While a tooth is prepared or having root canal treatment, do I have to eat only soft food like porridge?

You do not have to limit yourself to porridge, but the rule is not to chew with the tooth under treatment. A tooth having root canal treatment is hollow inside and sealed temporarily, so chewing something hard can crack it or dislodge the temporary filling. 1) Eat on the other side, 2) avoid tough or hard foods such as nuts, dried squid, and hard bread until treatment is finished, and 3) be careful with very hot or cold foods, which can irritate it. Once treatment is complete and a crown is fitted, you can eat normally.