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 increases in urinary Ca and Mg excretion.


METHODS
Before-after trial of caffeine abstinance and consumption was conducted on two consecutive days in a metabolic ward while subjects ate a controlled diet containing 11.3 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 concentrations and urinary Ca, Mg, sodium and creatinine excretion were measured.


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 elevated 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 and p = 0.01). Creatinine excretion was not different at any time.


CONCLUSIONS
Nighttime compensatory renal conservation was insufficient to offset morning caffeine-induced mineral losses, resulting in net 24-hour urinary increases of 0.32 mmol Ca and 0.16 mmol Mg.
Washington State University, Spokane 99204-0399.
S A
SA
Kynast-Gales
L K
LK
Massey
D000293:Adolescent; D000328:Adult; D002110:Caffeine / Q000008:administration & dosage / Q000493:pharmacokinetics / Q000494:pharmacology*; D002118:Calcium / Q000652:urine*; D002940:Circadian Rhythm / Q000187:drug effects*; D003404:Creatinine / Q000652:urine; D005260:Female; D006801:Humans; D008274:Magnesium / Q000652:urine*; D008297:Male; D012463:Saliva / Q000378:metabolism; D012964:Sodium / Q000652:urine
D002110:Caffeine; D012964:Sodium; D003404:Creatinine; D008274:Magnesium; D002118:Calcium