Men and women who tip the scale at more than 220 pounds are significantly more likely to develop kidney stones than people who weigh less than 150 pounds, according to new study findings reported Tuesday.

Based on data from nearly 250,000 people collected over almost 50 years, the study’s authors also found that men who gained more than 35 pounds since turning 21 were nearly 40 percent more likely to develop kidney stones. Women who gained the same amount after age 18 had up to an 80 percent increase in risk.

“Our study provides additional evidence that people should maintain a healthy body weight,” study author Dr. Eric N. Taylor of Brigham and Women’s Hospital in Boston told AMN Health. “Kidney stones are common, expensive, and extremely painful.”

Taylor explained that the current study - published in the Journal of the American Medical Association - was not designed to figure out why overweight people develop more kidney stones.

However, he noted that research shows that people who are obese often develop a pre-diabetic problem called insulin resistance. This may cause changes in urine that increase the risk of kidney stones, Taylor said.

Taylor added that during the study, not enough people lost weight to determine whether shedding excess pounds will reduce the risk of kidney stones among obese people. “However, our study indicates that weight loss should be tested as a strategy to prevent kidney stones,” he noted.

Approximately 1 in 10 men and 1 in 20 women will develop a kidney stone over the course of their lives. Each year, more than $2 billion is spent treating this condition, Taylor and his team note.

Kidney stones form as a result of a “complex process,” Taylor noted. Typically, stones form when people accumulate excess amounts of calcium and other substances in their urine, which triggers the formation of crystals.

To investigate whether obesity causes changes in the body that encourage kidney stones, Taylor and his team reviewed data collected from 241,623 men and women as part of 3 different studies, who were followed for a total of 46 years.

The investigators found that increases in body weight, body mass index (a measure of weight that factors in height), and waist size were all associated with a larger risk of kidney stones.

“The fact that higher body mass index, larger waist size and weight gain were all related to increased risk strongly suggests that adiposity, or fat tissue, plays an important role in the development of kidney stones,” Taylor noted.

SOURCE: Journal of the American Medical Association, January 26, 2005.