[1] UCL, Canc Res UK Hlth Behav Res Ctr, Dept Epidemiol & Publ Hlth, London WC1E 6BT, England
[2] NW London Hosp NHS Trust, Northwick Pk Hosp, Dept Clin Biochem, Harrow, Middx, England
[3] London Sch Hyg & Trop Med, Noncommunicable Dis Epidemiol Unit, Canc Res UK Canc Survival Grp, Dept Epidemiol & Populat Hlth, London WC1, England
[4] Univ Coll London Hosp, Dept Gastroenterol, London, England