Butter is one of the more fattening foods you can eat because it is primarily made up of animal fat and also contains a lot of saturated fat. Although a lot of people have misconceptions about butter because it does not have any trans-fats or carbohydrates, it still contains quite a bit of fat. This is why it is highly recommended that you find healthier alternatives to use when looking for something delicious to butter your bread with, such as margarine.
Margarine tends to be cheaper and in many cases healthier for you because it contains little to no saturated fat, which is what dieters really want to avoid the most. Butter has a very high fat content and those who want to shed those extra pounds or prevent from gaining more will want to avoid getting the natural stuff whenever possible.