A poor diet is the leading cause of ill-health in the UK yet it is also one of the most contested areas of science. Is eating less saturated fat or sugar the most important factor or would the problems be solved if we ate more fruit and vegetables? Join us to hear Prof Susan Jebb, Professor of Diet and Population Health of the University of Oxford discuss what we know about nutrition and ask the question: once we know the theory of a healthy diet, how can we help more people to achieve it in practice-education or taxation?