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Nutrition & Product Information
Calcium Counseling Resource
Absorption/Utilization Issues
Calcium nutriture depends not only on calcium intake, but also on numerous dietary and nondietary factors that influence calcium metabolism.
The body's calcium status depends more on overall nutritional factors than just calcium intake (1). Some nutritive factors influence the body's absorption of calcium, while other factors affect calcium retention or urinary calcium excretion. Urinary losses of calcium are a big determinant of calcium loss (1). Helping clients achieve a calcium status consistent with reduced risk of major chronic diseases requires knowledge of the impact a variety of factors have on calcium metabolism.
DIETARY FACTORS AIDING CALCIUM ABSORPTION AND RETENTION
Vitamin D
Adequate vitamin D promotes calcium absorption and helps the body adapt to a low calcium intake (1,2,3,4). Vitamin D status of the body depends on exposure to sunlight and, to a lesser extent, on dietary intake of this vitamin (2,3).
To be biologically active, vitamin D must be converted in the kidney to calcitriol (1,25(OH)2 D). This hormonal form of vitamin D is responsible for regulating the intestinal absorption of calcium. Low blood levels of serum 25(OH)D, a clinical indicator of vitamin D status, are often observed in mature adults, especially those who are homebound or institutionalized (5,6). Lack of exposure to sunlight, low intake of vitamin D, and/or decreased skin and endogenous synthesis of the metabolically active form of vitamin D contribute to vitamin D deficiency.
In the absence of vitamin D, less than 10% of dietary calcium may be absorbed, compared to the typical 30% (4). Low estrogen activity, found in postmenopausal women not receiving hormonal replacement therapy, increases the impact of vitamin D deficiency on calcium absorption and bone health (7).
The skin's synthesis of vitamin D is influenced by factors that affect the ability of solar ultraviolet photons to reach the skin's surface (2,3). These factors include:
- Melanin - for groups such as African Americansand Asians who have a higher content of melanin pigment in their skinthan Caucasians, a longer exposure to ultraviolet light is needed toachieve the same degree of cutaneous synthesis of vitamin D.
- Sunscreens - topically applied sunscreens witha sun protective factor of 8 or more block sun-induced cutaneous vitaminD production.
- Season and latitude - during the winter monthsand at northern latitudes there is little or no production of vitaminD in the skin due to limited exposure to sunlight.
- Atmospheric pollution and clothing - these blockthe cutaneous production of vitamin D.
- Aging - by 70 years of age, the skin's abilityto manufacture vitamin D is decreased by fourfold compared to duringyoung adult years.
Numerous studies demonstrate that increasing vitamin D intake improves calcium absorption and bone health (3,8,9,10,11). Dietary recommendations (AIs) for vitamin D are based on the amount of vitamin D to maintain blood levels of 25-hydroxyvitamin D associated with bone health (3). For everyone 50 years and younger, 5 µg/day (200 IU/day) of vitamin D is recommended (3). For adults ages 51 through 70 years, the vitamin D recommendation increases to 10 µg/day (400 IU/day). For those older than 70 years, 15 µg/day (600 IU/day) is recommended (3). These recommendations assume limited availability of vitamin D from exposure to sunlight. A high prevalence of vitamin D deficiency was reported among older American adults admitted to hospitals, even among those with high intakes of vitamin D (12). This finding indicates that current recommended intakes for vitamin D may be insufficient for some individuals (12), a conclusion reached by other investigators (13,14,15).
Vitamin D needs can be met by exposure to sunlight as well as by diet. Despite the age-related decrease in the skin's ability to manufacture vitamin D via exposure to sunlight, older adults can still benefit from limited exposure to sunshine (16). If excess vitamin D is produced in the skin, it can be stored in body fat and be available during the winter months when the sun is incapable of producing vitamin D in the skin.
Few foods naturally contain vitamin D - egg yolks, fatty fish (salmon, swordfish), and cod liver oil (2,3). For this reason, milk, some cereals, and some juices are fortified with vitamin D. Almost all processed fluid milk in the United States is voluntarily fortified with vitamin D to obtain the standardized amount of 400 IU per quart (2). For some individuals, such as the homebound or institutionalized, a daily multivitamin tablet containing vitamin D may be necessary to help meet vitamin D needs. Because vitamin D in very high doses is toxic (i.e., 6,000 to 8,000 IU) and may promote bone loss, caution must be taken to avoid excessive intakes of this vitamin (3). A "tolerable upper intake level" of 50 µg vitamin D/day (2,000 IU/day) is established (3). This is the maximal level of vitamin D intake unlikely to be harmful to health. It is not a recommended intake.
