The finding comes from a year-long study based on the principles of the " Volumetrics" diet proposed by Penn State University researcher Barbara J. Rolls, PhD. Consumer Reports recently gave Volumetrics the top rating among popular diet plans.

The diet cuts way back on fats, which ounce-for-ounce carry more calories, making them more calorie dense than any other food. But the study offers a big bonus: You get to eat lots of water-rich, low-density foods such as fruits, vegetables, and soups.

It works in short-term studies. But can it work for longer periods?

Rolls, Julia A. Ello-Martin, PhD, and colleagues enrolled 97 obese women whose average weight was about 200 pounds. The women were stratified by age and severity of obesity and then randomly assigned to one of two diets.

Half the women went on a low-fat diet. The other half went on the same low-fat diet but was told to eat more water-rich fruits, vegetables, soups.

Of course, the women didn't just go on a diet. They got lots and lots of help.

For the first six months, women in both groups had individual, 30-minute, weekly sessions with a dietitian. They also were taught new cooking techniques, including how to modify their favorite recipes to reduce their fat content. And they got lessons on grocery shopping, dining-out strategies, and meal/snack ideas.

Moreover, the women received behavior therapy stressing self-monitoring, goal-setting, social-support networks, coping with emotional eating, managing stress and the environment, overcoming obstacles, problem solving, and handling setbacks.

Physical wasn't left out. All the women were given pedometers and were asked to increase their walking by 2,000 steps a day until they walked at least 10,000 steps a day.

For the second six months, the women attended one small group session and one individual session with a dietitian each month.

The only difference between the groups was that half the women got extra training in how to add low-density foods to their diets.

What happened? Seventy-one of the original 97 women finished the study. All the women lost weight. Those on the low-fat diet lost an average of 14 pounds. Those on the low-fat, low-density diet lost 17.5 pounds -- even though they ate 25% more food by weight.

"Eating a diet that is low in calorie density allows people to eat satisfying portions of food, and this may decrease feelings of hunger and deprivation while reducing calories, Ello-Martin says in a news release.

The study appears in the June issue of the American Journal of Clinical Nutrition