On the one side, the only way to gain weight is to eat and drink more calories than you can use up in energy for one day. On the other side, the only way to lose weight is to eat fewer calories than you use up in energy per day.
Having in mind this, one had to know that fats are the highest calorie foods of all. This is so because they are twice as high in calories as sugary and starchy carbohydrates. Some of the fats cannot be seen. For instance: butter, margarine, salad oils. All of them make a high-calorie contribution to the diet. So, for example, per 30 gm you have the following calories: 210 in butter, 210 in margarine, 255 in olive oil, the same amount in corn oil. In comparison per 30 gm you have the following calories: 4 in melon,4 in mushrooms, 7 in orange, 10 in apple, 120 in Cheddar cheese,42 in Roast chicken (skinned).
If you want to change high-fat foods to low-fat foods you will not make a great effort. You can take very little food but it depends on the kind of food and calories which this food includes in.
The most important thing that everybody has to know is that the average man needs only 2500 calories per day and the average woman-2000.