As long as you have food in the fridge, your peeps will never starve. Once the Fed bar gets half empty, it will not go too much lower as long as food is stocked.. it sometimes even goes a little higher while the game is closed.

Every year in the game (approx. 3 hours, 30 minutes) they will consume food by themselves but this will not necessary raise the fed bar. You need to get them to prepare a meal and sit down for the meal while you're watching them for that half empty fed bar to go back up near full.