Answered Oct 26
The first option is to burn glucose, because it is the least complex, and more direct.
If you run short on glucose, you have a store of glycogen in your liver and muscles. Glycogen is made of linked glucose monomers, which can be separated and burned.
You would also use other sugars, such as lactose, which take a bit more energy input to do.
Next, you would burn lipids. These are disassembled into 2-carbon units.
Last, you would start burning proteins. This may be a bad thing, for a couple of reasons. You need to get rid of the excess nitrogen, which takes resources. And, ultimately, you would start cannibalising things like your muscle tissue. In a starvation scenario, your body would be simultaneously burning tissue proteins, and using those carbon skeletons to manufacture glucose to send to the brain, which can only use glucose.