There is no penalty for not getting a balanced diet. You cannot really survive on only grain everyday, but the game doesn’t care. Managing the diet of your company is not the focus of the game.
Heh, just got event: “Your purchases of diverse foods has the men gleefully chowing down and feeling good about life” -all brothers have “in good spirit” morale. I ques there is some event for having monotonous food too with some negative consequences.
There is a penalty for only buying grain too btw. After buying nothing but grain since it’s so cheap for about 20 days I got an event with dissatisfied brothers and the gist of it was “Can we at least have a little meat every now and again?”