EFFECT OF CAFFEINE ON CIRCADIAN EXCRETION OF URINARY CALCIUM AND MAGNESIUM

Objective: To determine if later renal conservation occurs in calcium (Ca) and magnesium (Mg) excretion after morning caffeine-induced incre ases in urinary Ca and Mg excretion. Design: Before-after trial of caf feine abstinance and consumption was conducted on two consecutive days in a metabolic ward while subjects ate a controlled diet containing 1 1.3 mmol Ca and 12.7 mmol Mg. 17 healthy males and females, ages 17-41 yr volunteered. Two caffeine doses of 3 mg/kg lean body mass caffeine were consumed at 7 and 10 a.m. on second day. Salivary caffeine conce ntrations and urinary Ca, Mg, sodium and creatinine excretion were mea sured. Results: Salivary caffeine peaked at 4.7 umol/ml at 11:30 a.m. and declined with a half-life of 7.3 hours. Urinary Ca and Mg were ele vated significantly (p = 0.01 and p = 0.04) for six h after the second caffeine dose. Caffeine had no significant effect on urinary calcium or magnesium excretion between 4 p.m. and 1 a.m. Between 1 and 4 a.m., urinary Ca and Mg excretion was decreased after caffeine (p = 0.04 an d p = 0.01). Creatinine excretion was not different at any time. Concl usions: Nighttime compensatory renal conservation was insufficient to offset morning caffeine-induced mineral losses, resulting in net 24-ho ur urinary increases of 0.32 mmol Ca and 0.16 mmol Mg,