This code is only available each time you return to a US City after taking a World Tour. This allows all food to be digestable. It also means that normal items that would make one of the monsters puke before, will now restore some health like normal food.

Code - Effect

Hold Down and press Jump button 3 times - George
Hold Down and press Punch button 3 times - Lizzy
Hold Down and press Kick button 3 times - Ralph