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The New Dietary Reference Intakes for Calcium: Implications for Osteoporosis

Purdue University, West Lafayette, Indiana

Address reprint requests to: Connie Weaver, Purdue University, 1264 Stone Hall, West Lafayette, IN 47907-1264.

	ABSTRACT

TOP ABSTRACT DIETARY REFERENCE INTAKES THE CALCIUM REQUIREMENT ACHIEVING ADEQUATE CALCIUM... ROLE OF GENETICS RACIAL DIFFERENCES CONCLUSION REFERENCES

 
The new Dietary Reference Intakes (DRI) for calcium were released August 13, 1997 by the Food and Nutrition Board of the National Academy of Science. Unlike the RDA’s which established the minimal amounts of nutrients needed to be protective against possible deficiency, the new values are designed to reflect the latest understanding about nutrient requirements directed at optimizing health in individuals and groups. Adequate calcium intake recommendations were set at levels associated with desirable retention of body calcium since high bone density is known to be less susceptible to fractures. Recommended intake for adults is 1000 to 1200 mg/day. The majority of people do not consume these levels of calcium. The upper tolerable limit for calcium was set at 2.5 g/day.

Key words: calcium requirement, calcium retention, calcium balance, bone mineral density, osteoporosis

Key teaching points:

• The new Dietary Reference Intakes for calcium reflect the latest understanding about nutrient requirements based on optimizing health in individuals and groups.

• The relationship of calcium intake to bone health and osteoporosis prevention was the primary consideration for setting the new requirement.

• Osteoporosis, a condition of reduced bone mass resulting in increased skeletal fragility, affects 25 to 30 million Americans.

• An adequate calcium intake across the life span maximizes calcium retention for highest bone strength within the genetic potential, reducing the risk of osteoporosis and osteoporotic fracture risk later in life.

	DIETARY REFERENCE INTAKES

TOP ABSTRACT DIETARY REFERENCE INTAKES THE CALCIUM REQUIREMENT ACHIEVING ADEQUATE CALCIUM... ROLE OF GENETICS RACIAL DIFFERENCES CONCLUSION REFERENCES

 
In August 1997, the Food and Nutrition Board of the Institute of Medicine, National Academy of Sciences, recently released the new Dietary Reference Intakes (DRIs) for calcium, magnesium, phosphorus, fluoride and vitamin D [1]. The committee made recommendations aimed at helping individuals at different stages of life obtain enough of a nutrient to promote bone strength and to maintain normal nutritional status. The report for bone-related nutrients was the first in a series of reports presenting dietary reference values for the intake of nutrients by Americans and Canadians.

The DRIs supersede the Recommended Dietary Allowances (RDAs) that have been published since 1941 by the Food and Nutrition Board. The new recommendations were made by a group of more than 30 U.S. and Canadian scientists who examined the peer-reviewed literature on both the beneficial aspects of nutrients and the potential toxic effects of overconsumption. The DRIs were based upon current concepts about the role of nutrients and food components in long-term health, going beyond prevention of deficiencies. The relationship of calcium intake to bone health and osteoporosis prevention was the primary consideration for setting the new requirement [1]. The four components of the new DRIs are Recommended Dietary Allowances (RDA), Estimated Average Requirements (EAR), Adequate Intakes (AI) and Tolerable Upper Intake Levels (UL).

The RDA is the average daily dietary intake level that is adequate to meet the nutrient requirement of approximately 97% to 98% of healthy individuals in a particular life stage, taking into consideration age and, if applicable, pregnancy and/or lactation. The sole purpose of the RDA for a nutrient is as a goal for daily dietary intake by individuals. It is not intended that it be used for nutritional assessment purposes, nor for planning diets for groups.

The EAR is defined as the nutrient intake value estimated to meet the requirement of fifty percent of the healthy individuals in a life-stage and gender group [1]. For groups, it is used in conjunction with data on the group’s distribution of intake, adjusted for day-to-day variation in intake. The EAR is the primary criterion for assessing adequacy of intake of groups. The EAR is also used to develop the RDAs, which are set at two standard deviations above the EAR when the standard deviation of the EAR is known.

An AI level was set for nutrients for which there was insufficient scientific data to establish an EAR and, therefore, an RDA. The AI is derived from observational or experimental estimates of nutrient intake, by a defined population or group, that apparently sustains a certain nutritional state. For calcium, the AI for each life-stage group is an experimentally derived approximate group mean value that appears to support maximal calcium retention, as determined by balance studies.

The UL is defined as the maximal level of daily nutrient intake that is likely to pose no risk of adverse health effects to almost all individuals in a specific life-stage and gender [1]. As intake increases above the UL, the potential for adverse effects increases. The setting of an upper intake level was necessitated by the increasing use of dietary supplements among the population and the increased practice of food fortification with nutrients. The term "tolerable" was included in order to stress that the UL is not a recommended intake level; there are no established benefits for healthy individuals associated with consumption of nutrients above the RDA or AI.

