Kidney stones form when calcium, oxalate and uric acid crystals precipitate in the tissues of the kidney. This is thought to occur due to too an inability of the kidney to dilute the urine enough to allows these substances to dilute completely and permanently in the urine. Because of this it has been suggested that increased fluid intake may decrease the risk of kidney stones. Both coffee and tea consumption facilitate increased fluid consumption and so may be beneficial at protecting from kidney stones. However, coffee and tea are also diuretics and so theoretically they may exacerbate the precipitation of salts in the kidney in at risk individuals. In addition, some evidence suggests that caffeine intake may be associated with increased calcium excretion and theoretically, this may also increase the risk of kidney stones. Because of the uncertainty of the effects of caffeine beverages on kidney stone formation, researchers have investigated the association between the two variables.
For example, in one study, researchers1 prospectively analysed the association between the intake of caffeine and the incidence of kidney stones in a large group of over 250,000 individuals from three different cohorts of subjects. After adjusting the data to take account of known risk factors, those individuals with the highest caffeine intake had a 26, 29 and 31 % reduced risk of developing kidney stones in the three cohorts. In one of the cohorts, decaffeinated or low caffeinated coffee intake was also inversely associated with kidney stone formation. In a subgroup of subjects, higher caffeine intake was associated with more urine volume, higher calcium excretion and higher potassium excretion. In addition, higher caffeine intake was associated with lower urine oxalate levels and lower supersaturation of calcium oxalate and uric acid. Tea and coffee therefore may dilute urine and reduce the risk of precipitation of salts into the kidney, thereby reducing the risk of kidney stones.
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