Urinary Calcium

MacFadyen, I.J., Nordin, B.E.C., Smith, D.A., Wayne, D.J. & Rae, S.L. 1965. Effect of variation in dietary calcium on plasma concentration and urinary excretion of calcium. Br. Med. J., 1: 161-164.

Urinary calcium

Urinary calcium is the fraction of the filtered plasma water calcium, which is not reabsorbed in the renal tubules. At a normal glomerular filtration rate of 120 ml/min and ultrafiltrable calcium of 6.4 mg/100 ml (1.60 mmol/l), the filtered load of calcium is about 8mg/min (0.20 mmol/min) or 11.6 g/day (290 mmol/day).

Because the usual 24-hour calcium excretion in developed countries is about 160–200 mg (4–5 mmol), it follows that 98–99 percent of the filtered calcium is usually reabsorbed in the renal tubules. However, calcium excretion is extremely sensitive to changes in filtered load. A decrease in plasma water calcium of only 0.17 mg/100 ml (0.043 mmol/l), which is barely detectable, was sufficient to account for a decrease in urinary calcium of 63 mg (1.51 mmol) when 27 subjects changed from a normal- to a low-calcium diet.

This very sensitive renal response to calcium deprivation combines with the inverse relationship between calcium intake and absorption to stabilise the plasma ionised calcium concentration and to preserve the equilibrium between calcium entering and leaving the ECF over a wide range of calcium intakes.

Marshall, D.H., Nordin, B.E.C. & Speed, R. 1976. Calcium, phosphorus and magnesium requirement. Proc. Nutr. Soc., 35: 163-173.

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. & Ryan, R.A. 1999. Urinary calcium in perimenopausal women: normative values. Osteoporos. Int., 9: 13-18.

However, there is always a significant obligatory loss of calcium in the urine (as there is in the faeces), even on a low calcium intake, simply because maintenance of the plasma ionised calcium and,therefore, of the filtered load, prevents total elimination of the calcium from the urine. The lower limit for urinary calcium in developed countries is about 140 mg (3.5 mmol) but depends on protein and salt intakes. From this obligatory minimum, urinary calcium increases on intake with a slope of about 5–10 percent. 

In the graph derived from 210 balance studies referred to above (Figure 14), the relationship between urinary calcium excretion and calcium intake is represented by the line which intersects the absorbed calcium line at an intake of 520 mg.

Insensible losses

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.

Urinary and endogenous faecal calcium are not the only forms of excreted calcium; losses through skin, hair, and nails need to be taken into account. These are not easily measured, but a combined balance and isotope procedure has yielded estimates of daily insensible calcium losses in the range of 40–80 mg (1–2 mmol), which are unrelated to calcium intake. 

The addition of a loss of 60 mg (1.5 mmol) as a constant to urinary calcium loss raises the dietary calcium at which absorbed and excreted calcium reach equilibrium from 520 to 840 mg (13 to 21 mmol) (Figure 14).