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Although kidney stones may be expelled from the body, they may require surgical removal.

Sugary beverages are known to contribute to obesity and tooth decay, but there is another reason to kick the soft-drink habit: Daily consumption of sugar-sweetened beverages may increase the risk of kidney stones.

Researchers identified the link in a long-term study involving 194,095 participants, published online this month in the Clinical Journal of the American Society of Nephrology. Study participants who consumed one or more servings of sugar-sweetened, non-cola soft drinks a day had a 33-per-cent higher chance of developing a kidney stone, compared to those who had less than one serving a week. Other sugar-sweetened beverages, such as cola and punch, were also associated with a higher risk.

The increased risk was the same for men and women, said co-author Gary Curhan, a researcher at Boston's Brigham and Women's Hospital and professor at Harvard Medical School. "The results were quite consistent," he said.

In the same study, however, researchers found a lower risk of kidney stones among participants who consumed the most coffee, tea, beer, wine and orange juice.

Sugar in the form of fructose "seems to increase the risk," Curhan said. But sugar content isn't the only factor, he said. Orange juice, for example, has a high fructose content but also contains potassium citrate, which may counteract the effects of fructose, Curhan explained.

An estimated 20 per cent of men and 10 per cent of women will develop a kidney stone at some point in their lives. The "stone" is a solid mass that forms in the kidneys from dietary minerals in the urine. Kidney stones can grow large enough to block the tubes that propel the urine from the kidneys to the bladder. Although kidney stones may be expelled from the body though urination, they may require surgical removal, and often recur.

Doctors routinely advise patients to drink more fluids to prevent kidney stones. But the effect of fluid intake on kidney stones depends on the type of beverage consumed. "It's important to look at individual beverages," Curhan said.

In the study, researchers tracked beverage consumption in three ongoing cohorts, the Health Professionals Follow-Up Study and both the Nurses' Health Study I and II. Participants in all three longitudinal studies provided information about their eating and drinking habits every four years. Beverages listed in a questionnaire included sugar-sweetened and artificially sweetened colas and non-cola soft drinks, coffee, tea, wine, beer, liquor, milk and water. Juices included apple, orange, grapefruit and tomato, as well as "other juice" and "punch" (not defined in the study).

In the analysis, individuals were divided into categories based on the number of servings consumed weekly and daily of each individual beverage.

Consumption of artificially sweetened soft drinks was only marginally associated with kidney stones. But participants who consumed one or more sugar-sweetened servings of cola a day had a 23-per-cent higher risk for kidney stones compared with those who consumed less than one serving a week. Participants who drank at least one serving a day of punch had an 18 per cent higher risk than those who had punch less than once a week.

Fructose has been shown to increase the urinary excretion of calcium, uric acid and oxalate (a type of acid), thus increasing the risk of kidney stones, the authors noted.

In beverages associated with a lower risk of kidney stones, however, the mechanisms involved are less clear. Comparing participants in the highest category of consumption with those in the lowest, researchers found a decreased risk of 41 per cent for beer, 31 to 33 per cent for wine, 26 per cent for coffee, 16 per cent for decaffeinated coffee, 12 per cent for orange juice and 11 per cent for tea.

They theorized that caffeine in coffee and tea may lower the risk of kidney stones by increasing urine production. But that did not explain the reduced risk of stones associated with decaffeinated coffee. Other mechanisms may be involved, the authors wrote, "possibly related to the presence in caffeinated and decaffeinated coffee and tea of phytochemicals with potent antioxidant properties."

Although beer and wine appear to have a protective effect, "we are not advocating that people drink alcohol to prevent kidney stones," Curhan said. Instead, the take-home message of the study is to drink plenty of fluids and "avoid sugar-sweetened beverages," he said.