User blog comment:FairyRaven/Fairy Tail Chapter 352: The Flame's Voice, Review and Commentary/@comment-1359322-20130920104532/@comment-7418318-20130920110825

When he ate his flames during the GMG arc he didn't show any visible change.

For all we know Atlas Flame could be intentionally enveloping himself in flames at all times because he likes it, not because it's his natural condition or part of his magic.