>>11914495I understand the premise of your question, but I will add that the definition of what a hydrocarbon is - so for all intents and purposes a carbohydrate is a hydrocarbon, it fact glucose is just an analog of cyclohexane. Oils, too, are hydrocarbons and we can digest them. However, I feel when you say hydrocarbon* you are referring to basic straight-chain alkanes that may or may not be liquid at room temperature, I believe the answer lies with the fact that our body does not provide the enzymes required for the metabolic breakdown of these substances. If you consider the body an environment, the enviroment does not provide the specific conditions required for the breakdown of these molecules. All in all, we can some hydrocarbons can be broken down in the body - however, some can not - this is because the energy required to break the bonds of these basic hydrocarbons* far exceeds the bodies capacity.
*For consistency, when you say hydrocarbon are you referring to liquid hydrocarbons like pentane, hexane, etc.? If it is the case keep in mind too that these substances in of themselves can result in adverse biological effects.
