Normal food, it's bad for them, for the most part, ghouls can only enjoy Coffee and water the same way humans do. If Ghouls go too long without eating, this causes them to go crazy, as was seen with ''Ken Kaneki'' in season 1 when he was struggling with being a ghoul. It's also shown how hard it is to keep food like chicken dumplings down for a ghoul, which was shown by Touka. This also made her extremely weaker than normal. Showing the drawbacks of not eating properly as a ghoul. With that being said, if a ghoul decides to participate in acts of cannibalism, in this case, eating ghouls. This can cause them to develop a mutation in their kagune, turning it into a Kakuja.