Someone correct me if I'm wrong, but I think the body is always burning Glucose (From Glycogen), Fatty Acids (directly), and Ketones (indirect from Fatty Acids), in various proportions.

On a high carb diet the above proportions are skewed toward Glucose, while on a low carb diet the proportions are skewed toward Fatty Acids and Ketones.

I'd like to know the percentages in each case if anyone knows.