Why do some yeast cells produce ethanol? Scientists have wondered about this apparent waste of resources for decades. Now, University of Groningen scientists think they have a solution: yeast cells produce ethanol as a ‘safety valve’ to prevent overload when their metabolic operation reaches a critical level. The new theory, which was published in Nature Metabolism on 7 January, could have far-reaching implications, as it also explains why cancer cells waste energy by producing lactate, known as the Warburg effect.