Maybe you could make it so that you get the option to buy the donut store from a coworker so players won't be able to miss the feature and it would make it a more natural part of the game.
Maybe the restaurant has barrels of addictive and ultra fattening food additives hidden away in the basement, with the option of using them or not. That way players could open the shop but opt out of the society fattening aspects if they want to without it seeming forced. Of course, the player would make more money if they choose to use the additive. You could call the stuff, "Glycero-adipocyte enzyme extract no.114 : Diet Supplement Additive." To make it seem more realistic.