The effect of whole-grain (WG) versus refined-grain (RG) diets on glucose-stimulated insulin secretion (GSIS) and β-cell function is unclear.In a double-blind crossover randomized controlled trial, 13 prediabetic adults (37.2 ± 1.8 y, BMI: 33.6 ± 1.4 kg m , 2 h glucose: 146.9 ± 11.6 mg dL ) are provided isocaloric-matched WG and RG diets for 8-weeks each, with an 8-10 week washout between diets. Glucose, insulin, and C-peptide are studied over 240 min following a 75 g OGTT. Incretins (GLP-1 and GIP), PYY, and total ghrelin are assessed at 0, 30, and 60 min. Mixed-meal diets for carbohydrate (54%), fat (28%), and protein (18%) contain either WG (50 g/1000 kcal) or equivalent RG.Both diets induce fat loss (≈2 kg). While neither diet impacts early phase GSIS, the WG diet increases total GSIS (iAUC of C-peptide /Glc , p = 0.02) and β-cell function (disposition index; GSIS × insulin sensitivity, p = 0.02). GIP and PYY are unaltered by either diet, but GLP-1 is higher at 30 min following RG versus WG (p = 0.04). Ghrelin levels are higher at 60 min of the OGTT following both interventions (p = 0.01).A WG-rich diet increases β-cell function independent of gut hormones in adults with prediabetes.