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Nutrition & Product Information
Calcium Counseling Resource
Optimal Calcium Intake
Because 99% of total body calcium is found in bone, the need for calcium is determined almost entirely by skeletal requirements.
Calcium needs vary throughout life with greater needs during periods of rapid growth in childhood and adolescence and in later adult years to compensate for age-related bone-loss (1).
DEFINING OPTIMAL CALCIUM INTAKE
"Optimal" calcium intake for bone health refers to the level of consumption that is necessary for an individual to maximize genetically determined peak adult bone mass, usually by about age 30; to maintain adult bone mass; and to minimize bone loss in later years (1,2). Because the size of the body's skeletal calcium reserve is genetically limited, calcium functions as a threshold nutrient. This means that below a critical level, the effect of calcium on bone mass is limited by the amount of calcium available. Above that level - the so-called "threshold"- increasing calcium intake does not confer additional benefit.
In a broader sense, an optimal calcium intake is the level consistent with reduced risk of chronic diseases such as osteoporosis, hypertension, and colon cancer (3). The long-term goal of recommendations for optimal calcium intake is to extend the number of years that an individual can remain active, functional, and independent (3).
In 1997, for the first time since the 1989 publication of the Recommended Dietary Allowances (RDAs) (4), the National Academy of Sciences (NAS) Food and Nutrition Board (FNB) of the Institute of Medicine (IOM) (1) issued updated dietary recommendations for calcium and related nutrients. These calcium recommendations -expressed as Adequate Intakes (AIs) - are based on the amount of calcium to support maximal retention of body calcium (1). Maximal calcium retention is associated with increased bone mass and reduced risk of osteoporosis. In contrast, the previous RDAs (4) represented the amount of calcium to prevent a deficiency. The AIs for calcium are higher than the 1989 RDAs for most age categories to reflect current scientific information. Recommended levels of daily calcium intake are now closer to the recommendations made in 1994 by an Expert Panel on Optimal Calcium Intake convened by the National Institutes of Health (NIH) (2, Table 1). The American Medical Association (AMA) (5) supports the NIH calcium recommendations.
AGE-SPECIFIC REQUIREMENTS FOR CALCIUM
Infants
During the first year of life, the body's calcium content increases faster in relation to body size than at any other time (6). Skeletal calcium increases from about 30 g at birth to about 80 g at one year of age (1). Exclusively breast-fed infants retain about 128 mg calcium/day, assuming that 61% of the calcium in human milk is absorbed (1,7). Calcium recommendations for infants are based on the average intake of calcium from breast milk (1). Considering that breast-fed infants consume an average of 780 ml/day during the first six months and that breast milk contains 264 mg of calcium/liter, the AI for calcium is 210 mg/day (1). The absorption of calcium from commercial cow's milk or soy protein formulas is less efficient, but these formulas contain more calcium than human milk (8). For infants fed a cow's milk formula, 315 mg calcium/day is recommended to achieve a net calcium retention similar to that obtained with human milk (1). Both human milk and commercial formulas support infants' growth and mineral accretion (9).
During the second six months of life, 270 mg calcium/day is recommended for breast-fed infants (1). This recommendation is based on a human milk intake of 600 ml/day with a calcium concentration of about 210 mg/liter or approximately 130 mg/day in addition to the calcium supplied by solid foods (140mg/day) (1,10). For 6-to 12-month-old infants fed cow's milk formula and solid foods, 335 mg of calcium/day is recommended (1).
Because of the relatively low bioavailability of calcium from special formulas such as soy protein-based and protein hydrolysate formulas, 378 mg calcium, or 20% above the 315 mg calcium/day for infants fed cow's milk formulas, is recommended (1). There is no evidence of calcium deficiency in breast-fed infants receiving sufficient vitamin D (1).
Children
Children ages 1 to 10 retain less calcium in the body than infants, but need two to four times as much calcium per unit of body weight as adults (11). Optimal calcium intake is important during these years to achieve peak adult bone mass, which is reached by about 30 years of age (1).
Ensuring adequate calcium nutrition early in life may be responsible for a 5% to 10% difference in peak adult bone mass and a 50% difference in the incidence of hip fractures in later years (11).
