Background: Postprandial vascular endothelial dysfunction is an early marker of atherosclerosis. Meal protein has been reported to reduce endothelial dysfunction in adults and the effect could be mediated by the amino acid content. Objective: This trial aims to assess the effect of a specifically designed plant protein blend that contains high leucine, arginine and cysteine on postprandial endothelial function in the elderly. Methods: In a randomized, double-blind, 3-period crossover (2-wk washout), controlled trial, we compared the vascular effects of 3 high-saturated-fat high-sucrose (HFHS) meals based on either our specific plant-protein blend, milk protein, or without added protein. The trial was conducted on 29 healthy adults >65y presenting at least 2 cardiometabolic risk factors. Postprandial vascular function was evaluated at fasting, 3 hours, and 5 hours postprandially, using brachial flow-mediated dilation (FMD), hand microvascular reactivity (by Flowmetry Laser Doppler, FLD), and finger reactive hyperemia index (using Peripheral Arterial Tonometry, RHI). Immune cells count and gene expression in peripheral blood mononuclear cells (PBMC) was also assessed postprandially. Data were analyzed using mixed linear models with repeated measurements on participants for meal composition and time of sampling. This trial was registered at clinicaltrials.gov as NCT04923555. Results: FMD iAUC decreased after meals (time effect P<0.01), with no significant differences between meals. RHI also decreased with time (P<0.01). PBMC count and MCP1, IL-1b, IL-6 expression increased after meals showing postprandial endothelial activation (P<0.05). Overall, meal composition had no effect on any of the postprandial changes (Ps>0.10). Conclusions: In healthy adults over 65 years presenting cardiometabolic risk, adding protein to an HFHS challenge meal does not mitigate postprandial impairments in vascular endothelial function and inflammatory activation. Further studies are needed to explore the potential differences with younger adults.