It is possible to cut calorie intake without feeling as though you are missing out, say experts.
Smaller portions don't have to leave you hungry
In a four-week experiment, women who ate 800 fewer calories per day felt just as full and satisfied.
This shows switching to lower fat alternatives or sizing down portions need not be difficult, according to the Penn University scientists.
They described the diet to the North American Association for the Study of Obesity.
The researchers recruited 24 young women aged 19 to 35 to take part in their study.
On two consecutive days of each of the four weeks the women followed a diet set by the scientists.
On these days the same breakfast, lunch and dinner menu was served but the foods were varied in energy density (calories) and portion size.
For example, a meal of pizza and salad was made less calorific by incorporating more vegetables and using reduced fat ingredients.
The meal on the left contains fewer calories
Reducing the calorie content of foods by 30% led to a 23% decrease in daily calorie intake.
Reducing the portion size by 25% led to a 12% decrease in calorie intake.
Despite the large variation in intake, the women rated their hunger or fullness the same.
Lead researcher Dr Barbara Rolls said: "The subjects found the smaller, lower energy density meals just as palatable, filling and satisfying as the big, high calorie menu items - and they didn't compensate for lowered intake on the first day by eating more on the second day of the study.
"This shows that even small reductions in the energy density or portion size of foods are likely to decrease energy intake.
"The results suggests that home cooks and restaurants could take an easy step toward obesity prevention by adding more fruits and vegetables and trimming the fat to decrease energy density without having to serve tiny portions," she said.
Dr David Haslam, chairman of the National Obesity Forum, said cutting calorie intake by 800 in this way should be easy to sustain and was enough to shed unwanted weight.
"There are certainly changes in our diet that we can make that are utterly sustainable and that you would never dream about going back on.
"Nobody in the world would find it particularly difficult to switch from full cream milk to semi skimmed milk in their tea and on their cereal.
"If you like cheese sandwiches you can cut down to half the amount of cheese if you grate it rather than using big slabs of cheese. It's just the same, if not better.
"They are certainly simple changes that are completely sustainable and if you make enough of these changes you can make a very significant drop in calories without even noticing, with no effect on quality of life whatsoever."