People who carry a small "spare tyre" around their waist but are otherwise a healthy weight are at triple the risk of dying from heart disease or stroke, an American study has found.

Men and women who are not overweight but store most of their fat around their waist are at greater risk of heart disease or stroke than the clinically obese.

This could be because those who are overweight or obese have more weight on their thighs and hips which helps offset the problem, researchers said.

Doctors from the Mayo Clinic in the United States examined the health records of 12,785 people with an average age of 44, over a 14-year period.

They recorded patients' body mass index (BMI) - their ratio of weight relative to height - as well as their waist-to-hip ratio, which signifies how much of their weight they store on their belly.

During the study, 2,562 of the patients died, including 1,138 as a result of a cardiovascular problem such as heart disease or stroke. The findings suggest that people with a normal BMI but a high waist-to-hip ratio were 2.75 times more likely to die from a cardiovascular condition than people who were normal on both scales. Even people who were clinically obese and had a high proportion of fat stored around their middle had only 2.34 times the risk of dying from heart disease or stroke compared with the healthiest group.

Speaking at the European Society of Cardiology's annual congress in Munich, Dr Karine Sahakyan said having a normal BMI "should not reassure them that their risk for heart disease is low". "Where their fat is distributed on their body can mean a lot.?.?. even if their body weight is within normal limits," she said.

Fat which accumulates between the organs in the abdomen, and causes the waistline to expand, is made of a different type of cell from that which accumulates around the legs and thighs. Cells in belly fat release chemicals that raise insulin resistance and are thought to increase the risk of cardiovascular problems.

People who are overweight and obese have more muscle mass and store some of their fat on their legs and hips, which Dr Sahakyan said was "actually protective". Slimmer people are more likely to carry extra weight on the waist, she said.

Patients with a high waist-to-hip ratio can offset their risk by exercising more and sticking to a healthy diet. Prof Peter Weissberg, the medical director of the British Heart Foundation, said waist-to-hip measurement was a "stronger indicator" of cardiovascular risk than BMI.