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Scientists have uncovered yet another reason to maintain a healthy weight. Obesity and weight gain are linked to an increased risk of developing kidney stones.

According to a study published in the Journal of the American Medical Association, the risk was higher in women than in men. "In order to reduce the risk of developing kidney stones, people should really maintain a healthy weight," said study author Dr. Eric N. Taylor, a nephrology expert at Brigham and Women's Hospital in Boston. "There's no question that kidney stones are linked to obesity," said Dr. Glenn Preminger (a professor of urologic surgery at Duke University and director of the Duke Comprehensive Kidney Stone Center). Preminger co-authored a study with similar findings that appear in the Journal of Urology.

Kidney stones are a widespread condition, with about 10% of men and 5% of women in the United States developing this painful malady during their lifetime. The annual cost to society is about $2 billion. Previous research demonstrated an association between body mass index (BMI) and the risk of developing kidney stones in older women. However, this relationship was unclear in men and had never been studied in younger women. There were no studies on a possible link between weight gain and kidney stones.

Taylor and his colleagues tracked 250,000 medical professionals with no history of kidney stones who were participating in three large groups: The Health Professionals Follow-up Study, the Nurses' Health Study in 1976, and the Nurses' Health Study in 1989. After adjusting for other factors, men weighing more than 220 pounds had a 44% increased risk of developing kidney stones compared to men weighing less than 150 pounds. Older women in the same height-weight category had an 89% increased risk for kidney stones, while heavy younger women had a 92% increased risk.

Body mass index (BMI) was also associated with an increased risk of kidney stones. Men in the highest category of BMI had a 33% higher risk compared with men in the lowest category. Older women in the highest category had a 90% higher rate and younger women more than double the risk. Women with the largest waist circumferences had a 71% greater risk of kidney stones compared to women with the lowest waist circumferences. Men had a 48% greater risk.

Men who had gained more than 35 pounds since they were 21 had a 39% higher risk of getting stones compared with men whose weight remained stable. Similarly, older women who had gained more than 35 pounds since they were 18 had a 70% increased risk while younger women had an 82% increased risk.

Scientists wondered if higher lean body mass as opposed to higher fat or adiposity was the critical link with kidney stones. "Given that most weight gain in adulthood is due to increases in fat, rather than muscle or bone, the relation between weight gain and increased risk suggests that fat tissue plays an important role in kidney stone formation," Taylor said. “At the same time,” he added, “this study did not rule out a role for lean body mass.”

It's not clear what the underlying biological mechanism is, but recent research has suggested that insulin resistance (a growing problem in an increasingly weight-wise society) may have an effect on urine composition. "Insulin resistance is a big problem with obese persons. It can have pretty dramatic effects on urine composition and effects that can actually cause the formation of kidney stones," Taylor said.

SOURCES: Eric N. Taylor, M.D., clinical and research fellow, nephrology, Brigham and Women's Hospital, Boston; Glenn Preminger, M.D., professor, urologic surgery, Duke University, and director, Duke Comprehensive Kidney Stone Center, Durham, N.C.; Jan. 26, 2005, Journal of the American Medical Association.

*HealthDay Reporter