Meat

Steak is the food that replaced Mushrooms in Paper Mario: The Thousand-Year Door during Bowser's special sidescrolling levels. Like the Mushrooms that make Mario grow, steak makes Bowser bigger. Steak can be found by hitting certain ? Blocks. If Bowser takes a hit from an enemy, he will shrink down by one level. Getting enough pieces of steak in one level will make Bowser gigantic, allowing him to smash through everything in the level, similar to Mega Mario.

Interestingly, the steak does not appear outside of Bowser's sidescrolling events.