You may sacrifice a Food. If you do, target creature gets +5/+5 until end of turn. Otherwise, that creature gets +3/+3 until end of turn.
If an effect refers to a Food, it means any Food artifact, not just a Food artifact token. For example, you can sacrifice Gingerbrute to activate the last ability of Tempting Witch.
If the target creature is an illegal target by the time Insatiable Appetite tries to resolve, the spell won't resolve. You won't sacrifice a Food.
You choose a target creature as you cast Insatiable Appetite, but you don't choose whether to sacrifice a Food until the spell resolves.