Postmenopausal women who exercise regularly won’t necessarily see dramatic changes on their bathroom scales or in their dress sizes, but according to a new study the workouts can have a significant beneficial impact.

They can “exorcise” invisible intra-abdominal body fat that wraps itself around internal organs and may pose a greater health risk than more obvious “love handles” or bulging bellies. It is dangerous, researchers say, partly because it’s invisible.

“When you look in the mirror, you don’t know how much you have,” said Melinda L. Irwin, M.P.H., Ph.D., assistant professor of epidemiology at the School of Public Health and lead author of the study published in the January 15 issue of JAMA: The Journal of the American Medical Association.

Women, who gain an average of a pound per year, tend to accumulate it after menopause, and men are also susceptible to health risks as they gain weight around their middles. Although thin women can have intra-abdominal fat, those with waist circumferences of more than 35 inches are the most likely candidates. According to Irwin this hidden fat has been linked to insulin resistance, type 2 diabetes, hypertension, cardiovascular disease and high cholesterol levels, and increases the risk of breast and colon cancers. Intra-abdominal body fat is a metabolically active fat tissue because of its shared circulation with the organs it surrounds.

The study, conducted by Irwin and colleagues at the Fred Hutchinson Cancer Research Center in Seattle, Wash., looked at 173 physically inactive postmenopausal women between the ages of 50 and 75. The women were assigned to one of two groups; one exercised at a moderate intensity five days a week and the other merely stretched one day a week. The researchers measured changes in weight and body fat at the start of the study and after one year.

“While overall weight loss was modest for the women who exercised, intra-abdominal body fat loss was statistically and clinically significant,” said Irwin. The study found that women who exercised moderately five times a week saw a 6 to 11 percent decrease in intra-abdominal body fat. “That would translate into a reduced risk of cardiovascular disease and cancer,” Irwin said. “The good news,” she said, “is that even if exercise doesn’t seem to be making any visible difference—women only lost, on average, three pounds—moderate exercise such as brisk walking reduced hidden fat.”

According to Irwin, most studies that examine the correlation between weight and exercise just weigh the test subjects and measure their waist circumferences. This merely measures weight, not total body fat or fat distribution. “Usually they conclude that exercise has minimal or no effect on body weight,” Irwin said. “We agree with them, except they’re not taking into consideration body fat and how it is distributed on the body.”

Irwin said she and her colleagues used computed tomography—“which is a lot more sensitive than just getting on a scale and measuring weight”—to gauge the test subjects’ amount of intra-abdominal tissue. Using this method, the researchers were able to observe a statistically significant effect of exercise on the intra-abdominal tissue.

The message to take away from this study, Irwin said, is that if you are getting frustrated because you are exercising but not losing any weight, keep at it. “Even if you think you aren’t getting any benefit, you really are.” She also noted that when weight is lost through exercise, rather than diet, you have a better chance of keeping it off. “Dieting hasn’t been shown to be good for weight maintenance; you gain it back, whereas if someone exercises to lose weight, they’re more likely to maintain the weight loss,” Irwin said.