Here's the problem with Clark and Kara metabolizing %100 of the food they eat...
Do they need it?
What happens if they don't eat?
if they metabolize the bad stuff to do specific necessary stuff, does that mean that their body will break down if they are not constantly eating bad food, as well as healthy food.... Oh.
Everything they eat is shoveled into the furnace for their heat rays.
Heat Ray Vision is pooping.
The Kryptonian's ability to absorb and store solar power might be their primary source of energy. they don't need to eat because they get everything they need from the sun. Krypton's dull red star provides them with enough energy for basic Earth-ish human performance. But under the more energetic "yellow' sun of Earth their vast superpowers are triggered. They retain their (still functional but largely unnecessary) digestive system as a vestige of their past lives.
They don't NEED to eat but they can and do, mainly for taste.