A Max Nut is a healing item from the Mario & Luigi series. They heal the most of all the nut items.
Mario & Luigi: Superstar Saga
Max Nuts heal all of Mario and Luigi's health.
Mario & Luigi: Bowser's Inside Story
Max Nuts heal 160 health instead of all in the previous game.
Mario & Luigi: Dream Team
Max Nuts heal only 80 health, which is only half of the last game.