A spare tyre round the middle of an otherwise average-sized body is more dangerous than being fat all over, research has shown.
The startling discovery was made by scientists who analysed data on 15,184 American men and women with an average age of 45.
Over a period of 14 years, normal-weight individuals with bulging middles had a worse survival record than participants officially classified as overweight or obese.
Men in this category were twice as likely to die than others defined as overweight or obese by their body mass index (BMI), which relates weight and height.
For women, the effect of having a spare tyre was less pronounced but still increased the risk of death by up to 40%.
The US scientists led by Dr Francisco Lopez-Jiminez, from the Mayo Clinic, wrote in the journal Annals of Internal Medicine: "Our analyses .. show that normal-weight US adults with central obesity have the worst long-term survival compared with participants with normal fat distribution, regardless of BMI category, even after adjustment for potential mediators.
"Our findings suggest that persons with normal-weight central obesity may represent an important target population for lifestyle modification and other preventive strategies.
"Future studies should focus on identifying factors associated with the development of normal-weight central obesity and better understanding the effect of normal-weight central obesity on health outcomes."
The researchers pointed out that "spare tyre" obesity was associated with the accumulation of "visceral" fat around internal organs, which is known to be especially harmful to health.
Excess visceral fat was associated with insulin resistance - which can lead to diabetes - higher levels of cholesterol and blood fats, and inflammation.
Study participants, who had an average age of 45, were recruited for the Third National Health and Nutrition Examination Survey (NHANES III), an on-going investigation collecting health and lifestyle data in the US.
Christopher Allen, senior cardiac nurse at the British Heart Foundation, said: "We all know that watching our weight is important, but often it's forgotten that where you carry the weight makes a difference too. Having more fat around your middle can lead to Type 2 diabetes, which greatly increases your risk of having a heart attack or stroke, but there is lots you can do to get rid of this excess weight and lower your risk.
"Keeping physically active and eating a healthy, balanced diet will help control your weight and reduce your risk of diabetes and heart and circulatory diseases. If you're concerned about your weight or need further support, make an appointment to see your GP or practice nurse."
Of the 3,222 deaths recorded in the study, 1,404 were due to heart disease.