[gtrubetskoy]

To illustrate the point: dietary fiber, which contains a lot of calories, is not digestible. Eat as much as you can, but you will not get one calorie of energy from it. So just based on that, where calories come from matters.

There is a difference between fat metabolism and sugar metabolism. They use different metabolic pathways, in other words the process by which a fatty acid molecule becomes ATP (the ultimate source of cellular energy) is very different from how fructose (which has to first be converted to glucose in the liver) becomes ATP.