Abstract Context The influence of dietary pattern trajectories from youth to adulthood on adult glucose metabolism is unknown. Objective To identify dietary pattern trajectories from youth to adulthood and examine their associations with adult impaired fasting glucose (IFG). Methods Thirty-one-year population-based cohort study among 1007 youths aged 3-18 years at baseline in Finland. Diet intake was assessed in 1980, 1986, 2001, 2007, and 2011. Group-based trajectory modelling was used to identify dietary pattern (identified by factor analysis) trajectories. Adult IFG was measured by the latest available data from 2001, 2007, and 2011. Results Among 1007 participants, 202 (20.1%) developed IFG and 27 (2.7%) developed type 2 diabetes in adulthood (mean follow-up of 30.7 years; mean [SD] age 40.5 [5.0] years). Three dietary patterns were identified at baseline and were retained in 1986 and 2001: “Traditional Finnish,” “High carbohydrate,” and “Vegetables and dairy products.” Three different patterns were identified in 2007, which remained similar in 2011: “Traditional Finnish and high carbohydrate,” “Red meat,” and “Healthy.” Trajectories of increased or stably medium “red meat” pattern scores from youth to adulthood were detrimentally associated with IFG (relative risk 1.46, 95% CI 1.12-1.90 for Medium (M)-stable/M-large increase vs low-stable trajectory) after adjusting for confounders. This association was slightly reduced after further adjusting for long-term dietary fiber intake. Conclusion Trajectories of an increased or stably moderate adherence to a “red meat” dietary pattern from youth to adulthood are associated with higher risk of adult IFG. This association is partly explained by low dietary fiber intake.