heart Lung and Blood Institute adopted the DASH diet as the official diet in its current recommendations to prevent hypertension (66).
The Calcium/Sodium Connection
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CARDIOVASCULAR DISEASE
Coronary heart disease (CHD) is the most common and serious form of cardiovascular disease in the United States. Cigarette smoking, high blood pressure (hypertension), and elevated blood cholesterol levels are the major risk factors for this disease (92). Calcium may protect against CHD by its effect on two of these risk factors: hypertension and blood lipid levels. In patients with mild to moderate hypercholesterolemia, an increased calcium intake lowers total and LDL (low-density lipoprotein) blood cholesterol levels (92,93,94).
Milk and milk products have been demonstrated to either lower or have no effect on blood lipid levels (84,92). Research findings demonstrate that increasing calcium intake to 1,600 mg/day via milk and milk products does not raise blood lipid levels or body weight, which are other risk factors for coronary heart disease (82,83). When older adults increased their intake of fat free or 1% reduced fat milk by three cups/day for 12 weeks, there was no change in total and low density lipoprotein (LDL) cholesterol levels, or the ratio of total cholesterol to high density lipoprotein (HDL) (84).
Milk intake also may reduce the risk of stroke (95,96). Men who consumed at least two, eight-ounce glasses of milk a day were two times less likely to have a stroke than men who consumed no milk. This was the conclusion of a study involving more than 3,100 Japanese men ages 55 to 68 who were enrolled in the Honolulu Heart Program and followed for 22 years (95). The findings were independent of blood pressure and other risk factors for stroke. In another study of nearly 86,000 middle-aged women who participated in the Nurses' Health Study, calcium intake was inversely associated with the incidence of stroke (96). Further, this inverse association was stronger for dairy calcium (i.e., calcium from milk, yogurt, hard cheese, ice cream) than from nondairy calcium or calcium supplements (96).
Men who consumed at least two, eight-ounce glasses of milk a day were two times less likely to have a stroke than men who consumed no milk (95).
Researchers speculated that in addition to calcium, other nutrients in milk or the lifestyles of the milk drinkers might protect them from strokes (95,96). Intake of milk is associated with improved intake of other nutrients in addition to calcium (82,83,84). Further investigation is necessary to confirm these findings and to provide an explanation for the inverse association between milk intake and stroke risk.
COLON CANCER
Colon cancer is the third leading cause of cancer deaths in the United States (97). Both genetics and environmental factors contribute to this disease. While some dietary factors are suspected of contributing to colon cancer, others are thought to be protective. Several components in cow's milk fat such as conjugated linoleic acid, sphingolipids, and butyric acid have been found to protect against colon cancer in experimental animal and laboratory studies (98). For example, when mice exposed to a carcinogen that causes colon cancer were fed sphingolipids (i.e., dairy sphingomyelin), the proliferation of colonic cells and the appearance of aberrant crypt foci decreased (99).
Findings from a number of different types of scientific studies support a beneficial role for calcium against colon cancer (100, 101, 102, 103, 104).
Epidemiological findings demonstrate that risk of colon cancer is low in populations or groups that consume high amounts of calcium (100,101). More than 15 years ago, researchers in Chicago found that colon cancer was reduced by 75% in men who consumed 1,200 mg of calcium a day and by 50% when they consumed more than 150 IU vitamin D a day (105). Observations that intake of milk and milk products, which are a major source of calcium and vitamin D (if fortified), may reduce risk of colon cancer are consistent with a protective role for calcium in colon cancer (106). In a population study in Texas, a calcium intake of over 1,300 mg/day was associated with reduced risk of colorectal adenomatous polyps, which are precursors of colon cancer (107). A similar beneficial effect of calcium was found in patients with adenomas of the colon and rectum living in Uruguay (108).
In laboratory animals, increasing dietary calcium reduces chemically induced colon tumors (102,109). The protective effect of calcium on colon cancer appears to be greater when animals are fed a high fat, rather than low fat, diet (109).
The most cited hypothesis regarding a possible mechanism for calcium's protective effect against colon cancer involves its ability to counteract the effect of excess dietary fat (100,102). High intakes of dietary fat increase free fatty acids and bile acids in the colonic lumen. These acids, in turn, are thought to stimulate colonic cell proliferation, which promotes cancer. Calcium has been demonstrated to bind with the potentially toxic free fatty acids and bile acids to form insoluble calcium soaps, which are less toxic to the colonic mucosa.
In humans at risk of colon cancer, increasing intake of calcium or lowfat dairy foods has been demonstrated to reduce their risk for this disease (103,104). When 70 patients with a history of developing polyps or noncancerous growths in the colon increased their intake of food sources of calcium (i.e., by 1,500 mg/day), specifically lowfat dairy foods, risk for colon cancer decreased (103). Likewise in another investigation of 930 adults with a recent history of colorectal adenomas, increasing calcium intake by 1,200 mg/day reduced the incidence of recurrent adenomatous polyps by 19% and the total number of tumors by 24% in less than one year (104). Although these findings indicate a protective effect of calcium or lowfat dairy foods against colon cancer, further studies are necessary to substantiate this role for calcium.
In humans at risk for colon cancer, increasing intake of dietary calcium or lowfat dairy foods reduces biological markers for this disease (103,104).
KIDNEY STONES
Patients with kidney stones are often advised to limit their intake of calcium. Yet, for the general healthy population, there is no evidence that dietary calcium intake promotes kidney stones (110,111,112). In fact, a low calcium intake may increase the risk of this painful and costly medical condition (110,111,112,113).
