I believe it's specifically fructose that is causing this problem or anything that contains fructose (white sugar, HFCS, fruit juices, etc.). Your body cannot burn fructose and has to convert it. It almost always gets converted into fat since the other options involve not having glucose. Fructose is typically consumed with plenty of glucose to go with it. People of the past used to eat a lot of starch (bundled up glucose) and were not turning out a bunch of lard asses everywhere.