"Women who tend to have more fat deep inside are at higher risk for heart disease, diabetes as well as cancer," said Dr. Pamela Peeke, author of "Fight Fat After Forty."

Researchers at the Fred Hutchinson Cancer Research Center in Seattle, Washington, studied 173 sedentary, overweight women between 50 and 75 years old over the course of a year to see whether exercise that shaves off outward bulges can reduce internal fat as well.

Half of the women participated in aerobic activity for a year, while the rest attended a weekly stretching class. Those in the aerobic exercise class saw significant results.

"The women who were in the exercise group significantly lowered their amount of body fat and significantly lowered the amount of intra-abdominal fat," said Dr. Anne McTiernan, director of Fred Hutchinson's Prevention Center and lead researcher of the study.

After a year, the exercisers lost between 3.4 percent and 6.9 percent of intra-abdominal fat while maintaining their calorie intake, according to the study, published in this week's issue of Journal of the American Medical Association and funded by the National Cancer Institute.

"Women got the most benefit if they did the full program -- 45 minutes a day, five days a week -- but women still got a benefit if they did as little as 30 minutes a day," McTiernan said.

For years, doctors have known that "apple-shaped" people who store their fat around the stomach are at higher risk for chronic conditions than "pear-shaped" people -- those who store fat in their buttocks and thighs. But intra-abdominal obesity doesn't always correlate with body shape, McTiernan said.

So how can people tell if they have too much fat deep in the belly? Peeke offers a simple, do-it-yourself test.

"Take your fingers, touch your pelvic bones, contract your abdominal muscle wall and you can basically just kind of sneak it across, right across your abdominal muscle wall and see if it's your abdominal muscle wall that's sticking out," she said. It's the fat that pushes those muscles out.

There may not be a lot of movement on the scale when burning off this kind of fat, but researchers said the results will be years added to a person's life.

"Even if a woman who exercises regularly doesn't see the benefits of dramatic weight loss on her scale, ... we now know that exercise reduces hidden intra-abdominal fat, the most dangerous type," McTiernan said in a statement.