For children 1 to 3 years of age, 500 mg of calcium/day are recommended to support a calcium retention of 100 mg/day (1). For 4- through 8-year-old children, a calcium intake of 800 mg/day is recommended to support a maximal calcium retention of 130 to 174 mg/day (1). In one intervention study, when 6- to 10-year-olds consumed 1,600 mg calcium/day, bone mass accumulation was 3% to 5% higher than when calcium intake was 1,000 mg/day or less (12). Other intervention studies indicate that at least 1,250 mg calcium/day is needed to maximize calcium retention in 6- to 10-year-old children (13,14,15). This positive effect of increased intake of calcium on children's bones appears to depend on a continued high calcium intake (8,16,17,18). According to the American Academy of Pediatrics (8), this observation "emphasizes the importance of diet in achieving adequate calcium intake and in establishing dietary patterns consistent with a calcium intake near recommended levels throughout childhood and adolescence." A diet rich in calcium could also help reduce the risk of lead poisoning in young urban children (19).
Adolescents
As children reach the adolescent growth spurt, skeletal accumulation of calcium (calcium retention) increases dramatically. Between 140 mg and 165 mg calcium/day is deposited in the skeleton during preadolescence and as much as 350 mg calcium/day during adolescence (5,14). Compared to young adult women ages 19 to 30 years, teenage girls retain approximately four times more of the calcium they consume, or an average calcium retention of 326 mg/day (20).
At least 40% or more of the body's total skeletal mass is formed during the adolescent growth spurt (5,13,21). Although bone mass may accumulate through the third decade of life, peak adult bone mass may be reached as early as late adolescence in certain bones (proximal femur and vertebrae) (21). Research indicates that the peak calcium accretion rate occurs at age 12.5 for girls and 14 years for boys (22). The teenage years are therefore a critical time to optimize peak bone mass. For children and adolescents ages 9 through 18, a calcium intake of 1,300 mg/day is recommended to maximize calcium retention (1).
Calcium intervention studies support a calcium intake of at least 1,300 mg/day for adolescents. A study of adolescent girls ages 12 to 14 years found that 1,300 mg calcium/day was the minimal amount needed for maximal calcium retention (23). In an investigation involving 12-year-old girls, increasing calcium intake from 935 mg to 1,370 mg/day for 18 months significantly increased total and spine bone mineral density (24).
Additional studies support a calcium intake greater than 1,300 mg/day during adolescence to increase bone mass (14,25,26,27). When 9- to 13-year-old prepubertal girls in Utah increased their calcium intake from 728 mg/day to 1,400 mg or higher/day for one year by consuming more milk and milk products, total and spinal bone density increased. Plus, they were also able to meet their calcium needs via foods from the Milk Group without an increase in fat intake, body weight, or percentage of body fat (25). In another controlled intervention trial, a calcium intake of 1,450 mg/day was estimated to optimize bone mass in adolescents (14). According to an Australian study, spinal and hip bone density increased within six months in 10- to 17-year-old twins who increased their calcium intake to more than 1,600 mg/day (27). A calcium intake of 1,600 mg/day during the teenage years was associated with increased hip bone density in women aged 30 to 39 years (26). These gains in bone mass, if maintained, could potentially offer some protection against future hip fractures. Findings from calcium intervention trials are consistent with calcium balance studies that indicate a calcium threshold of 1,480 mg/day for adolescents (13).
Based on current scientific evidence, a calcium intake of at least 1,300 mg/day and perhaps as high as the upper optimal calcium intake of 1,500 mg/day recommended by the NIH (2) and supported by the American Medical Association (5) and the American Academy of Pediatrics (8) is needed during adolescence to maximize peak adult bone mass. This level of calcium intake also may minimize teens' risk of accidental bone fractures (28).
According to a recent NIH Consensus Statement on Osteoporosis, "bone mass attained early in life is perhaps the most important determinant of lifelong skeletal health" (29). The National Institute of Child Health and Human Development recognizes "inadequate calcium consumption among children and adolescents to be a growing problem and a serious threat to their later healthy growth and development" (30). This federal government agency recommends that children and teens consume more calcium to protect their bones from fractures in later years. Health professionals acknowledge that it is difficult for children to meet their calcium needs without including milk in their diets (31). Drinking more milk during childhood and adolescence not only increases bone density, but also increases the likelihood of a high milk intake in young adulthood (32).
