Free glucose is not the most common source of fuel in most animal diets, including the human diet. Each of the basic food types can be used as a source of energy. Proteins must first be digested to their constituent amino acids. The amino acids are then transformed into various compounds, which enter the middle of glycolysis or the Krebs cycle. Toxic parts of amino acids are stripped off and eliminated in urine or used to synthesize other compounds.
Lipids contain almost twice as much energy per unit weight than carbohydrates.
They must first be digested to glycerol, which enters in the middle of glycolysis, and fatty acids, which are converted to acetyl CoA and, thus, enter the Krebs cycle.