Nut

Nuts, or more specifically Peanuts, are satisfying snacks used by the Mario Bros. during their adventures in the Mario & Luigi series. Nuts restore HP to both Mario and Luigi. In Mario & Luigi: Partners in Time, they are replaced by Mushroom Drops.

Mario & Luigi: Superstar Saga
Nuts first appeared in Mario & Luigi: Superstar Saga. Regular Nuts will heal 20 HP for both brothers. Super Nuts, a stronger version, will heal 40 HP to each brother. Ultra Nuts will heal 100 HP to both bros. Max Nuts, like Max Mushrooms and Max Syrup, will heal all HP to both Mario & Luigi.

Mario & Luigi: Bowser's Inside Story
Nuts make a return in Mario & Luigi: Bowser's Inside Story, after their absence from the previous game. Just like in Superstar Saga, they are able to heal a certain amount to both bros. Regular Nuts and Super Nuts do the same but Ultra Nuts, which the last nuts the players are able to buy in a store, heal 80 HP while Max Nuts, which can only be obtained from defeated enemies, ? blocks, or from successful Mighty Meteor attacks, heal 160 HP instead of all HP like in Superstar Saga.

Mario & Luigi: Dream Team
Nuts return yet again for Mario & Luigi: Dream Team, which works for same, but all four types heals different amount of HP: 15HP for the regular, 30HP for the Super, 50HP for the Ultra and 80HP for the Max.

Trivia

 * In Mario & Luigi: Bowser's Inside Story, the player can purchase all of the max items (Max Mushroom, Max Syrup and TNT Drumstick) except for the Max Nut.
 * Also, the player can use Nuts even if only one Bro. is below his max HP.