Most gallstones are nuggets of built-up cholesterol. In affluent countries like the United States, gallstones remain the most common abdominal illness. In fact, Americans undergo more than 800,000 gallstone operations each year.

　　In a study led by Dr. Chung-Jyi Tsai, of the division of digestive diseases and nutrition at the University of Kentucky Medical Center, Lexington, researchers collected data on more than 42,000 women participating in the Nurses' Health Study.

　　The women in the study were 39 to 66 years of age in 1986, when the gallstone study began. At that time, none of them had had gallstones. As part of the study, the women provided waist and hip measurements and details of their normal diet.

　　Reporting in the Feb. 13 online issue of Gut, the researchers found that nearly 3,200 of the women needed to undergo gallstone surgery by the year 2000.

　　Compared to women with waistlines at or under 26 inches, women with waistlines of 36 inches or more faced double the risk of developing gallstones requiring surgical removal, the researchers found.

　　In addition, higher waist-to-hip ratio —— a calculation of waist size divided by hip size —— also increased the risk of gallstones. Women with a ratio of 0.86 were 40 percent more likely to have gallstones compared with women whose ratio was 0.70 or less.

　　How might belly fat boost gallstone risk? The researchers point out that this type of fat is more "metabolically active" than fat found elsewhere on the body. In addition, earlier research has also tied gallstones to the metabolic syndrome —— a constellation of risk factors for heart disease that includes excess abdominal fat.

　　And what about risks for men? According to experts at the National Digestive Diseases Information Clearinghouse, women between 20 and 60 years of age are twice as likely to develop gallstones compared with men. However, that disparity fades with age, meaning that men and women are equally likely to develop stones once they pass 60.

　　One expert said the study shows that widening waistlines are an important risk factor for gallstones in women.

　　"This is a way of fine-tuning obesity-related causes of gallstones," said Dr. Joanne A. P. Wilson, a professor of gastroenterology at Duke University. A bulging midriff is also associated with a risk of heart disease, she added.

　　"The bottom line is that women should control their weight," Wilson said.

　　Another expert stressed that most gallstones won't require surgery, however.

　　"At the present time, ultrasound screening for gallstones is not recommended because the majority of patients with gallstones get through life without ever having problems or symptoms," said Dr. Alan F. Hofmann, a research professor of medicine at the University of California, San Diego.

　　"Someday, we may screen and treat gallstones medically, but that has been judged not to be cost-ineffective because so many patients would be treated unnecessarily," he added.