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 calciu m (Ca) isotopes for the measurement of intestinal Ca absorption. The a im of this study was to compare the time course and fractional absorpt ion 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 failu re after treatment with calcitriol (1 mu g daily for 10 days). 3. The time course of Ca absorption was determined using the Sr-85 (intraveno us)/Ca-45 (oral) dual-tracer technique, and the time course of Sr abso rption using Sr-85 (intravenous)/stable Sr (oral). Oral tracers were a dministered on consecutive days with a test meal containing 5.3 mmol o f Ca and 2.5 mmol of either stable Sr or Ca carrier. The fractional ab sorption 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 simil ar to that of Ca. There was a significant correlation between the FA(3 60) for Ca-45 and Sr, although the relationship was improved by includ ing a quadratic term (R(2) = 0.89, P < 0.001, significance of quadrati c 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 incre ased only 2.4-fold. 5. Although the fractional absorption of Sr determ ined by dual-tracer deconvolution was the best predictor of FA(360) fo r Ca-45, little was lost by confining the analysis to a single serum S r measurement taken 3 h or more after oral administration. 6. We concl ude that Sr absorption is qualitatively similar to that of Ca, althoug h absorption of Sr is much lower than that of Ca. Furthermore, the rel ationship does not appear to be linear. Stable Sr may be useful in pla ce of Ca isotopes in the routine clinical evaluation of Ca absorption.