Two prospective observational studies have demonstrated that increasing dietary calcium, particularly from dairy foods, reduces the incidence of kidney stones in adults (111,112). In a study of 45,000 male health professionals 40 to 75 years of age with no history of kidney stones, the men who consumed a calcium-rich diet (1,326 mg calcium, or the amount in more than four glasses of milk) had a 44% lower risk of developing kidney stones over the next four years than men who consumed 516 mg calcium (the amount in less than two glasses of milk) (111). Similarly in a 12 year study of nearly 92,000 nurses with no history of kidney stones, those who consumed more than 1,110 mg calcium/day were about one-third less likely to develop kidney stones than the nurses who consumed 430 mg calcium/day or less (112). In these studies, a greater intake of dairy products, the major source of dietary calcium, was associated with decreased risk of kidney stones (111,112). In contrast, intake of calcium supplements increased risk of kidney stones (111,112).
Calcium restriction may increase risk of developing kidney stones by increasing the intestinal absorption and urinary excretion of oxalate.
Urinary oxalate is more important than urinary calcium for kidney stone formation (113,114). Consuming milk and oxalate-rich foods (e.g., vegetables, beans, whole grains) at the same meal allows the calcium in milk to bind with oxalate, rendering oxalate unavailable for absorption (113). The result is that urinary oxalate excretion and risk of kidney stones are reduced (113). Researchers speculate that calcium supplements may increase kidney stone risk because they tend to be taken between meals when there is little or no opportunity to bind with oxalate in the intestine.
For individuals at risk of kidney stones, one serving of dairy foods with a gradual increase to three servings/day, as indicated by urinary calcium, is recommended (110). For the general healthy population, recommended intakes of dietary calcium from foods such as milk should be consumed not only to reduce risk of kidney stones, but also other diseases such as osteoporosis.
Because there is no one cause or type of kidney stone, treatment should be individualized.
PREMENSTRUAL SYNDROME
Increasing calcium intake may help to alleviate, at least in part, symptoms of premenstrual syndrome (PMS) (115,116). In a double-blind, cross-over trial involving 466 women, those who consumed an extra 1,200 mg calcium/day for three months experienced a 48% decrease in PMS symptoms compared to a 30% decrease in the placebo group (115). Additional research is necessary to substantiate calcium's beneficial role in PMS. Nevertheless, consuming recommended dietary intakes of calcium may be a safe, natural approach to reducing PMS symptoms.
POLYCYSTIC OVARY SYNDROME
Calcium (and vitamin D) may protect against polycystic ovary syndrome (PCO) (117). This female endocrine disorder is a common cause of menstrual dysfunction and infertility. Findings from laboratory animal studies indicate that disordered calcium regulation may contribute to PCO. High parathyroid hormone and very low vitamin D levels in the blood were identified in 13 premenopausal women with PCO (117). Moreover, treatment with calcium (1,500 mg/day) and vitamin D reversed calcium abnormalities and led to remission of symptoms in the majority of women after two months (117). Clinical trials are needed to confirm this potential beneficial health effect of calcium.
WEIGHT CONTROL
Calcium, and particularly dairy foods, may play a beneficial role in controlling body fat and reducing the risk for obesity (118). When mice genetically predisposed to obesity were fed diets varying in the amount and source (i.e., calcium carbonate or nonfat dry milk) of calcium for six weeks, the high calcium diets reduced weight gain and fat pad mass by 26 to 39% (118). Also, in the mice fed the high calcium diets, the expression and activity of adipocyte fatty acid synthase, as well as stimulation of lipolysis, were reduced by 3- to 5-fold. In this study, calcium in the form of dairy foods (i.e., nonfat dry milk) reduced the animals' fat deposition more than did elemental calcium (118).
To determine whether these findings in experimental animals apply to humans, the researchers examined epidemiological data from the National Health and Nutrition Examination Survey (NHANES) III (118). Calcium intake was inversely associated with body fat, especially for women, after controlling for energy intake, physical activity, age, and other variables (118). The findings from the experimental animal study and this epidemiological investigation led the researchers to suggest that increasing dietary calcium reduces diet-induced adiposity by decreasing energy efficiency and increasing thermogenesis (117).
Other investigators recently re-evaluated data from five previously conducted clinical trials - four observational studies and one double blind, placebo controlled, randomized trial - to explore the association between calcium intake and body weight (119). A lower calcium intake was linked to higher body weight in all 780 women participating in the studies (119). In the randomized, controlled trial, a calcium intake of 1,000 mg/day was associated with a nearly 18 pound lower average body weight over four years (119). Calcium intake explained approximately 3% of the variance in body weight. These investigators suggest that the tendency of dieters to reduce or eliminate milk from the diet, thereby decreasing their calcium intake, may contribute to the frequent failures of weight-reducing diets (119).
RICKETS
Intake of vitamin D fortified milk can help to prevent nutritional rickets, a vitamin D deficiency disease. Although thought to have been eliminated in the U.S., rickets has recently been identified in a number of African American infants and young children (120,121). Re-emergence of rickets may be explained by increased breastfeeding among African American women, fewer infants receiving vitamin D supplements, and limited exposure to sunlight by mothers and their infants (121). Intake of two cups of vitamin D fortified milk can meet the vitamin D recommendation (200 I.U.) for toddlers and young children ages 1 to 8 years.
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