The catabolism of lipids occurs through beta-oxidation

The catabolism of lipids occurs through beta-oxidation that occurs in the matrix of mitochondria and changes fatty acids into acetyl groups. Tissues then incorporate these groups into CoA to create Acetyl CoA that continues further into the Krebs cycle.

In certain situations, the body oxidizes carbohydrates to create both the fatty acid and glycerol portions of triacylglycerol (TAG). The transformation of fatty acids into carbohydrates is however impossible as the dehydrogenase reaction of pyruvate goes in one direction and is irreversible.