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TABLE 2-1 1997 DRI Factorial Approach for Determining Calcium Requirements During Peak Calcium Accretion in White Adolescents

 

Number of Observations

Female Calcium Requirements (mg/day)

Number of Observations

Male Calcium Requirements (mg/day)

Peak calcium accretion

507

212a

471

282a

Urinary losses

28

106b

14

127c

Endogenous fecal calcium

14

112d

3

108e

Sweat Losses

 

55f

 

55f

Total

 

485

 

572

Total adjusted for absorptiong

 

1,276

 

1,505

aMartin et al. (1997) using peak BMC velocity.

bGreger et al. (1978); Weaver et al. (1995).

cMatkovic (1991).

dWastney et al. (1996) for mean age 13 years on calcium intakes of 1,330 mg/day.

eAbrams et al. (1992).

fTaken from Peacock (1991) who adjusted the adult data of Charles et al. (1983) for body weight.

gAbsorption is 38% for mean age 13 years on calcium intakes of 1,330 mg/day (Wastney et al., 1996).

a better snapshot of long-term calcium intake than does the combination of accretion/retention data.

In children, change in BMC is a useful indicator of calcium retention; change in BMD is less suitable, because it overestimates mineral content as a result of changes in skeletal size from growth (IOM, 1997). In adults, with their generally stable skeletal size, changes in either BMD or BMC are useful measures. In the context of longitudinal calcium intervention trials that measure change in BMC, the measures can provide data on the long-term impact of calcium intake not only on the total skeleton, but also on skeletal sites that are subject to osteoporotic fracture (IOM, 1997). However, because DXA does not distinguish between calcium that is within bone and calcium on the surface (e.g., osteophytes, calcifications in other tissues) or within blood vessels (e.g., calcified aorta), an increase in BMC or BMD, particularly in the spine, may result in false positive readings suggesting high bone mass (Banks et al., 1994).

In DXA, fan beam dual-energy X-ray beams are used to measure bone mass, with correction for overlying soft tissue. Data are converted to BMC and the area represented is measured. The BMD measurement is annotated in grams of mineral per square centimeter. BMC represents the amount of mineral in a volume of bone without consideration of total body



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