Sarah Schenker from the British Nutrition Foundation replies: The only thing that makes you gain weight is taking in more energy (calories) than you use up. In other words, it doesn't matter where the excess energy comes from, theoretically eating too much lettuce could make you gain weight.
Fat is considered fattening because of the four energy-providing nutrients (fat, protein, alcohol and carbohydrate) and fat is the most energy dense food. One gram of fat provides nine calories, compared with seven calories from one gram of alcohol, four calories from one gram of protein and 3.75 calories from one gram of carbs. It makes sense then that cutting fat from your diet is the easiest way to reduce your energy intake.
Sugar is much lower in calories. The trouble is, it's very palatable and is neither satisfying nor filling, so it's easy to eat too much. It also tends to be found in foods with large amounts of fat, such as biscuits, cakes and sweets. If you want to lose weight you don't need to give up sugary foods completely - stick to fruit and veg and starchy carbs, which release energy slowly and help ease hunger pangs.