As the body progresses from the absorptive to the postabsorptive state, the continues to burn glucose while virtually every other organ in the body switches to fatty acids as its major energy source

As the body progresses from the absorptive to the postabsorptive state, the Brain continues to burn glucose while virtually every other organ in the body switches to fatty acids as its major energy source. During the absorptive state the ingested nutrients enter blood from GI tract while the postabsorptive state the GI tract is empty of nutrient and body's own stores must supply energy.