AbstractBackgroundType 2 diabetes mellitus (T2DM) risk is markedly higher in UK South Asians (SA) and African Caribbeans (AC) compared to Europeans. Explanations for this excess are unclear. We therefore compared risks and determinants of T2DM in first- and second-generation (born in the UK) migrants, and in those of mixed ethnicity populations.MethodsData from the UK Biobank, a large population-based cohort of volunteers aged 40-69, were used. T2DM was assigned using self-report and glycated haemoglobin. Ethnicity was self-assigned. Using logistic regression and mediation analysis, we compared T2DM between first- and second-generation migrants, and between mixed European/South Asians (MixESA), or mixed European/African Caribbeans (MixEAC) with both Europeans and SA or AC respectively.ResultsT2DM prevalence was three to five times higher in SA and AC compared with Europeans [OR (95%CI): 4·80(3·60,6·40) and 3·30(2·70,4·10) respectively]. T2DM was 20-30% lower in second versus first generation SA and AC migrants [0·78(0·60,1·01) and 0·71(0·57,0·87) respectively]. T2DM in mixed populations was lower than comparator ethnic minority groups [MixESA versus SA 0·29(0·21,0·39), MixEAC versus AC 0·48(0·37,0·62)] and higher than Europeans, in MixESA 1·55(1·11, 2·17), and in MixEAC 2·06 (1·53, 2·78). Improved adiposity patterns in second generation migrants made an important contribution to risk reduction. Greater socioeconomic deprivation accounted for 17% and 42% of the excess risk of T2DM in MixESA and MixEAC compared to Europeans, respectively.ConclusionExcess T2DM risks in South Asians and African Caribbeans compared with Europeans in the UK are attenuated by ∼20% in second-generation migrants, demonstrating the marked benefits of favourable changes in environmental risk factors. T2DM prevalence in people of mixed ethnicity was also raised compared with Europeans, but considerably less than in the ethnic minority group; persistent socioeconomic disadvantage accounted for some of the residual excess.