THE NATURE AND SIGNIFICANCE OF THE RELATIONSHIP BETWEEN URINARY SODIUM AND URINARY CALCIUM IN WOMEN

Orally or parenterally administered sodium is known to increase urinar y calcium in experimental animals and humans, and there is well-docume nted correlation between urinary sodium and calcium in 24-h urine coll ections from normal subjects and renal stone formers. The correlation between urinary sodium and calcium is generally sodium driven, i.e., i t is the sodium load that influences urinary calcium rather than vice versa, but the converse may also occur, as after an oral calcium load or in hypercalcemia. When sodium is the determinant, 100 mmol of sodiu m takes out approximately 1 mmol of calcium in the urine. When calcium load is the determinant, each millimole of calcium appearing in the u rine is associated with an extra 10-20 mmol of sodium. Sodium-dependen t calcium loss may continue indefinitely, but calcium-dependent natriu resis is self-limiting. There is a significant correlation between cal cium and sodium in fasting urine from both pre- and postmenopausal wom en, but there is more calcium relative to sodium in postmenopausal wom en than in premenopausal women. In postmenopausal but not premenopausa l women, urinary hydroxyproline is also related to obligatory sodium a nd calcium output, and restriction of salt intake lowers not only urin ary sodium but also calcium and hydroxyproline. There is not only an i ncrease in obligatory calcium excretion at the menopause, but also an increase in the fasting urinary sodium, which in turn accounts for som e of the increase in calcium output. This rise in fasting urinary sodi um represents a delay in sodium excretion that may have a significant effect on calcium homeostasis.