Obese people are more likely to develop kidney stones than normal weight individuals, but severe obesity doesn’t seem to further increase risk, research in the Journal of Urology shows.

Kidney stones are solidified deposits of minerals and acid salts. “Passing” the stones in the urine can be extremely painful. Rarely, a person will require surgery or other types of treatment if they are unable to pass the stone on their own.

Studies have shown a rise in kidney stone disease paralleling the increase in obesity, Dr. Brian R. Matlaga of Johns Hopkins University in Baltimore and his colleagues write. It’s unclear why weight would affect kidney stone rates, although researchers have a number of theories involving different levels of substances in the blood among the obese.

Matlaga’s team analyzed 2002-2006 records for more than 95,000 people included in a national private insurance database.

The researchers identified 3,257 people with kidney stones. Men were twice as likely as women to have kidney stones, while the risk of stones increased with age.

They used the body mass index (BMI) - a standard measure used to gauge how fat or thin a person is - to classify patients’ weight. (You can use http://www.health.am/calculator/1.php to calculate your own BMI.)

Among people classified as either normal weight (BMI 18.5-24.9) or overweight (BMI 25-29.9) but not obese, about one in 40 had been diagnosed with kidney stones, compared to about one in 20 of those classified as obese (BMI above 30). But there was no significant difference in risk among obese individuals, no matter how heavy they were.

Those who were obese were more likely than the non-obese to undergo kidney stone removal, but those who were severely obese were no more likely to undergo these procedures than their normal weight peers.

It’s possible, the researchers note, that the heaviest people were more likely to have their kidney stones treated with medication than surgery.

“Dietary modification and weight loss should be encouraged in the obese population for a multitude of reasons but also to reduce stone risk,” they conclude.

SOURCE: The Journal of Urology, February 2010.