The reason Ghouls can't eat the same foods as Humans is that the way their digestive system is built differently. Also its their tongue. To a ghoul, food like steak, chicken, pork, cake, and other food like that tastes disgusting to them, and it's really bad for them. Causing really bad physical drawbacks when eating human foods. The exception to this rule is Coffee, Water, and human flesh; these are the only foods that their bodies will allow them to keep down without making them sick.
KAKUGAN (THE EYE)
When ghouls are excited, hungry, or using their kagune, a ghoul's eye will change. The sclera turns pitch black, and the iris glows crimson due to the sudden rush of Rc cells to the ocular pathways.
Artificial ghouls and half-ghouls will have this effect, too, but only in one eye.
GHOULS DURABILITY
Ghouls have extremely dense muscle tissue and skin; due to this, normal weapons like kitchen knives, bullets, sword exc. wont break their skin. When a weapon like knives or bullets is used against them, it usually breaks the weapon or just doesn't work. Minor injuries heal very quickly, like getting bitten by an animal would heal in seconds. Losing things like your arms or legs can heal in a matter of days. Though if a ghoul is hungry, this can take longer. The exception to this durability rule is places like the eyes, tongue, and ears.
GHOUL STRENGTH
Ghouls possess a huge amount of strength, being 4-5 times stronger than a normal human. Because of this, a ghoul can punch through things like brick walls and easily tear through the tissue of a human like it's paper.
GHOUL Vulnerabilities
Ghouls are weak to Quinques, which are melee weapons made from a special type of steel called quinque. There are also Q-bullets, which are made from the same type of steel as Quinques. They're also weak to RC Suppressants, which basically make the RC count in a ghoul body drop down to human levels, basically making them pass out. Items that use RC suppressants are shown as smoke grenades, but they're very rarely used.