THE RESPONSE TO CALCITRIOL THERAPY IN POSTMENOPAUSAL OSTEOPOROTIC WOMEN IS A FUNCTION OF INITIAL CALCIUM ABSORPTIVE STATUS

Calcitriol is used in the treatment of osteoporosis but the indication s for its use have not been clearly defined. Because it stimulates cal cium absorption, we have tended to select osteoporotic patients with l ow calcium absorption for this therapy and now report the results. We measured the hourly fractional rate of calcium absorption (alpha) with Ca-45 and fasting urinary calcium/creatinine (Ca/Cr) and hydroxyproli ne/creatinine (OHPr/Cr) in 103 postmenopausal women aged 68 (0.67SE) y ears with vertebral compression fractures (77) or forearm or vertebral bone density below the young normal range (26). They were given 0.25 mu g daily of calcitriol (Rocaltrol, Poche, Basle, Switzerland) with a 1 g calcium supplement daily for 6-12 weeks, when the biochemical tes ts were repeated. Initial OHPr/Cr was inversely related to initial alp ha (P = 0.001) and positively to initial Ca/Cr (P < 0.001). alpha rose on therapy from 0.47 (0.018) to 0.59 (0.018) per hour (P < 0.001) and OHPr/Cr fell in the whole group from 19.1 (0.83) to 13.8 (0.58) (P < 0.001). The change in alpha on therapy (corrected for the ''regression to the mean effect'') was inversely related to initial alpha (P < 0.0 01) as was the change in OHPr/Cr (P = 0.001), There was no relationshi p, however, between initial Ca/Cr and either the rise in a or the fall in OHPr/Cr on therapy. The data support the concept that low calcium absorption is a cause of negative calcium balance in postmenopausal os teoporosis and that the effectiveness of calcitriol therapy is inverse ly related to the initial rate of calcium absorption.