Background: Urine calcium after an overnight fast is higher in osteoporotic than in normal post-menopausal women. The question is whether this is the cause or effect of the bone-losing state.
Objective: To establish whether the elevated obligatory calcium loss in osteoporotic women is due to a raised filtered load of calcium or to reduced renal tubular reabsorption of calcium.
Design: Covariance analysis using total plasma calcium and its fractions as the covariates.
Patients: Eighty-two untreated post-menopausal women without vertebral compression and 137 untreated post-menopausal with vertebral compression all between the ages of 61 and 75 years.
Measurements: After an overnight fast, calcium, albumin, globulins, anion gap and bicarbonate were measured in the plasma, and calcium, sodium and creatinine in the urine. The calcium fractions in plasma and the calcium/creatinine and sodium/creatinine ratios in urine were calculated. Bone density was measured in the distal forearm.
Results: The ultrafiltrable and ionized calcium in the plasma and the calcium/creatinine ratio in the urine were significantly higher in the women with vertebral compression than in those without. On covariance analysis, neither total plasma calcium nor any of the plasma calcium fractions made a significant contribution to the difference in fasting urine calcium between normal and osteoporotic women, whether bone status was defined by vertebral compression or by bone density.
Conclusions: The increased obligatory calcium loss in osteoporotic women is not due to an increase in the filtered load of calcium and must therefore reflect reduced renal tubular reabsorption. This implies that the calcium loss in the urine is not the result of increased bone resorption but is more likely to be causal.