Here is a study that proves what I believe and most of my fitness gurus out there do as well, that diet alone is not enough to lose weight.  It is a combination of diet and exercise that works the best.  In some cases dieting alone can actually work against you.  If you cut your calories too much your body goes into a starvation mode and starts holding onto its fat because it is afraid it is not going to get the calories it needs.  In many cases a lot of diets don’t consist of the true calories and nutrition our body needs.  That is why the best way to lose body fat is to moderately lower your caloric intake and exercise 5-6 days a week for 30-60 minutes a day.  The combination of both is what will work the best and the fastest!

http://www.sciencedaily.com/releases/2010/04/100413170710.htm