MRC Epidemiology Unit & UKCRC Centre for Diet and Activity Research (CEDAR), School of Clinical Medicine, University of Cambridge, Box 285 Institute of Metabolic Science, Cambridge Biomedical Campus, Cambridge, CB2 0QQ, UK. lf354@medschl.cam.ac.uk. Norwich Medical School and CEDAR, University of East Anglia, Norwich, NR4 7TJ, UK. Unaffiliated, Cambridge, UK. Department of Social Policy and Intervention, University of Oxford, Barnett House, 32 Wellington Square, Oxford, OX1 2ER, UK. MRC/CSO Social and Public Health Sciences Unit and Centre for Research on Environment, Society and Health, University of Glasgow, 200 Renfield Street, Glasgow, G2 3QB, UK. MRC Epidemiology Unit & UKCRC Centre for Diet and Activity Research (CEDAR), School of Clinical Medicine, University of Cambridge, Box 285 Institute of Metabolic Science, Cambridge Biomedical Campus, Cambridge, CB2 0QQ, UK.