Fat will only be stored if there is fat present. Malonyl -CoA exists in high amounts when there is plenty of metabolic fuel present. Thus, carnitine acyltransferase is inhibited and
this in turn prevents acyl-CoA from crossing into the cell?s mitochondria. Another enzyme is inhibited by the presence of NADH and Thiolase is also inhibited by the presence of Acetyl-COA. In short, when a lot of glucose is present, fatty acid metabolism is inhibited.
It is the last sentence that clues us in here. Basically, a cell will not convert fats into energy if there is glucose present. When the cell has carbs and sugar to work on, it will not convert the fat to energy, thus the fat gets stored.
This is why the low carb diets work, with little to zero carbohydrates and
subsequently glucose to work on, the fat will be used for energy. This is exactly why the high carb diet works too. When no or little fat is present, it won't be stored as fat.
In addition to this, it is important to realize that it costs the body quite a bit of energy to take carbs and store them as fat. This alone is actually a positive. There really needs to be some form of fat present to make it easier.