UNLABELLED We evaluated the effects of low calcium in the diets of young adolescent girls. We measured calcium absorption and excretion using stable isotopes. We found partial adaptation to low intakes but a persistent large deficit relative to recommended intakes. Low calcium intakes pose a substantial risk of inadequate calcium retention. INTRODUCTION A substantial number of adolescent girls in the United States have habitual calcium intakes <500 mg/day (about 40% of the current recommended intake). The ability to adapt to these very low intakes by increasing calcium absorption and decreasing calcium excretion is not known. We sought to determine the effects of recommended (REC-Ca) versus very low (LO-Ca) calcium intakes on calcium absorption and excretion in white and black girls. MATERIALS AND METHODS Pubertal, but premenarcheal girls, were adapted to low or recommended calcium intakes for at least 2 weeks before each study. Calcium absorption (n = 51) and endogenous fecal calcium excretion (n = 36 of the 51) were determined by dual-tracer stable isotope studies. Subjects were then switched to the other diet for at least 6 weeks, and the study was repeated. RESULTS Calcium intake was 389 +/- 10 mg/day on LO-Ca and 1259 +/- 35 mg/day on REC-Ca diets. Fractional absorption increased from 44.9 +/- 1.9% on REC-Ca to 63.4 +/- 1.7% on LO-Ca (p < 0.01), but the net calcium absorption remained less than one-half the value on LO-Ca as on REC-Ca. Despite decreases in both endogenous fecal calcium excretion and urinary calcium excretion, net calcium balance was much lower on LO-Ca compared with REC-Ca1 (131 +/- 14 versus 349 +/- 32 mg/day, respectively; p < 0.001). We found significantly lower urinary calcium excretion but not calcium absorption in black girls compared with white girls. CONCLUSIONS Very low calcium intakes are only partially adapted to by increased absorption and decreased excretion. Very low calcium intakes place both white and black pubertal girls at substantial risk for inadequate calcium retention.