	THE CALCIUM REQUIREMENT

TOP ABSTRACT DIETARY REFERENCE INTAKES THE CALCIUM REQUIREMENT ACHIEVING ADEQUATE CALCIUM... ROLE OF GENETICS RACIAL DIFFERENCES CONCLUSION REFERENCES

 
Adequate calcium intake is thought to reduce the risk of a number of chronic diseases, but the relationship of calcium deficiency and osteoporosis is the most studied. The panel concluded that intakes which were optimal for bone health provided adequate protection for hypertension and colon cancer. Osteoporosis, a condition of reduced bone mass resulting in increased skeletal fragility, affects 25 to 30 million Americans. Approximately twenty-five percent of women of Anglo-Saxon origin over the age of fifty have osteoporosis. Within the female adult population there are more than 1.5 million bone fractures annually, resulting in healthcare costs in excess of $13 billion every year [2]. The three sites most vulnerable to osteoporatic fracture are the spine, the wrist and the hip. Hip fractures are the most serious in terms of disability, loss of independence, mortality and medical costs.

Skeletal bone density is a marker for fracture risk at any age. The lower the skeletal bone density, the greater the risk for fracture. The goal for setting the calcium requirement was the intake which would promote development of maximal peak bone mass, or bone density, during growth and would minimize loss thereafter. This is the intake which maximizes calcium retention for highest bone strength within the genetic potential. To determine intakes which lead to maximal retention, a range of intakes needs to be studied. Longitudinal studies of bone density over a range of calcium intakes are unavailable. Therefore, for all ages for which data were available, balance studies were employed over a range of calcium intakes for the determination of maximal calcium retention. Calcium retention increases with intake until a plateau occurs beyond which there are little appreciable additional benefits.

In addition to maximal retention intake calculations, evidence used to set the calcium requirements came from randomized clinical trials, which measured changes in bone density and, to a lesser extent, cross-sectional and epidemiological studies. The calcium requirements are given in Table 1. The criteria used for setting the DRI for groups by life-stage are listed in Table 2 [1]. Calcium requirements across the life span are not uniform due to changes in skeletal growth and age-related changes in absorption and excretion. Calcium retention is high during growth and, ideally, is in balance with intake during adulthood, but becomes negative when calcium intakes are inadequate, since the skeleton serves as the primary mineral reservoir for the maintenance of serum calcium levels.

The most important factor for determining the calcium required for children and adolescents is the attainment of peak bone mass within the genetic potential. At the end of consolidation, when the maximum amount of bone has been accumulated, certain sites may "peak" during adolescence or shortly thereafter. Maximizing peak bone density is thought important to protect against fracture later in life, but bone density may also predict fracture risk in children. Children aged 3 to 15 years who had suffered forearm fractures had substantially lower bone density than age-matched, non-fracture controls [11]. The calcium intake which leads to maximal calcium retention in prepubertal children is 800 mg/day [12] and 1300 mg/day for white adolescent girls [13] as determined by balance studies over a range of calcium intakes.

Adulthood
In adults, total body bone mass remains relatively constant over the reproductive years, as decreases in the proximal femur and other sites after age 18 are offset by continued growth of the forearm, total spine and head. Bone loss proceeds at the rate of about 0.5% to 1% per year from the age of approximately 40 years in both genders, although in women this loss is increased in the first 3 to 5 years after menopause. Age-related decreases in calcium absorption and increases in urinary calcium contribute to this loss.

In premenopausal women supplementation with 1 g calcium significantly reduced vertebral bone loss and 2 g calcium supplementation provided no further benefit [14]. In postmenopausal women, Dawson-Hughes [15] reviewed the randomized clinical trials and concluded that calcium supplementation helped those on previously low intakes of calcium and was also beneficial when given more than five years post-menopausal. Recker et al. [16] further found that increasing dietary calcium through supplementation or calcium-rich foods resulted in higher bone density and reduced incidence of fracture later in life. Calcium also augments the effect of hormone replacement therapy [17].

The calcium intake required by younger adults aged 19 to 50 years to achieve mean maximal retention or minimal loss was determined to be 1000 mg/day [11]. In older adults aged 51 to 70, the calcium intake required to achieve maximal retention was 1200 mg/day, using the balance data of Spencer et al. [18].

