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.