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The Nature and Significance of the Relationship between Urinary Sodium and Urinary Calcium in Women^1,2,3,

B. E. Christopher Nordin^*,,4, Allan G. Need^*, Howard A. Morris^* and Michael Horowitz

^* Division of Clinical Chemistry, Institute of Medical and Veterinary Science, Adelaide 5000, South Australia Department of Pathology Department of Medicine, University of Adelaide, Adelaide 5000, South Australia

Orally or parenterally administered sodium is known to increase urinary calcium in experimental animals and humans, and there is well-documented correlation between urinary sodium and calcium in 24-h urine collections from normal subjects and renal stone formers. The correlation between urinary sodium and calcium is generally sodium driven, i.e., it 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 sodium takes out 1 mmol of calcium in the urine. When calcium load is the determinant, each millimole of calcium appearing in the urine is associated with an extra 10–20 mmol of sodium. Sodium-dependent calcium loss may continue indefinitely, but calcium-dependent natriuresis is self-limiting. There is a significant correlation between calcium and sodium in fasting urine from both pre- and postmenopausal women, but there is more calcium relative to sodium in postmenopausal women than in premenopausal women. In postmenopausal but not premenopausal women, urinary hydroxyproline is also related to obligatory sodium and calcium output, and restriction of salt intake lowers not only urinary sodium but also calcium and hydroxyproline. There is not only an increase in obligatory calcium excretion at the menopause, but also an increase in the fasting urinary sodium, which in turn accounts for some of the increase in calcium output. This rise in fasting urinary sodium represents a delay in sodium excretion that may have a significant effect on calcium homeostasis.

KEY WORDS: • urinary sodium • urinary calcium • menopause • osteoporosis

¹ Presented as part of the third Annual Workshop of Nutrition and Bone Health Group given at the Annual Meeting of the American Society for Bone and Mineral Research, October 3, 1992, Minneapolis, MN. Financial support for the workshop was provided by a grant from the National Dairy Council, Rosemont, IL. Guest editor for this supplement was L. K. Massey, Department of Food Science and Human Nutrition, Washington State University, Spokane, WA.

² Portions of this article were published previously in the Proceedings of a French Dairy Corporation Symposium: Osteoporose. Pour une prevention nutritionnelle du risque? CERIN, 1992. (Centre de Recherche et d'Information Nutritionnelles, 27 rue de la Procession, 25015 Paris, France.)

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ To whom correspondence should be addressed.

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