People who have had gallstone disease are more likely than others to develop coronary heart disease, according to a large analysis of past studies.

Hardened deposits known as gallstones form in the gallbladder when the bile contains too much cholesterol or other abnormal substances, and while the cause is not fully understood, factors like obesity, high-calorie diets and metabolic syndrome are associated with gallstone risk.

The same factors increase the risk for coronary heart disease, which kills 370,000 Americans per year, according to the Centers for Disease Control and Prevention.

"Several previous studies have suggested a potential link between gallstones and cardiovascular disease," said senior author Dr. Lu Qi, professor of epidemiology at Tulane University in New Orleans. "Our study provides the first consistent evidence in a U.S. population."

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There are more than three million cases of gallstones in the U.S. each year, according to the National Institutes of Health. If they cause pain the whole organ can be surgically removed.

The researchers combined the results of seven U.S. studies including more than 800,000 people and 51,000 cases of coronary heart disease - defined in the studies as heart attack or having a procedure to clear a blocked artery to the heart.

Six percent of women and 3 percent of men had a history of gallstones. These people tended to be older, were more often smokers or regular aspirin users, were less physically active, had a higher body mass index and more often had a history of high blood pressure, diabetes or high cholesterol than others, according to the results in Arteriosclerosis, Thrombosis, and Vascular Biology.

After adjusting for the common heart disease risk factors, as well as age, lifestyle and other factors, researchers found that people with a history of gallstones had a 23 percent higher risk of coronary heart disease compared to others.

"Our study is observational in nature," Qi said, which can't determine cause and effect. "Further investigations are needed to demonstrate whether the observed relation is causal."

Obesity, diabetes, being female, and eating foods high in fat and cholesterol and low in fiber all increase the risk of gallstones, which affects up to 25 percent of adults in the Western world, Qi said.

Patients with gallstones should be monitored closely based on a careful assessment of gallstone and heart disease risk factors, Qi said.