Talk:One-eyed ghoul/@comment-26306690-20150930235958/@comment-10733658-20151001050114

I would suggest going back to the early arcs of the series for your answers. It's shown, discussed, and explained multiple times.

Kaneki describes human food in rather graphic detail several times.

Nishiki and Touka both eat human food to pass, and Yoshimura tries to teach Kaneki how they do it.

Touka gets sick from eating Yoriko's food, because her body cannot digest it. So she's essentially giving herself food poisoning repeatedly. It's implied Arata had the same issue, as he frequently ate anything given to him by his neighbors and was described as being "sickly".