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Background: Plasma linoleic acid (C18:2n-6, LA) has been associated with a lower risk of type 2 diabetes mellitus (T2DM) in prospective cohort studies, but associations of dietary LA with T2DM risk are inconsistent.Objective: To study both plasma and dietary LA in relation to incident T2DM in post-myocardial infarction (MI) patients who received state-of-the-art drug treatment.Methods: We included 3,377 patients (80% male) from the Alpha Omega Cohort aged 60-80 y who had an MI <10 y before study enrollment and who were initially free of T2DM. At baseline, LA was measured in plasma cholesterol esters as a proportion of total fatty acids. Dietary LA intake was estimated from a validated 203-item food-frequency questionnaire that had been specifically designed for measuring fatty acid intake. Incident T2DM was ascertained through self-reported physician diagnosis and medication use. HRs for incident T2DM in quintiles of plasma LA and dietary LA were obtained from multivariable Cox models that included age, sex, lifestyle and dietary factors. For dietary LA, we used a substitution model in which LA was iso-calorically replaced with the sum of saturated plus trans fatty acids.Results: From the lowest to the highest quintile, dietary LA ranged from 3.5 to 8.8 percent of energy and the proportion of plasma LA ranged from 43.7 to 56.3%. Plasma and dietary LA were weakly correlated (Spearman r = 0.14, P <.0001). During a median follow-up of 3.4 years, 171 cases of T2DM occurred. Plasma LA was associated with a significantly lower risk of T2DM (HRQ5vsQ1: 0.45; 95% CI: 0.27 - 0.76; P for trend = 0.002). Dietary LA was not associated with T2DM risk (HRQ5vsQ1:0.97; 95% CI: 0.53 - 1.76; P for trend = 0.85).Conclusion:Plasma LA but not dietary LA was associated with lower T2DM risk in post-MI patients. The utility of plasma LA as a biomarker of dietary LA warrants further investigation.