The Diet Blog has an interesting post about a study done at the University of Pennsylvania. They split dieters into two groups; one was told to increase the amount of fruits and vegetables in their diet while the other was told just to reduce the total calories they ate every day. The result was that the first group lost 13 lbs in in 6 months while the other only lost 5 lbs.
To me it’s common sense; it’s easier to stay on a diet if you don’t feel hungry and by eating foods low in calories that still satiate you, the more easily you can stick to it. From my personal experience, I would add that foods with a low glycemic index tend to stay in your stomach longer and make you feel full longer as well.