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Obesity ups risk of kidney stones

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

The risk was higher in women than in men, according to a study published in the Jan. 26 issue of the Journal of the American Medical Association.

"In terms of kidney stone risk, 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 about it [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 appeared in July in the Journal of Urology.

The problem of kidney stones is widespread, with about 10 percent of men and 5 percent of women in the United States developing this painful malady during their lifetime. The annual cost to society is about $2 billion.

Previous research had shown an association between body mass index (BMI) and the risk of developing kidney stones in older women. However, this relation was unclear in men and had never been studied in younger women. Nor had any studies been done on a 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 begun in 1976 and the Nurses` Health Study begun in 1989.

After adjusting for other factors, they found that men weighing more than 220 pounds had a 44 percent increased risk of developing kidney stones compared with men weighing less than 150 pounds. Older women in the same high-weight category had an 89 percent increased risk for kidney stones, while heavy younger women had a 92 percent 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 percent increased risk compared with men in the lowest category. Older women in the highest category had a 90 percent increased rate and younger women more than double the risk. Women with the largest waist circumferences had a 71 percent greater risk of kidney stones compared with women with the lowest waist circumferences. Men had a 48 percent greater risk.

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

Scientists had 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 a growing (weight-wise) society, may have an effect on urine composition.

"Insulin resistance is a big problem in obesity and can have pretty dramatic effects on urine composition and effects that can actually cause the growth of kidney stones," Taylor said.