Food is what you need to keep you alive, and if you can't eat it it isn't food. If you are in a seige you will give up all your gold and paper for a plump rat. That is very basic use-value. The classical economists took the labour theory of value for granted when it came to less immediate values than that, and it was only when its implications (the capitalists steal surplus value) became evident that later ones slithered away from it into far less defensible value systems surely.