Young Adults
In the early adult years 19 through 30, bone mass accumulation continues for approximately 10 years after linear growth has stopped, but generally at a lower rate than during adolescence (1,33). Although data are limited, 1,000 mg calcium/day is recommended for men and women ages 19 through 30 years to maximize calcium retention (1). However, higher calcium intakes may be needed (33,34). A study found that when 156 college-aged women in their 20s were followed for four years, increasing calcium intake to 1,400 mg/day or higher increased spinal bone mineral density (33). In another investigation involving female college students ages 18 to 22, an intake of 1,200 mg calcium, mostly from milk and cheese, increased arm bone calcium content and density (34). Based on the findings, a calcium intake of 1,200 to 1,500 mg/day for young adults may be optimal.
Men
While men are less likely than women to develop osteoporosis, one-third of hip fractures occur in men and mortality following hip fractures is greater in men than in women (35). Risk of osteoporosis is greater in men with low peak bone mass than in those with higher peak bone mass (36). Therefore, it is important that men, like women, consume sufficient calcium throughout life to optimize peak bone mass, minimize age-related bone loss, and reduce risk of osteoporotic fractures. Few data are available on the calcium needs of men. For this reason, current calcium recommendations are the same for similar aged men and women (1). Recommended calcium intakes (AIs) are 1,300 mg/day for pre-teens and teenagers 9 through 18 years, 1,000 mg/day for men 19 through 50 years old, and 1,200 mg/day for men 51 years of age and older (1). More studies are needed to determine the optimal calcium intake for men.
Hip bone mineral density, but not spinal bone mineral density, was reported to be higher in men who consumed over 1,200 mg calcium/day than in men whose intake was less than 800 mg/day (37). A meta-analysis of the effect of calcium intake on bone mass concluded that there are too few well-designed studies to draw any conclusions regarding optimal calcium intake in men (38). In the three studies including men ages 18 through 50 in the meta-analysis, the men consumed 1,300 to 1,400 mg calcium/day (38). Other studies in men link higher calcium intakes with lower risk for bone loss (39). Researchers at the University of Pittsburgh reported that men who drank milk at every meal, especially as teenagers and from ages 18 to 50, tended to have a somewhat higher bone mineral density than men who drank milk less frequently (40). A recent prospective study of more than 1,800 men in Southern Europe links higher intakes of milk and cheese with decreased risk of hip fractures (41).
Premenopausal Women
The window of opportunity for achieving peak bone mass remains open, at least in part, until about age 30 (42,43). Once peak adult bone mass is reached, bone turnover is stable or bone formation and bone resorption are balanced.
For premenopausal women ages 31 through 50 years, 1,000 mg calcium/day is recommended to maximize calcium retention (1). According to a meta-analysis of 27 well-designed studies involving women between 18 and 50 years of age, a calcium intake of approximately 1,000 mg/day or higher is positively associated with bone mass (38). One study links milk intake of women ages 25 to 34 years with increased forearm bone density (44). In another investigation involving nearly 1,000 healthy premenopausal women ages 45 to 49 years, spinal and hip bone mineral densities were somewhat higher among those who consumed more than 2 cups of milk a day during childhood and young adulthood than among women who consumed a cup or less of milk a day (45).
Menopausal Women
During menopause, a sharp decline in estrogen levels contributes to an acute change in calcium balance and accelerated bone loss lasting approximately five or more years (46). During the first five years of menopause, women lose about 3% of bone mineral a year (46). Thereafter, bone loss slows to 1% a year, similar to that seen in aging men. As a result of the initial rapid decline in estrogen at menopause, women lose about 15% of the bone they had before menopause (42). If nutrition is adequate, this loss continues for only a few years. Thereafter, the skeleton reaches a new steady state, albeit at a 15% lower bone mass (42). This menopausal bone loss highlights the importance of achieving a high peak skeletal bone mass earlier in life.
