Alternating between diet foods and the real thing may undermine the body's sense of caloric intake and invite overconsumption. In a study at Purdue University, rats ate full-fat potato chips daily or alternated days of full-fat chips and chips containing the fake fat Olestra. When offered a new high-fat snack, only the rats previously fed diet chips overate and failed to adjust their daily intake accordingly. Studies of artificial sweeteners have shown similar results. "The message isn't that you should never consume artificial sweeteners or fat substitutes," says researcher Susan E. Swithers. "But if you do, then you might have to count calories more carefully."