CICO is absolutely right, but it just so happens that the best way to actually implement this with people is to reduce carbs in the diet. Carbs are the most easily, and commonly, overeaten. After all, most people aren't going to get fat off of just veggies or butter. It'll be the sugary foods that they overeat, like breads, candies, sodas, etc. Watch this lecture by a UCSF professor about the effects of carbs and how they affect obesity: https://youtu.be/dBnniua6-oM