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 (42,46). Hormone replacement therapy may be the most effective way to slow bone loss, especially in the first six to eight years after menopause when the decline in estrogen is the predominant factor responsible for the accelerated bone loss. Estrogen appears to inhibit bone resorption and improve the efficiency of calcium absorption (2). Estrogen deficiency-related bone loss cannot be prevented by increasing calcium intake (1). However, dietary calcium improves the effectiveness of estrogen on bone (47,48,49). New research demonstrates that a low dose of hormone replacement therapy combined with increased intakes of calcium and vitamin D helps to protect bones as much as higher doses of hormone replacement therapy (49).The decision to take estrogen after menopause should be discussed with a physician because of the possible risks involved (50).
Although calcium is less effective than estrogen in slowing the rapid bone loss that occurs immediately after menopause (51), calcium intake is nevertheless important during the early postmenopausal period (52). 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 (52). Increasing calcium intake during the early menopausal years may protect women from lead exposure (53). Blood lead levels are reported to increase in women during the first four years of menopause when bone loss is accelerated, since excess lead is stored in bone (53).
Postmenopausal Women
Almost three-quarters of hip fractures occur in white postmenopausal women (29). This group also experiences the highest age-adjusted fracture incidence (29). For postmenopausal women ages 51 through 70 years, 1,200 mg of calcium/day is recommended for maximal calcium retention (1). This calcium recommendation is 200 mg/day higher than for 31- through 50-year-olds because of the decrease in calcium absorption with advancing age (1). Findings from calcium balance studies and calcium intervention trials were used to determine the calcium recommendation of 1,200 mg for women 51 through 70 years (1,54,55,56). Evidence is insufficient to support different calcium recommendations for women depending on their menopausal status or use of hormone replacement therapy (1). Researchers in Sweden recently suggested that postmenopausal women may need to consume more than 1,200 mg/day or as much as 1,400 mg/day to protect against osteoporosis (57).
Over 70 Years
For mature adults over 70 years of age, a calcium intake of 1,200 mg/day is recommended (1). Bone loss occurs in both mature male and female adults, regardless of their gonadal hormone level (42,58). Adults in their 50s and 60s generally lose about 0.5% to 1.0% of bone per year, but bone loss may be even greater in later years (42). In a study of women in their 80s who consumed a low-calcium diet (514mg/day), bone loss from the hip was 3% per year (59). This age-related bone loss emphasizes the importance of identifying interventions that retard further bone loss.
- Age-related physiological and lifestyle changescommon in later years contribute to bone loss in mature adults and increasetheir need for calcium (60). These factors include:
- Inadequate vitamin D status. With aging, vitaminD status, indicated by low levels of blood 25(OH)D, declines as a resultof less efficient skin synthesis of vitamin D, impaired conversion ofthe inactive to more active form of vitamin D in the kidney, reducedsun exposure, and low dietary intake of vitamin D (61,62). Vitamin Dinsufficiency causes reduced calcium absorption, low blood levels ofionized calcium, increased circulating parathyroid hormone levels, anda rise in bone resorption.
- Reduced intestinal absorption of calcium. Absorptionof calcium typically decreases with age in both men and women (51,63).This age-related decline in calcium absorption efficiency is relatedto the loss of estrogen, possibly other hormonal changes, and vitaminD insufficiency (60). In addition to reduced intestinal absorption ofcalcium, the body's ability to increase calcium absorption during periodsof low calcium intake is diminished in older adults.
- Low calcium intake. Some mature adults consumediets low in calcium as a result of reduced food intake (64). Physiological(e.g., declines in smell and taste), and socioeconomic factors (e.g.,isolation, poverty), chronic diseases, and use of medications can decreasemature adults' food intake.
- Findings from a variety of calcium studies supporta calcium intake of at least 1,200 mg/day for mature adults. Some calciumbalance studies reveal that a calcium intake of 1,500 to 1,700 mg/dayis necessary to attain calcium balance (i.e., calcium intake equalscalcium losses) in mature adults (42,65).
- Well-controlled calcium intervention trialsdemonstrate that a calcium intake of over 1,200 mg/day may reduce boneloss and fracture risk at various skeletal sites in mature adults (59,61,66,67,68,69).
