Effect of caffeine on circadian excretion of urinary calcium and magnesium.
Level 3 - non-randomized controlled study
Non-randomized before-after metabolic study with fixed intervention order
PubMed 7836625 · doi:10.1080/07315724.1994.10718436
What was done
17 healthy male and female volunteers (aged 17–41 years) were studied in a metabolic ward over two consecutive days while consuming a controlled diet (11.3 mmol Ca, 12.7 mmol Mg). Day 1 served as a baseline caffeine-abstinence control. On Day 2, participants consumed two caffeine doses (3 mg/kg lean body mass) at 7:00 AM and 10:00 AM. Researchers measured salivary caffeine concentrations and urinary excretion of calcium, magnesium, sodium, and creatinine across circadian intervals.
What was found
Salivary caffeine peaked at 4.7 µmol/mL at 11:30 AM (half-life 7.3 hours). Urinary calcium and magnesium excretion were significantly elevated during the 6 hours following the second caffeine dose (p = 0.01 and p = 0.04). Excretion was unchanged between 4:00 PM and 1:00 AM, but significantly decreased below baseline between 1:00 AM and 4:00 AM (p = 0.04 for Ca, p = 0.01 for Mg). Creatinine excretion did not change. Nocturnal conservation failed to balance daytime losses, resulting in net 24-hour urinary increases of 0.32 mmol Ca and 0.16 mmol Mg.
Why it matters
This study shows that while the kidneys mount a delayed compensatory response to conserve calcium and magnesium following caffeine-induced excretion, the compensation is incomplete over a 24-hour period.
Limits
Small sample size (n=17). The study used a non-randomized, fixed-order before-after design without a randomized crossover control day. The findings reflect acute effects over 24 hours and do not assess chronic adaptation or bone density outcomes.
Cited by
- context Caffeine intake inhibits the absorption of minerals, particularly magnesium.