Nonfat foods become add-on foods. When we add them to our diet, we actually increase the number of calories we eat per day and gain weight. That was borne out in a Pennsylvania State University study. For breakfast, Prof. Barbara Rolls gave two groups of women yogurt that contained exactly the same amount of calories. One group s yogurt label said “high fat” — the other, " low fat". The low fat yogurt group ate significantly more calories later in the day than the other-group. People think they've saved fat and can indulge themselves later in the day with no adverse consequences, says Richard Mattes, anutrition researcher at Purdue University. But when they do that, they don't compensate very precisely, and they often end up overdoing it.