For the last 20 years the focus of nutritional advice has been to reduce total fat intake and consume large amounts of carbohydrate. However, this advice is inconsistent with many lines of evidence indicating that unsaturated fats have beneficial metabolic effects and reduce risk of coronary heart disease. More recent evidence has also shown that the large majority of carbohydrates in current industrial diets, consisting of refined starches and sugars, have adverse metabolic effects and increase risks of obesity, heart disease and type 2 diabetes. Thus, in what appears to be an optimal diet, most calories would come from a balance of whole grains and plant oils, proteins would be provided by a mix of beans, nuts, fish, eggs, and poultry, and the remaining nutritional needs would be filled by plenty of vegetables and a few fruits. Important considerations include the role of dairy products, the interrelationships with physical activity and genetic variations, the implications of our food choices on environmental sustainability, and how we move from today’s pathological diet to a more optimal way of eating. Dr Walter Willett is Professor of Epidemiology and Nutrition and Chairman of the Department of Nutrition at Harvard School of Public Health and Professor of Medicine at Harvard Medical School. Dr Willett is the most cited nutritionist internationally, and is among the five most cited persons in all fields of clinical science. He is a member of the Institute of Medicine of the National Academy of Sciences and the recipient of many national and international awards for his research. Recorded on 12 June 2008 at the University of Edinburgh's George Square Lecture Theatre.