The effect of salt restriction on urine calcium and hydroxyproline in postmenopausal women

Abstract

Fasting urine sodium (Na/Cr), calcium (Ca/Cr) (molar ratios) and hydroxyproline (OHPr/Cr) (molar ratio x 1000) are interrelated in postmenopausal women. In order to establish whether urine sodium was contributing to bone resorption by taking calcium out in the urine, 31 normal postmenopausal women with Na/Cr values over 10 were asked to restrict their salt intake to less than 80 mmol/day for 7 days prior to remeasurement of the three relevant variables in a fasting urine sample. Mean Na/Cr fell from 18.1 ± 0.93 to 7.6 ± 0.77 (p<0.001) Ca/Cr from 0.27 ± 0.021 to 0.23 ± 0.022 (NS) and OHPr/Cr from 19.1 ± 0.83 to 17.0 ± 1.0 (p<0.01). Na/Cr fell in all but one subject, Ca/Cr in 21 out of 31 and OHPr/Cr in 25 out of 31. In the 20 subjects in whom Na/Cr fell by more than 5, Na/Cr fell from 19.8 ± 1.1 to 5.4 ± 0.6, Ca/Cr from 0.27 ± 0.026 to 0.18 ± 0.017 (p<0.001) and OHPr/Cr from 19.4 ± 1.2 to 15.9 ± 1.3 (p<0.001). This fall in hydroxy-proline was compatible with that predicted from the relation between hydroxyproline and sodium excretion in cross-sectional studies; the final value approached the premenopausal mean of 14.2 ± 0.83. We conclude that a substantial part of the high bone resorption in postmenopausal women represents a response to the sodium-dependent component of obligatory urine calcium. Fasting urine sodium (Na/Cr), calcium (Ca/Cr) (molar ratios) and hydroxyproline (OHPr/Cr) (molar ratio x 1000) are interrelated in postmenopausal women. In order to establish whether urine sodium was contributing to bone resorption by taking calcium out in the urine, 31 normal postmenopausal women with Na/Cr values over 10 were asked to restrict their salt intake to less than 80 mmol/day for 7 days prior to remeasurement of the three relevant variables in a fasting urine sample. Mean Na/Cr fell from 18.1 ± 0.93 to 7.6 ± 0.77 (p<0.001) Ca/Cr from 0.27 ± 0.021 to 0.23 ± 0.022 (NS) and OHPr/Cr from 19.1 ± 0.83 to 17.0 ± 1.0 (p<0.01). Na/Cr fell in all but one subject, Ca/Cr in 21 out of 31 and OHPr/Cr in 25 out of 31. In the 20 subjects in whom Na/Cr fell by more than 5, Na/Cr fell from 19.8 ± 1.1 to 5.4 ± 0.6, Ca/Cr from 0.27 ± 0.026 to 0.18 ± 0.017 (p<0.001) and OHPr/Cr from 19.4 ± 1.2 to 15.9 ± 1.3 (p<0.001). This fall in hydroxy-proline was compatible with that predicted from the relation between hydroxyproline and sodium excretion in cross-sectional studies; the final value approached the premenopausal mean of 14.2 ± 0.83. We conclude that a substantial part of the high bone resorption in postmenopausal women represents a response to the sodium-dependent component of obligatory urine calcium.