The treats are separate from the groceries as the groceries do include fruit in one of the categories. I'm just going by how the game labels the products

Treats can be sold while groceries cant be sold back. I do notice that they stopped complaining after giving them a big load of groceries or if I throw in a regular grocery here and there. In any case, groceries keep them alive and are essential but if they keep getting the same type it could get boring for them!