“Do you not perceive that whatever goes into the mouth passes into the digestive system, and is cast out into the draught?”
So His disciples need to recognise that when something is eaten it goes through the digestive system, and that what then remains, leaves their bodies as waste and goes into the latrine. It takes no uncleanness in and it leaves no uncleanness behind. Thus it cannot cause religious defilement. (This has nothing to do with whether it can cause physical problems). So the idea that food can pass on religious contamination is to be seen as a fallacy.