Isn't it false to assume the human body (and the human brain) is a 'closed system'? This is going to sound mighty stupid, but doesn't the fact that we ingest va

Isn't it false to assume the human body (and the human brain) is a 'closed system'? This is going to sound mighty stupid, but doesn't the fact that we ingest variable amounts of food (i.e. energy) suggest that in surplus situations this energy can be used 'on the spot', rather than converted to long-term storage (i.e. fat)?