Pregnancy and Lactation
The criteria used to determine the calcium requirement for pregnant and lactating women was bone mineral content. No data suggests that parity or lactation have permanent negative effects on bone density. Therefore, the panel set the AI for pregnant females less than 18 years to be the same as for nonpregnant adolescent females (1300 mg Ca/day). The AI for pregnant women aged 19 to 50 was also set at the same level (1000 mg Ca/day) as that for nonpregnant women and men in the same age category. At this time, calcium requirements are not higher due to pregnancy or lactation because calcium absorption efficiency and retention adjusts during the third trimester to accommodate skeletal needs of the fetus and during post-weaning periods. Net skeletal calcium loss during lactation is generally regained within six months post partum. Furthermore, calcium supplementation does not appear to prevent lactation-induced bone loss [19]. More research is needed to determine the effect of pregnancy and lactation on bone loss in adolescents and older mothers and the impact of inadequate calcium intake on bone mineral mass in the years following pregnancy and lactation. Requirements for some individuals may be higher during these altered states. Nevertheless, a larger public health concern is that calcium intakes of women are generally too low.

The Elderly
Adults aged 70 years or older have decreased physical activity which leads to reduced food intake. The tendency to spend most of their time indoors leads to lowered exposure to sunlight, which leads to reduced vitamin D status, a decline in calcium absorption efficiency and a decline in renal calcium conservation [20]. Hip fracture as well as nonvertebral fracture rates among elderly retirement home residents decreased when residents were supplemented with 1200 mg calcium and 800 IU vitamin D daily [21]. There was also a significant positive effect of calcium on the bone mineral density at the proximal femur in this study. In another randomized trial in men and women (mean age 71 years), Dawson-Hughes found that supplementation with 500 mg elemental calcium and 352 IU vitamin D significantly reduced nonvertebral fracture rates [22]. Because there are very few data in men and women at high calcium intakes to allow an estimation of the intake above which no further gains in calcium retention occur, the AI was set at 1200 mg/day [1].

	ACHIEVING ADEQUATE CALCIUM INTAKES

TOP ABSTRACT DIETARY REFERENCE INTAKES THE CALCIUM REQUIREMENT ACHIEVING ADEQUATE CALCIUM... ROLE OF GENETICS RACIAL DIFFERENCES CONCLUSION REFERENCES

 
Calcium intakes for most of the population from puberty and older are appreciably less than the requirement (Fig. 1). In the United States, the mean calcium intake of adolescents is likely inadequate to meet the demands of rapid skeletal growth, and they are destined to obtain a lower peak bone mass than their genetic potential [23]. The mean calcium intake for boys in the U.S. aged 9 to 13 years is 980 mg/day and 889 mg/day for girls. The requirement of 1300 mg/day falls above the seventy-fifth percentile of calcium intake for boys and the ninetieth percentile of intake for girls [1]. In older individuals, lower calcium intakes relative to the requirement typically lead to bone loss. The gap between actual calcium intake and AIs can be as high as hundreds of milligrams per day. Closing this gap is our responsibility and challenge as educators and health professionals.

View larger version (15K): [in this window] [in a new window]   Fig. 1. Mean calcium intake of American males and females compared to the NAS 1997 Dietary Reference Intakes-Adequate Intakes.

	ROLE OF GENETICS

TOP ABSTRACT DIETARY REFERENCE INTAKES THE CALCIUM REQUIREMENT ACHIEVING ADEQUATE CALCIUM... ROLE OF GENETICS RACIAL DIFFERENCES CONCLUSION REFERENCES

 
Although environmental factors have a significant influence on the development of bone mass, sixty to eighty percent of peak bone mass is genetically determined. Some recent reports begin to illustrate the role of genetics in calcium and bone metabolism. O’Brien et al. [26] hypothesized that bone turnover is less responsive to alterations in dietary calcium intake in both girls and adult women from families with histories of osteoporosis, compared to control families, in three generations of all families. They observed diminished responsiveness to calcium supplementation in girls from osteoporotic families due to an increased rate of resorption as well as formation compared to control girls from non-osteoporotic families (Fig. 2). In these individuals, calcium requirements are likely higher to achieve desirable peak bone mass and to minimize skeletal loss.

View larger version (25K): [in this window] [in a new window]   Fig. 2. Mean (±SEM) calcium retention on low calcium (LC) and high-calcium (HC) intakes in three generations of females (grandmothers, mothers, and granddaughters) from healthy (control) and osteoporotic families. Overall, balance in bone calcium turnover was significantly influenced by calcium intake (p<0.006), and was significantly higher in the granddaughters compared with grandmothers or mothers during the low-calcium period (p<0.001). Reproduced with permission of Journal of Bone and Mineral Research.