Lactose
Lactose stimulates the intestinal absorption of calcium in laboratory animals (17) and in human infants (18). But there is no scientific evidence that lactose improves calcium absorption in adults (19,20,21). When young healthy women consumed milk and milk products containing different amounts of lactose, no effect on calcium absorption was observed (20).
The increased prevalence of osteoporosis in lactose-intolerant persons is most likely due to a low intake of dietary calcium primarily as a result of low intake of milk and milk products, not decreased calcium absorption (22,23,24). Deficiency of the enzyme lactase, or lactose malabsorption, does not diminish calcium absorption (25) or increase the calcium requirement (3).
DIETARY FACTORS REDUCING CALCIUM ABSORPTION AND INCREASING EXCRETION
Fiber (wheat bran, phytate, and oxalate)
Fiber's effect on calcium absorption is variable and small at best, especially when adequate calcium is consumed (26,27). Many kinds of fiber, including the fiber in green, leafy vegetables, which are relatively high in calcium, have no effect on calcium absorption (28). In contrast, large amounts of wheat bran reduce the absorption of calcium ingested at the same time (29,30).
Two fiber constituents, phytate and oxalate, can reduce the availability of calcium contained in the same food (31,32,33). The lower absorption of calcium from beans (pinto, red, white), compared to milk, is explained by beans' content of phytate (32).
Oxalate, found in spinach, inhibits the absorption of calcium from this food (34). Only about 5% of the calcium in spinach is absorbed, compared to the absorption of calcium from milk, which is about 30% (34).
One would need to consume five cups of red beans or eight cups of spinach to obtain the same amount of absorbable calcium as in one cup of milk (33).
Oxalates in chocolate do not affect calcium absorption from chocolate milk (35). Chocolate milk contains a small amount (0.5% to 0.6%) of oxalic acid, a compound occurring naturally in cocoa beans and other plants. Oxalic acid can combine with calcium to form an insoluble salt. However, the absorption of calcium from chocolate milk has been shown to be similar to that from other milk and milk products such as unflavored milk, yogurt, and cheese (35). A recent study in adult women found that a low fat, high fiber diet reduced calcium absorption (36).
Phosphorus
The wide availability of phosphorus in the diet, especially in relation to calcium, has led to questions regarding its effect on calcium status. However, several human studies indicate that phosphorus intake has little or no effect on calcium status (1,37). A high phosphorus intake reduces urinary calcium loss, but because it also increases digestive juice secretion of calcium by an equal amount, calcium status is unaffected. There is no convincing evidence that dietary phosphorus within the range currently consumed in the U.S. adversely affects calcium bioavailability, calcium absorption, or bone health, especially when calcium intake is adequate (1,3,37,38). Calcium recommendations are no longer tied to phosphorus consumption (3).
Protein
Dietary protein, from very low to very high intakes, increases urinary calcium excretion (3,37,39). For every gram of protein metabolized, urinary calcium increases by about 1 mg (39). Protein intake, therefore, influences the requirement for calcium. Individuals who consume a low-protein diet, such as some vegetarians, may require less calcium than omnivores or individuals who consume a higher protein diet. However, the entire diet needs to be considered when establishing calcium requirements. Other factors, such as the source of calcium, or other dietary constituents such as potassium, influence calcium metabolism (40). Vegetarians often consume plant sources of calcium, which may reduce calcium absorption. However, this may be offset by the reduced urinary excretion of calcium associated with the low-protein diet. Although some high-protein foods increase loss of calcium in the urine, the presence of other components such as calcium, phosphorus, and potassium in protein-rich foods such as dairy foods appears to counterbalance or modify the calciuretic action of protein (37,39,40). When calcium intakes are adequate, high dietary protein has little effect on bone health (1,41). A dietary calcium-to-protein ratio of 20:1 (mg:g) or higher provides adequate protection for the skeleton (41). Calcium intake recommendations in the U.S. are not altered according to dietary protein intakes (3).
Sodium
Sodium, like protein, increases urinary calcium excretion (3,42,43,44). Urinary sodium excretion potentiates calcium loss in the urine, eventually reducing calcium retention in the body (44). However, when dietary calcium intake is adequate, sodium intake does not adversely affect calcium balance (1). The typically high intakes of protein and sodium in the U.S. diet contribute to the higher average calcium requirements in this country compared to other nations where protein and sodium intakes are much lower. Calcium recommendations do not differ according to individuals' sodium intake (3).
