User blog comment:FairyRaven/Fairy Tail Chapter 400: Wings of Hope, Review and Commentary/@comment-24729244-20140913120829/@comment-9595671-20140913121335

I can't tell if that's actual lava or not. It could just be like Igneel's domain inside him.

Then again Natsu does consume fire, and in that anime filler where he and Loke traded bodies Loke kept drooling lava.