Washington ? When the fat get even fatter, their risk of death jumps, too, especially if they have an apple-shaped waistline.

So concludes a study of 90,000 women in the United States, the first to look closely at the alarming trend of extreme obesity, being at least 90 pounds overweight.

“People think of obesity as a single thing, but your risk can be modified within that,” said lead researcher Dr. Kathleen McTigue of the University of Pittsburgh. She presented the study Saturday at a meeting of the American Heart Assn.

The good news is that losing even 20 pounds or so will help, she said.

For the fattest women, “it would definitely improve your health prognosis if you can move yourself just one weight category over,” McTigue said.

About 60 percent of American adults are either overweight or obese. The excess pounds increase the risk of diabetes, heart disease and other ailments.

The government equates obesity with a body mass index, or BMI, of at least 30. Someone who is 5-feet-4 would have to weigh 175 pounds to reach that threshold.

The index is calculated by dividing a person’s weight in pounds by his height in inches, squared, and multiplying that total by 703.

Doctors typically warn that obesity increases the risk of death twofold, mostly from heart disease.

Recent studies suggest about 4 million Americans are extremely obese, with a BMI of at least 40. That translates into 233 pounds for that 5-feet-4 person.

Apple-shaped women had higher health risks than pear-shaped women, even at the same weight, McTigue found.

Abdominal fat long has been associated with heart disease. Genetics are most responsible for where the body stores fat, but McTigue’s findings suggest that apple-shaped women “need to be more careful about your body weight than other people,” she said.

The study provides the most detailed look yet at the health risks of extreme obesity, offering crucial information as more people move into that once-rare category, said Catherine Loria, a nutritional epidemiologist at the National Heart, Lung and Blood Institute.

“It tends to be a very touchy subject” for doctors and patients to broach, she said. But, “we really do have to make the point that there are important health consequences that should be addressed.”