Scientists had known since the early 20th century that rats could not survive on a diet whose only protein source was zein, which comes from maize (corn), but recovered if they were fed casein from cow's milk.
Os científicos sabían desde comezos do século XX que as ratas non podían sobrevivir cunha dieta cuxa única fonte de proteínas fose a proteína zeína do millo (Zea mays), pero que se recuperaban se lles daban a proteína caseína do leite de vaca.