Diet can influence cancer survival, but the molecular reasons are largely unknown. Jing Chen at Emory University in Atlanta, Georgia, and his colleagues searched for clues in mice with tumours that carried the BRAF V600E mutation, which occurs in some human cancers. They found that high-fat, low-carbohydrate diets boosted concentrations of acetoacetate — which is made when the liver breaks down fat — in the animals' blood, and that this spurred tumour growth. Treatment with cholesterol-lowering agents slowed tumour growth by reducing acetoacetate levels. Moreover, an acetate inhibitor reversed the effects of a fatty diet on tumours.