The primary source of body fat for most Americans is not dietary fat ----- but carbohydrate, which is converted to blood sugar and then, with the aid of insulin, to fat by fat cells.
Remember, insulin is our main fat-building hormone. Eat a plate of pasta. Your blood sugar will rise and your insulin level (if you have type 2 diabetes or are not diabetic) will also rise in order to cover, or prevent, the jump in blood sugar. All the blood sugar that is not burned as energy or stored as glycogen is turned into fat.
So you could, in theory, acquire more body fat from eating a big carb "fat-free" dessert than you would from eating a tender steak nicely marbled with fat. Even the fat in the steak is more likely to be stored if it is accompanied by bread, potatoes, corn, and so on.
Now consider what would happen if you instead ate a “fat-free” dessert with exactly the same number of calories as that steak. Your insulin level will jump dramatically in order to cover the sugar and starches in the dessert. Remember, insulin is the fat-building and fat storage hormone. Since it’s dessert, you probably won’t be going out to run a marathon after eating, so the largest portion of your newly created blood sugar won’t get burned. Instead much of it will be turned into fat and stored.
Interestingly enough, eating fat with carbohydrate can actually slow the digestion of carbohydrate, so the jump in your blood sugar level might thereby be slowed. This would probably be relatively effective if you’re talking about eating a green salad with vinegar-and-oil dressing. But if you’re eating a regular dessert, or a baked potato with your steak, the slowdown in digestion would not prevent blood sugar elevation in a diabetic.
Read more about this the insulin/fat connection here.