When mature women 69 to 106 years of age increased their calcium intake from 500 to 1,700 mg/day and consumed an additional 800 IU of vitamin D for 18 months, hip bone density increased, the rate of hip fractures was reduced by 41%, and nonvertebral fractures declined by 30% (59). After 36 months, risk of hip fractures was lowered by 29% (68). Risk of nonvertebral fractures was lowered by 24%.
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Studies of changes in parathyroid hormone levels also support an increased requirement for calcium in later years (68,71). Elevated blood levels of parathyroid hormone, which increase bone resorption, are thought to be typical of advancing age. However, increasing older adults' calcium intake decreases parathyroid hormone levels and bone resorption (71,72,73). A recent eight-week study found that increasing intakes of calcium by 1,200 mg/day and vitamin D by 800 I.U./day reduced secondary hyperparathyroidism and risk of falls in older women (73).
Food sources of calcium, such as milk and other dairy products, are especially beneficial for bones (72). When older adults aged 55 to 85 years increased their usual low dairy intake (<1.5 adding="" adverse="" an="" and="" blood="" body="" bone="" by="" calcium="" conclude="" cost-efficient="" day="" decreased.="" diet="" effects="" findings="" fluid="" for="" glasses="" glucose="" had="" hormone="" improve="" improved="" increasing="" intake="" is="" led="" levels="" lipid="" little="" loss="" milk="" no="" nutrient="" nutrition.="" of="" older="" on="" or="" parathyroid="" particularly="" per="" researchers="" substantially="" that="" the="" their="" to="" vitamin="" way="" weight="">
Studies support a calcium intake of at least 1,200 mg/day, as currently recommended by the National Academy of Sciences (AIs) (1), and closer to an optimal calcium intake of 1,500 mg/day for mature adults recommended by the NIH (2) and AMA (5). Because the skeletal benefits of intermittent increases in calcium and vitamin D do not persist, older adults should consume dietary calcium and vitamin D at recommended intake levels on a continuous basis (75).
Pregnant Women
During pregnancy, alterations in calcium absorption, metabolism, and excretion occur to conserve calcium (1,76). In particular, intestinal absorption of calcium and blood 1,25(OH)2D (calcitriol) levels increases (1,77). Bone remodeling also accelerates during late pregnancy with an initial building phase followed by a rise in resorption. The net effect is maintenance of the mineral content of bone.
During pregnancy, especially the last trimester, the fetus accumulates about 330 mg calcium/day for a total of approximately 30g (1). If maternal bone mineral were the sole source of this calcium, the mother would lose approximately 3% of her skeletal calcium per pregnancy. However, because of the adaptive mechanisms that conserve calcium, pregnancy has no apparent adverse effect on the skeleton of well-nourished women (1). Also, because of these adaptive changes in the body's handling of calcium during pregnancy, recommended calcium intakes (AIs) are the same for pregnant and nonpregnant women - 1,000 mg/day for women 19 through 50 years and 1,300 mg/day for those 14 to 18 years (1). Increasing calcium intake during pregnancy, especially among women with low calcium intakes (<600 bone="" content="" increase="" may="" mineral="" the="">
Optimal calcium intake during pregnancy may help to regulate maternal blood pressure in some mothers (79,80,81,82,83). Pregnancy-induced hypertension occurs in approximately 5% to 10% of the four million pregnancies in the United States each year. This condition contributes to preterm delivery, low birth weight, and prenatal and maternal death. Preeclampsia is a life-threatening disorder of late pregnancy that occurs in 2% to 8% of all pregnancies and is characterized by high blood pressure, fluid retention, and protein in the urine, and can endanger the health of both mother and child (81).
In one study, consuming an adequate calcium intake during pregnancy reduced pregnant women's risk for preeclampsia by 62% (81), which could save several billion dollars a year in related health care costs (84). Further support for calcium's beneficial effect on blood pressure during pregnancy comes from a recent investigation in Spain (85). Pregnant women with low calcium intakes exhibited higher blood pressure levels than in those who were more likely to meet their calcium recommendations (85). Increasing calcium intake during pregnancy may also have a beneficial effect on blood pressure of the offspring (86).
According to a meta-analysis of 14 controlled trials involving 2,459 pregnant women, increasing calcium intake by 1,500 or 2,000 mg/day reduced the incidence of pregnancy-induced hypertension by 70% and the risk of preeclampsia by 62% (81).
