Endothelial dysfunction is an early indicator of vascular impairment and is associated with increased cardiovascular disease (CVD) risk (1). Dietary modifications may improve endothelial function, and observational evidence suggests that higher dietary fibre intake is associated with lower CVD risk (2). While dietary interventions have demonstrated benefits for cardiovascular risk factors, the effects of increasing dietary fibre intake through a whole-of-diet approach on endothelial function remain unclear. This study compared the effects of a high fibre diet (HFibre) versus a lower-carbohydrate, high unsaturated fat (LCHFat) diet on endothelial function in men aged ≥45 years and postmenopausal women with habitual dietary fibre intakes below 20 g/day. A parallel randomised controlled trial allocated participants to either a HFibre (≥40 g/day dietary fibre) or a LCHFat diet (~40% total energy from fat, <10% total energy from saturated fat) for 3 months, with a follow up at 6 months. Participants attended four consultations with the study dietitian during the intervention period. Dietary data were collected using a 4-day food diary at study visits, and 5 x 24-hour diet recalls between study visits across the 6 months. The primary outcome was endothelial function, assessed using brachial artery flow mediated dilation (FMD). Secondary outcomes included anthropometry, blood pressure, fasting glucose, and lipids. Changes from baseline to 3 months were analysed using paired t-tests, and between group differences with repeated measures ANOVA. Fifty-two participants were randomised (n = 26 each group), with 48 participants completing the 3-month dietary intervention (37 women, 11 men). At 3 months, fibre intake increased significantly in the HFibre group (+23.5 ± 10.9 g/day, p < 0.001), but remained <20 g/day in the LCHFat group, while total energy intake remained unchanged in both groups. FMD increased significantly in the HFibre group (+1.37 ± 2.04%, p = 0.004), but not in the LCHFat group (p = 0.285), with no between-group differences. Participants in the LCHFat group had a reduction in body weight (-1.16 ± 2.70 kg, p = 0.046), fat mass (-1.14 ± 2.47 kg, p = 0.038), and total cholesterol (-0.29 ± 0.65 mmol/L, p = 0.040) at 3 months, but these outcomes were unchanged in the HFibre group. No significant changes were observed for blood pressure, glucose, LDL cholesterol, HDL cholesterol, triglycerides, BMI, waist circumference, fat free mass, skeletal muscle mass, or visceral fat mass in either group. The HFibre dietary intervention significantly improved endothelial function over 3 months, whereas the LCHFat diet reduced body weight, fat mass, and total cholesterol. Although no significant between-group differences were observed for endothelial function, the findings suggest that these dietary patterns may influence cardiovascular risk through different mechanisms. Further investigation of potential mechanistic pathways should be explored.