What is the source of Sanji's flame power? I recently came up with a theory that Sanji might get a power because all his siblings did, even though he was called a dud. I was then watching One Piece Film: Gold and remembered how his flames didn't make any sense. Could his flames have come from his father's experiments?

He did say in Enies Lobby that he got heated up when he was mad and his powers related to fire keep growing a bit stronger than friction should be able to make. His siblings had powers similar to devil fruit users, so the more powerful Sanji's flames become, the less I believe it is natural (by natural I mean through Haki and friction).

What do you think?
Well, this is an opinion-based question so no clear answer can be given, but I can at least tell my own opinion.

This was a plausible theory for a time, but in my opinion it is now impossible.

Unless I missed something, nothing indicates that Sanji's Diable Jambe comes from anything other than friction. He's been able to activate Diable Jambe without spinning ever since the beginning of the second part of the series, as shown in the manga by his fight against Surume the kraken; long before his family was even mentioned.

He's been stated several times in the Totto Land arc to be a "dud" whose genetic enhancements failed, a statement greatly strengthened by a revelation from chapter 852.


  Sanji's mother Sora is the reason why his enhancements failed. When she was forced by her husband to subject herself to experiments to allow her then-unborn sons to be genetically altered, she took a drug potent enough to counteract those alterations, in an attempt to cancel them. While this didn't work with Sanji's brothers, it did work with Sanji himself, who turned out as normal and full of empathy and compassion as any normal human.


The above moment's entire meaning and point would be completely shattered if Sanji suddenly turned out to have altered genes after all, hence why I think the theory is impossible. However, this is only my opinion of course, as no clear answer can be given yet.