Effect of caffeine on circadian excretion of urinary calcium and magnesium

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Journal of the American College of Nutrition, Vol 13, Issue 5 467-472,

1994 by American College of Nutrition

_Effect of caffeine on circadian excretion of urinary calcium and magnesium

-- Kynast-Gales and Massey 13 (5): 467 -- Journal .._

(http://www.jacn.org/cgi/content/abstract/13/5/467)

Effect of caffeine on circadian excretion of urinary calcium and magnesium

S. A. Kynast-Gales and L. K. Massey

Washington State University, Spokane 99204-0399.

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. DESIGN: 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.