Even among people with a normal weight, having a big belly may be deadly, a new study shows.

"People should not only look at their weight, but also consider their waist," Dr. Annemarie Koster of the National Institute on Aging, the lead researcher on the study, told Reuters Health.

Being overweight or obese is clearly bad for one's health, but the best way to gauge whether a person's fatness is putting them at risk has been "controversial," Koster and her team write in the American Journal of Epidemiology.

Body mass index, or BMI, has been the standard measurement used, they add, but the way fat is distributed throughout the body - especially at the waistline - may be even more important than how many excess pounds a person is carrying.

To investigate the relationship among belly fat, BMI and mortality, the researchers followed 245,533 men and women participating in the National Institutes of Health-American Association of Retired Persons study. Study participants ranged in age from 51 to 72 at the study's outset, and were followed for nine years.

Among men, the researchers found, those in the top fifth based on their waist circumference were about 22 percent more likely to die during the study period than men with trimmer waistlines, independent of BMI. A similar risk was seen among women.

In addition, people considered to be abdominally obese based on World Health Organization guidelines - a waistline of 35 inches or more for women, or 40 inches or more for men - were 20 percent more likely to die over the nine-year study than their peers with slimmer waists.

The findings were true for smokers and non-smokers, healthy people and those with chronic illness, and across all the ethnic groups the researchers looked at, which included non-Hispanic whites, non-Hispanic blacks, Hispanics, and Asians. In fact, there was evidence that mortality risk climbed more quickly with waist circumference among Asians, particularly men.

While the danger of abdominal fat - in particular visceral fat, which collects around the internal organs in the abdomen - is becoming clear, Koster noted, the reason why a fat belly is bad for health is still not well understood. "More research is needed there," she said.