RENAL LEAK OF CALCIUM IN POSTMENOPAUSAL OSTEOPOROSIS

BACKGROUND Urine calcium after an overnight fast is higher in osteopor otic than in normal post-menopausal women. The question is whether thi s is the cause or effect of the bone-losing state. OBJECTIVE To establ ish whether the elevated obligatory calcium loss in osteoporotic women is due to a raised filtered load of calcium or to reduced renal tubul ar reabsorption of calcium. DESIGN Covariance analysis using total pla sma calcium and its fractions as the covariates. PATIENTS Eighty-two u ntreated post-menopausal women without vertebral compression and 137 u ntreated postmenopausal with vertebral compression all between the age s of 61 and 75 years. MEASUREMENTS After an overnight fast, calcium, a lbumin, globulins, anion gap and bicarbonate were measured in the plas ma, and calcium, sodium and creatinine in the urine. The calcium fract ions in plasma and the calcium/creatinine and sodium/creatinine ratios in urine were calculated. Bone density was measured in the distal for earm. RESULTS The ultrafiltrable and ionized calcium in the plasma and the calcium/creatinine ratio in the urine were significantly higher i n the women with vertebral compression than in those without. On covar iance analysis, neither total plasma calcium nor any of the plasma cal cium fractions made a significant contribution to the difference in fa sting urine calcium between normal and osteoporotic women, whether bon e status was defined by vertebral compression or by bone density. CONC LUSIONS The increased obligatory calcium loss in osteoporotic women is not due to an increase in the filtered load of calcium and must there fore reflect reduced renal tubular reabsorption. This implies that the calcium loss in the urine is not the result of increased bone resorpt ion but is more likely to be causal.