The interplay between genetic effects and diet on bone mass is unclear, although recent studies suggest that one interesting association is between calcium utilization and allelic differences related to the gene for the Vitamin D Receptor (VDR). In the study by Dawson-Hughes et al. [28], a differential relationship of VDR alleles was observed at high and low calcium intakes. It was found that women with BB allelic variants of the VDR have reduced calcium absorption efficiency on low calcium intake, but that this inherited trait is reduced at higher calcium intakes. A recent report from Switzerland in 143 Caucasian prepubertal females, randomized to a supplementation trial in which they received 850 mg supplemental calcium daily or a placebo for one year, showed a significant impact in girls with Bb and BB VDR alleles on BMD gain in six bone sites and on bone size and statural height [29]. Thus, genetic make-up in humans appears to influence ability to utilize nutrients. One wonders, with the advances being made in understanding the human genome, if nutrient requirements in the future will be differentiated by genetic typing at birth.

	RACIAL DIFFERENCES

TOP ABSTRACT DIETARY REFERENCE INTAKES THE CALCIUM REQUIREMENT ACHIEVING ADEQUATE CALCIUM... ROLE OF GENETICS RACIAL DIFFERENCES CONCLUSION REFERENCES

 
An issue related to the role of genetics is possible racial differences in calcium requirements. The scientific data used to determine calcium requirements was collected primarily in Caucasians.

African Americans have generally been reported to have higher bone mass and lower incidence of fracture than whites [30]. Although vertebral bone density did not differ between black and white girls before puberty [31], bone mineral content of the forearm was already higher in black than in white children by age one to six years [32]. It has been observed that blacks have lower serum 25 OH D, but elevated 1,25 (OH)₂ and PTH compared to whites [33,34]. It has also been observed that there are significant differences in bone formation between black and white women; bone formation rates within each remodeling unit are reduced, but there is a longer total formation period in blacks than in whites [35]. A possible mechanism for the difference between black and white bone metabolism was proposed by Parisien et al. [36]. In a comparative study of black and white women aged 25 to 40, parathyroid hormone (PTH) was infused and the skeletal response, using biochemical remodeling indices, was measured. Their data strongly suggested that blacks have decreased skeletal sensitivity to the acute resorptive effects of increased PTH. There are too few data to warrant a different DRI for calcium for blacks; further study is necessary to determine the relationship between calcium intake and calcium retention and bone density in blacks to determine whether calcium requirements are different.

The incidence of osteoporosis, as reflected by the number of hip fractures, is rising in Asia and Japan. Usual calcium intakes do not decline with age in Asian countries as they do in America. Calcium intakes in China appear to be similar to intakes of elderly Americans, but lower in the younger adults than in their American counterparts [37]. Vegetable calcium sources are more important in the Chinese diet than in the typical Western diet because of the lower intake of dairy products in the Chinese population. Vegetables and soy products provide 41% of the calcium intake of Chinese, in contrast to less than 10% for Americans [37]. Calcium absorption from several commonly consumed Chinese vegetables was higher than from milk by almost 10%, although the amount of calcium in these vegetables on a per-serving basis would require ingesting, for example, upwards of two servings of Chinese cabbage or five servings of broccoli to equal the amount of calcium in a one-cup serving of milk [38]. As Asians adopt more American-type diets, their incidence of osteoporosis would be expected to increase, as their consumption of high-quality calcium foods decreased.

More studies need to be done in black and Asian populations to determine how calcium metabolism and bone formation/resorption differ from those of whites on different dietary calcium intakes. This is essential in order to evaluate the appropriateness of the new AI for calcium for these populations.

	CONCLUSION

TOP ABSTRACT DIETARY REFERENCE INTAKES THE CALCIUM REQUIREMENT ACHIEVING ADEQUATE CALCIUM... ROLE OF GENETICS RACIAL DIFFERENCES CONCLUSION REFERENCES

 
Adequate calcium intake plays an important role in calcium retention, which dictates bone mass and, therefore, skeletal fragility. It is important to educate Americans that current calcium intakes fall far short of the recently established calcium requirements for most age groups. Many questions remain unanswered in our understanding of the requirements for calcium in individuals of different races and cultural environments. Comparative studies on calcium and bone metabolism with a sufficient sample size to have the necessary statistical power to detect differences are essential to expanding our knowledge and determining the adequacy of the AIs when considering blacks, Hispanics and Asians.

Physicians can exert significant influence in making patients aware of the new DRIs for calcium by asking questions regarding calcium intake. Health-care professionals are in a position to promote adequate nutrients, participation in physical activity, hormone therapy and alternate calcium sources, when necessary, for individuals who are unwilling or unable to consume adequate calcium. Patients who consume inadequate calcium, are not physically-active and/or are not receiving hormone therapy after menopause are at increased risk for osteoporosis.

Received May 1, 1999.

	REFERENCES

TOP ABSTRACT DIETARY REFERENCE INTAKES THE CALCIUM REQUIREMENT ACHIEVING ADEQUATE CALCIUM... ROLE OF GENETICS RACIAL DIFFERENCES CONCLUSION REFERENCES

 

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