Well the rules are indeed ambiguous and I have the same question. The rules state:
"Produce metal and/or energy from any 2 sectors without discarding a food resource."
Everything is in the use of the word "discarding".
To me it means that you need to have a food resource in the sectors, but you don't have to discard it.
As Dan Schnake helped design the game and the rules, I think I'll take his word for it.