I don't think Mario/Luigi eat the power-ups...
Because they are Mushroomian, they probably absorb them to get the ability to grow and/or shoot fireballs etc.
In Super Smash Bros. Luigi and Mario shoot fireballs without the help of the infamous red and orange flower, so maybe they have adapted to it, and the magical Mushroom Land fire courses through their veins!
Or..maybe not..
Anyone wonder why Mario didn't shoot fireballs in Mario 64? The left that out, along with Super Mushrooms and...Toad was a ghost...
SM64 was just wrong..
and.....pasta......
-Luigi
I'm-a Luigi, number-a one!