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The MedLey trial was funded by a National Health and Medical Research Council Grant (#APP1050949 to KJM). The InterAct project was funded by the EU FP6 programme (grant number LSHM_CT_2006_037197 to NJW). Biomarker measurements for carotenoids were funded jointly by the InterAct project, the EPIC-CVD project, and the MRC Cambridge Initiative (RG71466 and SJAH/004 to NJW, NGF, JD, AB). EPIC-CVD has been supported by the UK Medical Research Council (MR/L003120/1 to ASB and JD), the British Heart Foundation (RG/13/13/30194 and RG/18/13/33946 to ASB and JD), the European Commission Framework Programme 7 (HEALTH -F2-2012-279233 to ASB and JD), the European Research Council (268834 to ASB and JD), and the National Institute for Health Research (NIHR; Cambridge Biomedical Research Centre at the Cambridge University Hospitals NHS Foundation Trust, BRC-1215-20014 to ASB and JD). This work was also supported by Health Data Research UK (to ASB and JD), which is funded by the UK Medical Research Council, Engineering and Physical Sciences Research Council, Economic and Social Research Council, Department of Health and Social Care (England), Chief Scientist Office of the Scottish Government Health and Social Care Directorates, Health and Social Care Research and Development Division (Welsh Government), Public Health Agency (Northern Ireland), and Wellcome. The coordination of EPIC is financially supported by the International Agency for Research on Cancer (IARC) and also by the Department of Epidemiology and Biostatistics, School of Public Health, Imperial College London which has additional infrastructure support provided by the NIHR Imperial Biomedical Research Centre (BRC). The national cohorts are supported by: Danish Cancer Society (Denmark); Ligue Contre le Cancer, Institut Gustave Roussy, Mutuelle Ge'ne'rale de l'Education Nationale, Institut National de la Sante' et de la Recherche Me'dicale (INSERM) (France); German Cancer Aid, German Cancer Research Center (DKFZ), German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE), Federal Ministry of Education and Research (BMBF) (Germany); Associazione Iblea Ricerca Epidemiologica (A.I.R.E. - ONLUS) Ragusa, Associazione Italiana per la Ricerca sul Cancro-AIRC-Italy, Compagnia di San Paolo, National Research Council and Sicilian Regional Government (Italy); Dutch Ministry of Public Health, Welfare and Sports (VWS), Netherlands~Cancer Registry (NKR), LK Research Funds, Dutch Prevention Funds, Dutch ZON (Zorg Onderzoek Nederland), World Cancer Research Fund (WCRF), Statistics Netherlands (The Netherlands); Health Research Fund (FIS) -Instituto de Salud Carlos III (ISCIII), Regional Governments of Andaluci'a, Asturias, Basque Country, Murcia and Navarra, and the Catalan Institute of Oncology -ICO (Spain); Swedish Cancer Society, Swedish Research Council and County Councils of Skane and Vasterbotten (Sweden); Cancer Research UK (14136 to NJW; C8221/A29017), Medical Research Council (1000143 to NJW; MR/M012190/1) (United Kingdom). JGS was supported by the MRC PhD studentship. NJW, NGF, and FI acknowledge funding from the Medical Research Council Epidemiology Unit (MC_UU_00006/1, MC_UU_00006/3); and NJW, NGF and AK from the NIHR Cambridge Biomedical Research Centre (IS-BRC-1215-20014; NIHR203312). NGF and JD are NIHR Senior Investigators. JD holds a British Heart Foundation Professorship. MBS acknowledges funding from the Federal Ministry of Education and Research and the State of Brandenburg (DZD grant 82DZD03D03). JSZ has received funding from Westlake University (No YSYY0209) and European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 701708. PWF has received funding from Novo Nordisk, Swedish Diabetes Association, Swedish Heart-Lung Foundation, European Research Council. ER has received funding from Imperial College Biomedical Research Centre. The funders of the studies had no role in the study design, data collection, data analysis, data interpretation, or report preparation. Trial Australian., Background Self-reported adherence to the Mediterranean diet has been modestly inversely associated with incidence of type 2 diabetes (T2D) in cohort studies. There is uncertainty about the validity and magnitude of this association due to subjective reporting of diet. The association has not been evaluated using an objectively measured biomarker of the Mediterranean diet. Methods and findings We derived a biomarker score based on 5 circulating carotenoids and 24 fatty acids that discriminated between the Mediterranean or habitual diet arms of a parallel design, 6-month partial-feeding randomised controlled trial (RCT) conducted between 2013 and 2014, the MedLey trial (128 participants out of 166 randomised). We applied this biomarker score in an observational study, the European Prospective Investigation into Cancer and Nutrition (EPIC)-InterAct case-cohort study, to assess the association of the score with T2D incidence over an average of 9.7 years of follow-up since the baseline (1991 to 1998). We included 22,202 participants, of whom 9,453 were T2D cases, with relevant biomarkers from an original case-cohort of 27,779 participants sampled from a cohort of 340,234 people. As a secondary measure of the Mediterranean diet, we used a score estimated from dietary-self report. Within the trial, the biomarker score discriminated well between the 2 arms; the cross-validated C-statistic was 0.88 (95% confidence interval (CI) 0.82 to 0.94). The score was inversely associated with incident T2D in EPIC-InterAct: the hazard ratio (HR) per standard deviation of the score was 0.71 (95% CI: 0.65 to 0.77) following adjustment for sociodemographic, lifestyle and medical factors, and adiposity. In comparison, the HR per standard deviation of the self-reported Mediterranean diet was 0.90 (95% CI: 0.86 to 0.95). Assuming the score was causally associated with T2D, higher adherence to the Mediterranean diet in Western European adults by 10 percentiles of the score was estimated to reduce the incidence of T2D by 11% (95% CI: 7% to 14%). The study limitations included potential measurement error in nutritional biomarkers, unclear specificity of the biomarker score to the Mediterranean diet, and possible residual confounding. Conclusions These findings suggest that objectively assessed adherence to the Mediterranean diet is associated with lower risk of T2D and that even modestly higher adherence may have the potential to reduce the population burden of T2D meaningfully., EU FP6 programme LSHM_CT_2006_037197, European Commission Framework Programme 7 HEALTH-F2-2012-279233, European Research Council (ERC), World Health Organization, Health Research Fund (FIS) - Instituto de Salud Carlos III (ISCIII), Catalan Institute of Oncology - ICO (Spain), Spanish Government 701708, Marie Curie Actions 701708, European Union's Horizon 2020, Marie Sklodowska-Curie 701708