Use of oral contraceptives blunts the calciuric effect of caffeine in young adult women.

Caffeine consumption increases the urinary excretion of calcium and other minerals. Factors that affect caffeine metabolism such as steroid hormones may modify this effect. The purpose of this study was to evaluate the influence of oral contraceptive (OC) use on the 4-h urinary excretion of calcium, phosphorus, magnesium, zinc, sodium, potassium and caffeine metabolites in response to a high caffeine dose given as coffee beverage. Adult women, 20-29 y, users (+OC, n = 15) and nonusers (-OC, n = 15) of oral contraceptives, with calcium intake approximately 500 mg/d, participated in two tests, caffeine load (5 mg/kg body weight) and no-caffeine control, in a randomized crossover design. The net increase (caffeine load corrected by no caffeine) in urinary excretion of most minerals was significantly higher in -OC than in +OC (P < 0.05), with the larger group difference for calcium (ninefold) followed by magnesium (twofold), zinc (onefold) and potassium (onefold). Net increases in urinary excretion of 1-methylurate and paraxanthine were about three- and fivefold higher, respectively, in -OC than in +OC (P < 0.05) whereas net increases in urinary excretion of 5-acetylamino-6-formylamino-3-methyluracil (AFMU) and 1,7-dimethylurate were over twofold higher in the +OC than in -OC (P < 0.05). Following the caffeine load, most urinary minerals showed negative correlation with urinary 1-methylurate in -OC (R </= -0.78, P < 0.01), and with urinary AFMU and 1,7-dimethylurate in +OC (R </= -0.84, P < 0.01). Oral contraceptives appear to limit the renal effect of caffeine on mineral excretion possibly by reducing paraxanthine excretion, the most active caffeine metabolite.