Fat can help you lose weight on a low carb diet, by decreasing your protein calories, so you decrease your sweet and starch cravings. Protein is metabolized to glucose by gluconeogenesis, and the more protein you eat, the more sweet and starch cravings you can have. The problem is that fat has 9 calories per gram, compared to 4 calories per gram for protein and carbohydrate, and your calories can get to stupid very quickly, with uncontrolled fat intake. Ideally, 75% of your calories should come from fat on a low carb diet. On the other hand, fat can pour on the pounds on a full carbohydrate diet or on maintenance. If you want to keep weight off, a good trick is not to put fat (butter or olive oil or cheese or cream chhese or peanut butter) on bread, because fat can take calories to psychotic. There are often more calories in the peanut butter, cheese or cream cheese you put on a bagel, than in the bagel itself.