STABLE STRONTIUM ABSORPTION AS A MEASURE OF INTESTINAL CALCIUM-ABSORPTION - COMPARISON WITH THE DOUBLE-RADIOTRACER CALCIUM-ABSORPTION TEST

1. Stable strontium (Sr) has been proposed as an alternative to calcium (Ca) isotopes for the measurement of intestinal Ca absorption. The aim of this study was to compare the time course and fractional absorption of Ca and Sr, when both are measured using dual-tracer techniques. 2. Ca-45 and Sr absorption tests were carried out on consecutive days in patients with osteoporosis (n = 10) or chronic renal failure (n = 7). Both tests were repeated in four patients with chronic renal failure after treatment with calcitriol (1 mu g daily for 10 days). 3. The time course of Ca absorption was determined using the Sr-85 (intravenous)/Ca-45 (oral) dual-tracer technique, and the time course of Sr absorption using Sr-85 (intravenous)/stable Sr (oral). Oral tracers were administered on consecutive days with a test meal containing 5.3 mmol of Ca and 2.5 mmol of either stable Sr or Ca carrier. The fractional absorption of Ca-45 and Sr at 6 h (FA(360)) and the absorption rate as a function of time were calculated by deconvolution. 4. The mean FA(360) for Sr (20.2%) was lower than the mean FA(360) for Ca-45 (37.8%, P < 0.001, paired t-test), but the time course of Sr absorption was similar to that of Ca. There was a significant correlation between the FA(360) for Ca-45 and Sr, although the relationship was improved by including a quadratic term (R(2) = 0.89, P < 0.001, significance of quadratic term, P < 0.05). After 1,25-dihydroxyvitamin D treatment, the FA(360) of stable Sr increased 4.29-fold, whereas the FA(360) of Ca-45 increased only 2.4-fold. 5. Although the fractional absorption of Sr determined by dual-tracer deconvolution was the best predictor of FA(360) for Ca-45, little was lost by confining the analysis to a single serum Sr measurement taken 3 h or more after oral administration. 6. We conclude that Sr absorption is qualitatively similar to that of Ca, although absorption of Sr is much lower than that of Ca. Furthermore, the relationship does not appear to be linear. Stable Sr may be useful in place of Ca isotopes in the routine clinical evaluation of Ca absorption.