NO EFFECTS OF ADAPTATION TO DIETARY CAFFEINE ON CALCIUM EXCRETION IN YOUNG-WOMEN

Seventeen young women aged 24.4±3.8 years abstained from dietary methylxanthines for 3 weeks. Two urine collections were made on day 7 of each week: 1 hour after drinking 120 ml water, then 2 hours after 240 ml decaffeinated beverage. During the control week subjects had no caffeine during the week or day 7. For the "abstention" week no caffeine was consumed during the week then 6 mg caffeine per kg lean body mass (LBM) was consumed on day 7 in the second beverage. During an "adaptation" week subjects consumed 300 mg caffeine daily, then on day 7, 6 mg caffeine/kg LBM was consumed. After both abstention and adaptation weeks, addition of caffeine significantly increased excretion from 2.9 to 6.7 mg Ca/hr (p<.0001). Caffeine similarly affected urinary magnesium (p<.0001) and sodium (p<.005). Basal fasting excretion rates for the hour before the second beverage were similar for each mineral for all periods. There were no differences for post-caffeine urinary calcium, magnesium and sodium excretion rates between caffeine-naive and caffeine-adapted weeks. Young women chronically consuming 300 mg caffeine/day continued to excrete increased calcium, magnesium and sodium after a caffeine challenge when compared to a caffeine-free week. Thus, there is no indication of conservation of urinary calcium excretion with continuing caffeine use after short-term adaptation. © 1990 Pergamon Press plc.