From Kirbypedia

English Name:	Mint Leaf
Japanese Name:	Sweet Potato
First Game:	Kirby's Dream Land
Type:	Power Up

Mint Leaf, along with the Spicy Food, is an item from first Kirby game and also sometimes considered an early precursor to Copy Abilities.

There's quite a big change between the Japanese and English version of this item. Both cause Kirby to swell up like a balloon and use unlimited air puffs, but the reasoning varies. The Mint Leaf simply gives him 'minty fresh breath', while the Sweet Potato makes him gassy and belch uncontrollably.

Sweet Potatoes are known for making people gassy, hence the swelling and inexhaustible supply of air pellets. The name was changed in the American version for lack of common knowledge to what a Sweet Potato is or politeness.

Games

Kirby's Dream Land

Upon eating the item, Kirby is swelled up like he has taken to the air. He can fly and spit out air pellets just as he does when normally inflated. In this case, however, his supply is unlimited for a set amount of time. He can expel as many air puffs as he wants without losing his flight or needing to re-inflate. Once his time expires, he returns to normal.

Against the boss Kaboola, a variant of the Sweet Potato is found with boosted capabilities. This "super" item supplies Kirby's with an infinite amount of air bullets that will last throughout the battle's duration. Additionally, the shots produced travel the length of the screen rather than only a short distance in front of Kirby. This can be seen as the prelude to other item granted Final Abilities in games to follow, such as the Star Rod.

Anime

In this version it is far more apparent that the item is indeed a sweet potato. In episode 073 eating a big sweet potato causes Kirby to get a terrible case of hiccups, rendering him incapable of inhaling his enemies! (Which was exactly what Dedede wanted.)

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Items:

Power-Ups: Warp Star - 1UP - Maxim Tomato - Pep Brew - Candy - Mint Leaf Spicy Food - Mike - Cell Phone - Food - Switch - Air Ride Machine Treasure Chest - Misc. Items

Obstacles: Star Block - Bomb Block - Heave-Ho Block- Hard Block Dream Course Obstacles - Tilt 'n' Tumble Obstacles

Final Weapons: Star Rod - Love-Love Stick - Rainbow Sword Master - Triple Star - Block