About 10 percent of men and 5 percent of women are afflicted with kidney stones at some time in their life. In the quest for risk factors that might predict who's going to get the painful stones—and what can be done to prevent them—researchers at Harvard took a look at obesity.

What the researchers wanted to know: Does being overweight increase your chances of developing kidney stones?

What they did: The researchers looked at hundreds of thousands of people enrolled in three long-term studies on health that started in the 1970s and 1980s. Two of those studies are following female nurses, and the other follows male health professionals. The researchers looked at the data prospectively; in other words, they looked at people's body size before they were diagnosed with kidney stones (they excluded people who'd already had kidney stones). They considered body type in many ways, by looking at weight, weight change, waist circumference, and body mass index, which is a measure of obesity based on both weight and height.

What they found: Both men and women were more likely to get kidney stones if they weighed more, had a higher body mass index, had gained more weight since early adulthood, or had a larger waist circumference. For example, men who weighed more than 220 pounds were 44 percent more likely to develop kidney stones than men who weighed less than 150 pounds. Women with a BMI of 30 or higher (which is considered obese) were about twice as likely to develop kidney stones as women with a BMI of 21 to 22.9 (in the normal range).

What the study means to you: Being overweight seems to increase the risk for kidney stones. The researchers haven't shown why that happens, but there are biologically plausible ways it could work. (For example, many obese people have high levels of insulin in the blood, which may encourage excreting calcium in the urine; most kidney stones are made up mostly of a calcium compound.)

Caveats: The researchers couldn't tell if losing weight reduced the risk of developing kidney stones.

Find out more: Get explanations of how your kidneys filter blood to make urine and kidney stones from the National Institutes of Health.

Calculate your BMI.

Read the article: Taylor, E.N., Stampfer, M.J. and G.C. Curhan. "Obesity, Weight Gain, and the Risk of Kidney Stones." Journal of the American Medical Association. Jan. 26, 2005, Vol. 293, No. 4, pp. 455-462.

Abstract online: http://jama.ama-assn.org