In contrast to the beneficial effect of calcium in this meta-analysis (81), the Calcium for Preeclampsia Prevention (CPEP) trial, which included over 4,500 pregnant women nationwide, found only a slight, but nonsignificant beneficial effect of the 2,000 mg of supplemental calcium on the risk of preeclampsia, pregnancy-induced hypertension, or other adverse outcomes of pregnancy (87). The failure to find a significant beneficial effect of calcium in this trial may be explained by the women's already high intake of calcium (i.e., 1,100 mg/day) (83). Calcium may be most effective for women at high risk of preeclampsia, such as women pregnant with twins or who have diabetes, those who are teenagers, or women with low calcium intakes (82,83,88).
Adequate calcium intake during pregnancy, especially during the third trimester, may increase breast milk calcium levels during lactation (89). Also, meeting calcium needs during pregnancy may protect women from high blood lead levels (90). When pregnant women consume low calcium intakes, bone resorption releases both calcium and stored lead into the bloodstream, which could limit cognitive development of the child (90).
Meeting dietary calcium recommendations is important for pregnant adolescents who are still growing or increasing their own bone mass. Although theoretically the calcium needs of pregnant adolescents are higher to support their own bone health and that of their fetus, research on the calcium needs of pregnant adolescents is limited (1). At present, no additional increase for calcium is recommended during pregnancy for these young women (1,2). Low calcium intakes are documented in adolescent females, especially those with inadequate food intakes (91). There is concern that a low calcium intake (<600mg/day) adult="" adversely="" affect="" an="" are="" at="" bone="" calcium="" disorders="" during="" especially="" growing="" higher="" hypertension="" hypertensive="" inadequate="" increase="" intake="" may="" mineralization="" of="" pregnancy="" pregnancy-induced="" pregnant="" rapidly="" risk="" still="" teens="" than="" the="" who="" women.="" young="">
Lactating Women
For lactating women, an optimal calcium intake is important to meet their own calcium needs, as well as the requirements for milk production. Between 160 mg and 300 mg of calcium/day are lost through the production of breast milk (2). Unlike pregnancy, calcium absorption efficiency is not increased during lactation (1,92). However, calcium needs for milk production appear to be met by decreased urinary excretion of calcium and increased bone resorption (92). Women who breastfeed their infants for at least six months lose bone density at selected sites regardless of high intakes of dietary calcium (93,94). But by about three months after weaning, women regain the bone density lost during lactation (94,95,96). The relatively low level of estrogen during lactation is thought to be responsible for this transient bone loss and would explain why bone loss is not suppressed by high intakes of calcium. In well-nourished women, variation in calcium intake does not appear to be related to either the amount of bone lost during lactation nor its recovery after weaning. After weaning or the resumption of menses, calcium absorption and blood levels of 1,25(OH)2D (calcitriol) increase to rebuild bone lost during lactation (94).
Increased dietary calcium intake does not prevent calcium loss during lactation and calcium lost appears to be regained following weaning (1,95). For these reasons, calcium recommendations (AIs) are the same for lactating and nonlactating women of the same age - 1,000 mg/day for 19- through 50-year-olds and 1,300 mg/day for 14- through 18-year-olds (1). There is insufficient scientific evidence to support higher calcium intakes for lactating women with closely spaced pregnancies, for those who are nursing more than one infant, or for lactating adolescents (1).
CONCLUSION
A recent analysis of 139 papers relating to calcium intake and bone health published between 1975 and 1999 provides convincing evidence of the importance of adequate intake of calcium and calcium-rich foods throughout life in bone health (97). In 50 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 sole exceptions were a trial in women in which the control group consumed more than 1,100 mg calcium/day and a trial in early postmenopausal women in whom loss of estrogen was the principal causal factor for bone loss. The six studies that used dairy sources of calcium were all positive (97).
Individuals who increase calcium intake through calcium-rich foods such as milk and milk products consume not only more calcium, but also more of other nutrients such as protein, riboflavin, phosphorus, folate, thiamin, niacin, vitamin B6, vitamin B12, magnesium, zinc, and iron (74,98,99,100,101,102). As such, a diet lacking or limited in dairy products is often a poor diet low in calcium as well as in other nutrients (97).
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