Note: The regression line crosses the line of equality at an intake of 640 mg. The equation is: Cao = 0.779 Cai + 142 mg where Cao is calcium output.
Matkovic, V. & Heaney, R.P. 1992. Calcium balance during Human growth: evidence for threshold behavior. Am. J. Clin. Nutr., 55: 992-996.
An alternative way of calculating calcium requirement is to determine the intake at which the mean maximum positive balance occurs. This has been done with a two component, split, linear regression model in which calcium balance is regressed on intake to determine the threshold intake above which no further increase in calcium retention occurs.
This may well be an appropriate way of calculating the calcium requirement of children and adolescents (and perhaps pregnant and lactating women) who need to be in positive calcium balance and in whom the difference between calcium intake and output is therefore relatively large and measurable by the balance technique.
However, in normal adults the difference between calcium intake and output at high calcium intakes represents a very small difference between two large numbers, and this calculation therefore carries too great an error to calculate their requirement.
Charles, P., Taagehøj., F., Jensen, L., Mosekilde, L. & Hansen, H.H. 1983. Calcium metabolism evaluated by Ca45 kinetics: estimation of dermal calcium loss. Clin. Sci., 65: 415-422.
Hasling, C., Charles, P., Taagehøj., J. & Mosekilde, L. 1990. Calcium metabolism in postmenopausal osteoporosis: the influence of dietary calcium and net absorbed calcium. J. Bone Miner. Res., 5: 939 946.
We are inclined to think that the most satisfactory way of calculating calcium requirement from current data is as the intake at which excreted calcium equals net absorbed calcium, which has the advantage of permitting separate analysis of the effects of changes in calcium absorption and excretion. This intercept has been shown in Figure 14 to occur at an intake of about 520 mg, but when insensible losses of calcium of 60 mg (1.5 mmol)
Nordin, B.E.C., Need, A.G., Morris, H.A. & Horowitz, M. 1999. Biochemical variables in pre- and postmenopausal women: reconciling the calcium and estrogen hypotheses. Osteoporos. Int., 9: 351-357.
are taken into account, the intercept rises to 840 mg, which we believe is as close as it is possible to get at present to the calcium requirement of adults on Western-style diets. The addition to this excretion line of an additional obligatory urinary calcium of 30 mg (0.75mmol) at menopause raises the amount to about 1100 mg, which we suggest is the mean calcium requirement of postmenopausal women.
However, this type of calculation cannot easily be applied to other high-risk populations (such as children) because there are not sufficient published data from these groups to permit a similar analysis of the relationship among calcium intake, absorption, and excretion. An alternative is to estimate how much calcium each population group needs to absorb to meet obligatory calcium losses and desirable calcium retention and then to calculate the intake required to provide this rate of calcium absorption. This is what has been done in the following section.
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