previously downloaded podcasts prompt to download again on iPhone

Whenever I sync my iPhone to iTunes, my previously downloaded podcasts get removed. Any podcast that was downloaded prior to the sync prompts to download again when I go to play it (which is typically when I commute and I don't have wifi access). Unfortunately, I have one podcast that requires me to sync via iTunes. Otherwise, all the others sync great with the podcasts app. I am hoping there is a setting that is causing this issue. Any help would be appreciated!