The body intakes foods and will typically break them down into smaller parts. These parts are then absorbed by the digestive system. The majority of digestion takes place in the intestines. In simplistic terms, these food particles can be broken down into three basic categories, carbohydrates, fat, and protein. The body actually requires that fat, carbohydrate, and protein be consumed daily. Weight gain occurs when an incorrect consumption of these calorie producing groups are ingested in relation to what the body needs. Weight loss occurs when not enough foods containing these calorie producing groups, are ingested to meet the energy needs of the body.
Interestingly, not all types of foods (carbohydrates, fat, and protein) produce the same amounts of energy and therefore they have less caloric (energy) value. The body understands this and will ‘reserve' certain types of food categories preferentially over another. These preferential characteristics of the body to store some foods more than others, and to burn some foods faster than others is what the manufacturers of the different weight loss diets are trying to use to justify their weight loss diet over another companies weight loss diet plan.
Not only are the food groups (carbohydrates, fat, and protein) broken down and stored differently, some types of sub groups within a group (simple carbohydrate and complex carbohydrate) may be broken down and used more efficiently.