When your party is low on food,
That's when the players start repeating "red wizard needs food badly"
do you actually chop up some of the monsters you've killed for food?
But now to provide an actual somewhat-helpful-answer, it depends on the situation. If the party is "out and about" then the nature and dungeoneering skills offer some ways to forage for food (the exact mechanics of which depend on the system being used).
But, beyond that, if the DM says the environment does not support any sort of ecosystem, then, well, yeah, you have to get creative with what is there OR start dealing with hunger/fatigue/etc.