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.
You choose a target creature as you cast Insatiable Appetite, but you don’t choose whether to sacrifice a Food until the spell resolves.
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.
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.