A test has been designed which can show if people of normal weight are at an increased risk of heart disease because they are carrying "hidden" fat.
Women with fat around organs in the abdomen were at highest risk
Developed by South Korean researchers, it checks blood-flow via a cuff on the ankle or arm.
Writing in Clinical Endocrinology, the team say those with the poorest blood flow had more fat around their organs, and so were most at risk.
UK experts said the study reinforced the dangers of abdominal fat.
The test measures the flexibility of someone's artery walls. The more elastic they are, the better a person's circulation will be.
The researchers from the University of South Korea tested 150 women aged 22 to 67, who all had healthy lifestyles.
The women's weight was checked.
The researchers then also checked for visceral fat, which lies around the organs and which differs from the ordinary fat which lies underneath the skin, using both the cuff test and body scans - to double-check the results.
People can have too much visceral fat but still be a normal body weight.
Approximately 40% of people are thought to have too much visceral fat stored round their abdominal organs - the key area of concern.
The team found normal weight women who had high visceral fat levels scored 11% higher on the test than women who were overweight with normal visceral fat levels.
This suggests the normal weight women had a higher risk of heart disease.
The researchers say other methods of measuring visceral fat levels and the risk of heart disease have been expensive and time-consuming, and that the cuff test could offer a way of diagnosing those at risk early.
And they say identifying those at risk early means they could be encouraged to change their lifestyles to improve their health.
Professor Duk-Chul Lee, who led the research, said: "While having lots of fat around your organs is bad for your health, people can easily reduce their visceral fat levels by taking regular exercise.
"It is essential that people realise the importance of maintaining a healthy lifestyle as well as a healthy body weight."
Professor Paul Stewart, an obesity specialist at the Queen Elizabeth Hospital in Birmingham, said: "The take-home message of this study is that having more fat around your the belly area puts you at increased risk of heart disease.
"it's not so much about general obesity; it's about middle-aged spread and where fat lies that's the problem.
"This is one more test that can indicate risk."
Professor Peter Weissberg, medical director of the British Heart Foundation (BHF), said: "This study confirms what we have known for some time; good heart health seems to depend more on our shape than our weight. "
''Visceral' fat inside our abdominal cavity - surrounding our intestines - increases our heart disease risk more than fatness in other parts of our body. "It's possible in future that measures of waist circumference will be included to refine the heart disease risk assessment.
"But factors such as blood pressure, cholesterol and smoking - which are routinely taken into account - are more important."