Let’s consider, for example a case of a slender person who weighs 150 pounds. If we are talking about a male, then close to 20 percent of that weight would be fat. Normal slender females usually maintain closer to 25 percent of their weight as fat or 37.5 pounds for someone weighing 150 pounds. This is a minimum amount of energy reserve nature intended to have stored in our bodies as fat. And these amounts of energy reserves, which were needed early in man’s evolution, provided just enough for surviving during times of short-term famine or illness.
If carbohydrate was stored in the body for spare energy, it would weigh more than twice the amount that fat does for the same amount of energy that is provided, and it would be very bulky. That 150 pound male would weigh a bulky 180 pounds because he would have to carry about 60 pounds of carbohydrate to provide the spare energy.
People who have too much fat, or those people who are afraid of gaining too much fat, probably have a difficult time believing that they are better off carrying their necessary energy reserves as fat in the form of triglyceride rather than as bulky carbohydrate such as glycogen.
Protein also weighs more than twice as much as fat does for the same energy potential. Also, when protein is burned for energy there is extra nitrogen that the kidneys have to process and eliminate. By comparison, fat burns as a pretty clean fuel; it goes to carbon dioxide and water.