To examine the associations between change in plant-centered diet quality and type 2 diabetes risk and change in body size.A prospective study conducted in the U.S. enrolled adults ages 18-30 years in 1985-1986 (examination year [Y0]) and followed them through 2015-2016. We analyzed the associations between change in plant-centered diet quality over 20 years (Y0-Y20) and diabetes (Y20-30; n = 2,534) and change (Y0-Y20 and Y20-30) in BMI, waist circumference (WC), and weight (n > 2,434). Plant-centered diet quality was measured using the A Priori Diet Quality Score (APDQS); a higher score favors nutritionally rich plant foods. Cox regression models were used to assess diabetes risk, and linear regression models were used to examine change in body size.During a mean follow-up of 9.3 (± 1.7) years, 206 case subjects with incident diabetes were observed. In multivariable analysis, participants with the largest increase in APDQS over 20 years had a 48% (95% CI 0.31-0.85; P-trend < 0.001) lower risk of diabetes over the subsequent 10 years compared with participants whose score remained stable. Each 1-SD increment in APDQS over 20 years was associated with lower gains in BMI (-0.39 kg/m2; SE 0.14; P = 0.004), WC (-0.90 cm; SE 0.27; P < 0.001), and weight (-1.14 kg; SE 0.33; P < 0.001) during the same period, but not with subsequent changes.Young adults who increased plant-centered diet quality had a lower diabetes risk and gained less weight by middle adulthood.