Calcium Requirements and Recommended Intakes

Methodology

 FAO/WHO verbatim.

Although it is well established that calcium deficiency causes osteoporosis in experimental animals, the contribution that calcium deficiency makes to osteoporosis in humans is much more controversial, not least because of the great variation in calcium intakes across the world (Table 30), which does not appear to be associated with any corresponding variation in the prevalence of osteoporosis. This issue is dealt with at greater length below in the section on nutritional factors; in this section we will simply define what is meant by calcium requirement and how it may be calculated.

The calcium requirement of an adult is generally recognised to be the intake required to maintain calcium balance and therefore skeletal integrity. The mean calcium requirement of adults is therefore the mean intake at which intake and output are equal, which at present can only be determined by balance studies conducted with sufficient care and over a sufficiently long period to ensure reasonable accuracy and then corrected for insensible losses. The reputation of the balance technique has been harmed by a few studies with in adequate equilibration times and short collection periods, but this should not be allowed to detract from the value of the meticulous work of those who have collected faecal and urinary samples for weeks or months from subjects on well-defined diets. This meticulous work has produced valuable balance data, which are clearly valid; the mean duration of the balances in the 210 studies from eight publications used in this report was 90 days with a range of 6–480 days. (The four 6-day balances in the series used a non-absorbable marker and are therefore acceptable.)

Mitchell, H.H. & Curzon, E.G. 1939. The dietary requirements of calcium and its significance. Actualites Scientifique et Industrielles No. 771. p.36-101. Paris: Hermann.

The usual way of determining mean calcium requirement from balance studies has been by linear regression of calcium output (or calcium balance) on intake and calculation of the mean intake at which intake and output are equal (or balance is zero). This was probably first done in 1939 by Mitchell and Curzon,

Hegsted, J.M., Moscoso, I. & Collazos, C.H.C. 1952. Study of minimum calcium requirements by adult men. J. Nutr., 46:181-201.

who arrived at a mean requirement of 9.8mg/kg/day or about 640 mg (16 mmol) at a mean body weight of 65 kg. The same type of calculation was subsequently used by many other workers who arrived at requirements ranging from 200 mg/day (5 mmol/day) in male Peruvian prisoners 

Nordin, B.E.C. & Marshall, D.H. 1988. Dietary requirements for calcium. In: Calcium in Human Biology. Nordin, B.E.C., ed. p. 447-471. Berlin: Springer-Verlag,.

Heaney, R.P., Recker, R.R. & Saville, P.D. 1978. Menopausal changes in calcium balance performance. J. Lab. Clin. Med., 92: 953-963.

to 990 mg (24.75mmol) in premenopausal women, but most values were about 600 mg (15 mmol) without allowing for insensible losses. However, this type of simple linear regression yields ahigher mean calcium requirement (640 mg in the same 210 balances) (Figure 16a) than the intercept of absorbed and excreted calcium (520 mg) (Figure 14) because it tends to underestimate the negative calcium balance at low intake and overestimate the positive balance at high intake. 

A better reflection of biological reality is obtained by deriving calcium output from the functions given in the previous section and then regressing that output on calcium intake. This yields the result shown in Figure 16b where the negative balance is more severe at low intakes and less positive at high intakes than in the linear model and in which zero balance occurs at 520 mg as in Figure 14.