It wasn't a stupid question! Yes it does use it up...not that they produce less, but if you're storing the milk less goes into baby! Some people will pump off what's left after their baby has finished feeding (so that by the time they want fed again they've refilled). But Sam's pretty greedy and doesn't leave much in there so I pump a whole breast in the morning if I'm pumping one day. Feed him from one, pump the other, give him breakfast then feed him from the first side again - it's never as full as the pumped one but he's ok with it. After a few days of pumping every day I end up making enough x