Senior Assistant Editor Ben Burwood speaks with Dr Zuguo Mei (Division of Nutrition, Physical Activity, and Obesity, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, GA, USA)  and Dr Gary M Brittenham (Department of Pediatrics, College of Physicians and Surgeons, Columbia University, New York, NY, USA) about their work defining physiologically based serum ferritin thresholds for iron deficiency.