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The effect of different intakes of salt for 12 mo on bone calcium content and urinary excretion of calcium and hydroxyproline were examined in sham operated and oophorectomized (OX) rats to determine the long term effects of high sodium intake and its interaction with estrogen deficiency. Sham operated (n = 24) and OX (n = 24) rats were divided into groups of six rats in a 2 x 4 design. One group of sham and one of OX rats were given 0, 2, 6 or 18 g/L sodium chloride to drink. Urine samples were collected at 0, 2, 4, 6, 10 and 12 mo for the measurement of sodium, calcium, creatinine and hydroxyproline. At the end of 12 mo, blood was taken for measurement of calcium, albumin, alkaline phosphatase and creatinine and the left femur was removed and analyzed for calcium and phosphate. Body weights of the OX rats were higher than the sham operated controls. At the start of the experiment (10 d after OX) urinary excretions of calcium and hydroxyproline were significantly higher in OX rats. However, after 4-6 mo, they were significantly lower in OX rats. Calcium excretion and hydroxyproline excretion were increased by high salt intake, and there was a significant correlation between sodium and calcium excretion (r = 0.962). Bone calcium content of OX rats was lower than their corresponding sham-operated controls. Sodium intake also had a significant effect on bone calcium content. Multiple regression analysis showed that OX and sodium intake explained 7.6% and 1.5% of the variation in bone calcium content. We conclude that high sodium intake causes increased loss of calcium and reduces bone calcium content in sham-operated as well as OX rats.

The effect of feeding high protein (40%) and high salt intake (18 gm/L of sodium chloride in drinking water) for 4 months on the urinary excretion of calcium and hydroxyproline (OHP) in sham operated and oophorectomized (OX) rats was studied. OX rats had higher body weight and increased excretion of calcium and OHP. Feeding a high-salt diet caused an increase in the excretion of calcium, phosphate, and OHP. The high-protein diet had a similar effect. There was a significant interaction between the effects of salt and protein on calcium excretion. OX did not influence the effect of high salt or high protein intakes. It is concluded that high protein and/or high salt intake increases the excretion of calcium and OHP. The effect is seen even after 4 months of the diet. Oophorectomy did not cause any additional effect.

The effects of different amounts of salt intake on urinary calcium and hydroxyproline (OHP) excretion were studied in oophorectomized (O) and sham-operated (S) rats on normal calcium diet. Groups of O and S rats were given either tap water or 2, 6, or 18 g/liter of NaCl to drink. Urinary excretion of sodium, calcium, and OHP was monitored at 0, 2, and 4 months. Urinary excretion of calcium and OHP increased with increasing salt intake (P < 0.001 by ANOVA) in S and O groups. Oophorectomy did not have any additional effect on calcium or OHP excretion. We conclude that increased salt intake causes increased excretion of calcium and OHP and oophorectomy does not further aggravate the loss.

Potassium depletion increases and potassium supplementation decreases urinary calcium excretion. To determine whether these experimental observations have any significance in healthy free living subjects, we measured 24 h urinary calcium, sodium, potassium and serum parathyroid hormone (PTH) in 142 healthy young (aged 18-40 years) female Chinese subjects. Sodium excretion and potassium excretion were positively correlated with calcium excretion (r = 0.494, p < 0.0001 and r = 0.181, p = 0.031, respectively). However stepwise multiple regression analysis showed that only urinary sodium excretion contributed to the excretion of calcium. We conclude that in free living subjects potassium intake has little effect on calcium excretion.

The effect of sodium intake on the excretion of calcium, hydroxyproline (OHP) and serum parathyroid hormone (PTH) was examined. When seven healthy males increased their sodium intake by 200 mmol/day, excretion of calcium increased by 1.70 mmol/day and that of OHP by 27%. Serum PTH, however, did not increase significantly. In a cross-sectional study of 334 healthy female subjects there were strong positive correlations between fasting urinary sodium/creatinine (Na/Cr) and calcium/creatinine (Ca/Cr) ratios (r = 0.573) and between Na/Cr and urinary OHP/Cr ratios (r = 0.246). Serum PTH was negatively correlated with Na/Cr ratio (r = -0.161). We conclude that an increase in dietary sodium causes calciuria and an increase in hydroxyproline excretion. The results do not support the hypothesis that this is mediated by PTH.

Objective: To study the age-related changes in bone density, serum parathyroid hormone, calcium absorption and other indices of calcium metabolism in Chinese women who habitually have a low calcium intake., Design: Cross-sectional study., Subjects: One hundred and fifty-six healthy Chinese women aged 20-83 years. None were on any medication or vitamin supplements. Subjects over the age of 60 years were all living in a hostel; younger subjects were nurses or subjects attending a family clinic for minor illnesses., Measurements: Fasting blood and urine samples were collected for biochemical measurements and calcium absorption was measured using 45Ca by the method of Marshall and Nordin. Bone density was measured by dual energy X-ray densitometry (Norland X R20 X-ray bone densitometer) at the left hip and lumbar spine. Serum parathyroid hormone was measured by a chemiluminometric assay., Results: Plasma ionized calcium concentration, alkaline phosphatase, bicarbonate, plasma creatinine and serum B2 microglobulin were significantly higher in the elderly than in the young, whereas plasma phosphate and the anion gap were higher in the young. Urinary excretion of calcium, phosphate and hydroxyproline were all higher in older women. Plasma parathyroid hormone concentration was positively correlated with age even after taking into account the decline in renal function (as indicated by the rise in B2 microglobulin (r = 0.506, P less than 0.001). Serum concentrations of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D were not lower in the older women. Fractional calcium absorption from an oral load of radiolabelled calcium was significantly lower in the older women and 37% of the older women were below the 2.5 percentile found in the younger women. Bone density measured by dual energy X-ray densitometry was also significantly lower in the elderly., Conclusion: In Chinese women there is an increase in PTH and a decrease in calcium absorption with age in spite of the presence of normal vitamin D metabolites.