A 5-foot, 5-inch tall white woman who weighs more than 240 pounds is 86 percent more likely to die within seven years than her normal-weight peer, according to a new study.

Extremely obese white or black women are more likely to die of any cause, or to develop heart disease, diabetes or high blood pressure, said researcher Dr. Kathleen McTigue, of the University of Pittsburgh School of Medicine.

"The health risks of extreme obesity have really been poorly understood because the condition used to be so rare," she said.

But now the rate of extreme obesity is outpacing mild to moderate forms, and is likely to have an impact on prevention and treatment needs as well as healthcare costs, Dr. McTigue noted.

She led a multi-center team that tracked for an average of seven years more than 90,000 women ages 50 to 79 from 40 sites who participated in the Women's Health Initiative Observational Study. The findings are in today's Journal of the American Medical Association.

The researchers calculated each participant's body mass index, or BMI, which is based on of weight and height.

A 5-foot, 5-inch woman with a normal BMI of 22 would weigh 132 pounds. At a BMI of 27, which is in the overweight range, she'd weigh 162 pounds. Women are obese if their BMI is 30 or greater, but that category can be subdivided, Dr. McTigue said.

If the woman in the example weighed 192 pounds, she'd have a BMI of 32, within the early obese range of 30 to 34.9. At 222 pounds, she'd have a BMI of 37, in the mid-range of 35 to 39.9. And at 240 pounds or more, she'd have a BMI of 40 or higher, and fall in the extremely obese range.

Almost 10 percent of the African-American women in the study participants and about 3 percent of the white participants were considered extremely obese, Dr. McTigue said.

The researchers found over seven years of follow-up that black women in the middle range of obesity were twice as likely to die from any cause than their peers with normal BMIs. Extremely obese white women were 86 percent more likely to die than normal-weight white women.

Dr. McTigue's work shows that "obesity is not all the same," said Madelyn Fernstrom, director of UPMC weight management center. She was not a member of the research team.

"If you are heavy, even if you cannot lose weight, at least you should try to maintain that weight," because health risks will escalate with added pounds, Dr. Fernstrom noted.

Everyone is aware of the importance of a healthy diet and physical activity, she said. It's not the "what" but the "how" that is challenging.

According to the researchers, the number of people with a BMI greater than 40 quadrupled between 1986 and 2000.

"We need to look into this further because this population is growing fairly rapidly," Dr. McTigue noted. But "many of the intervention studies exclude extremely obese people."

Some obesity treatments, such as surgery, can be risky. The study findings might help obese women and their doctors "accurately balance the risks of being treated with the risks of not being treated," she said.

Dr. McTigue plans to study a larger number of participants in the Women's Health Initiative to better understand the relationships between obesity and its health impact in different racial and ethnic groups.