I have been thinking that if Natsu can't eat his own fire, can he set some object on fire using his magic and then eat that fire??
As per the Fairy Tail Wiki
In addition, the user can consume external sources of fire to restore their body to a healthy state and regain their reserves of strength, something that also makes them immune to most types of fire, due to their capability of nullifying fire-based attacks by sucking them in and eating them; the consumed fire also seems to possess a different “taste” according to its “quality”. However, the user can’t eat their own flames, or things set on fire by them, to reinvigorate themselves.