Effects on Renal Functions

In humans, the administration of a single dose of 4mgkg_1 caffeine increases the urinary excretion of sodium, calcium, magnesium, potassium, chloride, and urine volume. The mechanism of this mild diuresis has been attributed to an increase in renal blood flow, an increased glomerular filtration, and a decrease in tubular reabsorption of sodium ions and other ions. Although these effects appeared more pronounced for a higher acute dose of 10mgkg_1, a review concluded that caffeine consumption stimulates a mild diuresis similar to water. There was no evidence of a fluid-electrolyte imbalance as well as disturbed thermoregulation, and caffeine was not detrimental to exercise performance or health.

Tolerance to the diuretic action of caffeine was demonstrated more than 50 years ago and was shown to develop on chronic caffeine intake so that the clinical significance of hypokalemia and calciuria is difficult to evaluate. Although controversial, some epidemiological studies have implicated caffeine in the increased risk for poor calcium retention. For calcium intakes lower than 750 mg per day, increased rate of bone loss and lower bone density were reported. However, it has been suggested that the effect on bone of high caffeine intake requires a genetic predisposition toward osteoporosis. In individuals who ingest calcium recommended daily allowances, there is no evidence of any effect of caffeine on bone status and calcium economy.

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