Lead
Calcium absorption is influenced by lead and vice versa (45,46). An adequate intake of calcium reduces lead toxicity by decreasing the retention, absorption, and concentration of lead in tissues (45,46,47). Recognition of the importance of adequate dietary calcium in the prevention of lead toxicity led the Centers for Disease Control and Prevention to recommend adequate dietary calcium (48).
OTHER FACTORS THAT INFLUENCE CALCIUM ABSORPTION AND RETENTION
Age, estrogen status, physiological states such as pregnancy and lactation, lifestyle factors such as cigarette smoking, and disease and medications are other factors that can affect calcium absorption and retention (3).
Some of these factors such as excesses of protein and sodium can be overcome by increasing calcium intake. However, increasing calcium intake will not correct the adverse effects of other factors such as smoking, alcohol abuse, or physical inactivity.
Age
With increasing age, intestinal absorption of dietary calcium decreases in both men and women, particularly after age 60. In infants and young children, calcium absorption can be as high as 60%, whereas in late adolescence and adulthood calcium absorption decreases to about 25% to 30% (3,4,49). In later years, the efficiency of calcium absorption may be reduced even further. The body's ability to increase calcium absorption during periods of low calcium intake also diminishes with age. The result of this age-related decrease in calcium absorption and adaptability is an increased need for calcium. Estrogen deficiency in women and age-related changes in vitamin D metabolism contribute to the decrease in calcium absorption efficiency with age (3,4,49,50).
Estrogen
The decrease in estrogen levels at menopause or amenorrhea related to chronic endurance exercise or anorexia nervosa reduces the efficiency of calcium absorption (3). In addition to estrogen's direct effect on bone metabolic processes, this hormone enhances calcium absorption and improves renal calcium conservation. In the absence of estrogen, the efficiency of intestinal calcium absorption decreases and renal calcium losses increase (3). Between the ages of 40 and 60 years, the combination of increasing age and menopause reduces a woman's calcium absorption efficiency by 20% to 25% (51).
Pregnancy and Lactation
During pregnancy, intestinal calcium absorption increases from about 27% before pregnancy to 54% at five or six months of gestation and 42% at term (49,52). While information regarding calcium absorption during lactation is conflicting, the efficiency of calcium absorption appears to be increased once women stop breastfeeding or menses resumes (53).
This increase in calcium absorption from about 32% during lactation to 37% three months after weaning is considered to be a biological mechanism to compensate for the loss of calcium caused by breastfeeding (53).
Caffeine
Although once thought to have a detrimental effect on calcium status, moderate caffeine intake has little influence on calcium status (1,54,55). A single cup of brewed coffee causes a deterioration in calcium balance by only 2 mg to 3 mg, mainly by reducing calcium absorption. This effect is readily offset by consuming a tablespoon or two of milk (54). Caffeine appears to have little, if any, effect on calcium metabolism, calcium status, or bone density when recommended dietary intakes of calcium are also consumed (1).
Cigarette Smoking
Cigarette smoking is associated with decreased bone density and increased risk of bone fractures (56,57,58). Smoking may reduce calcium absorption or have a direct adverse effect on bone (57).
Alcohol
Excess alcohol intake can compromise calcium status by reducing the intestinal absorption of calcium, decreasing intake of calcium (and other nutrients), and causing liver damage (59). Damage to the liver impairs the production of the active metabolite of vitamin D (calciferol), which in turn reduces calcium absorption. The amount of alcohol necessary to reduce calcium status is unknown. However, the Dietary Guidelines for Americans (60) defines moderate alcohol intake as no more than one drink (12 ounces of regular beer or five ounces of wine) for women and no more than two drinks for men a day.
Diseases and Medications
A number of diseases affect calcium absorption and excretion (61,62). Diseases of the small intestine, liver, and pancreas can reduce calcium absorption. Idiopathic hypercalciuria is characterized by excessive excretion of calcium in the urine. Other diseases including hypoparathyroidism, diabetes, and thyrotoxicosis influence calcium absorption and retention by their effect on vitamin D metabolism.
Some surgical procedures, particularly those involving the small and large intestine, may adversely affect calcium absorption. Several commonly used medications, such as glucocorticoids (used to treat rheumatoid arthritis, ulcerative colitis, asthma), corticosteroids, anticonvulsants, and aluminum-containing antacids reduce calcium absorption and/or increase urinary calcium excretion (1,62). If long-term use of the above medications is necessary, bisphosphonate drugs may be needed to help preserve bone mass. Seek the advice of your physician.
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