Food is a non-toxic, organic substance that provides nutrition in the form of carbohydrates, protein, fibre, fats/oils, and/or vitamins and minerals. Sure there are some edge cases which you can argue the point in, such as a lump of rock salt maybe, but for the most part it is something which provides sustenance. Sure you could eat mud or plaster and it won’t kill you, but it won’t help keep you alive either, so its not food.