Both carbohydrates and proteins can be degraded by inherent biochemical pathways of our body. If there is no acutal need for energy released by further catabolic degradation, the produced intermediate 'platform' compounds are used to build up fatty acids. This then allows the body to store the uptaken energy in a rather efficient way, for later usage.
If you want to have a more precise illustration I can look up the reaction sequences that occurs in this case. I do not know it by hard right know. :)