Wholegrains consumption is advantageous for the prevention of cardiovascular diseases, partly due to their polyphenolic and fibre content. In this study, 14 raw wholegrains (Nigeria and UK grown) were screened for their phenolic acids (PAs) and fibre content. PAs were extracted into soluble and bound fractions and analysed for 26 PAs using Ultraperformance Liquid Chromatography-Tandem Mass Spectrometry (UPLC-MS/MS). Based on the PA screening of raw grains, selected grains from Nigeria were cooked and subjected to in vitro digestions, PA content of cooked and digested samples were determined. Subsequently, freeze-dried digestate of selected grains were fermented over a 24 h period using an in vitro colonic model. Fermented samples were analysed for their PA contents using UPLCMS/MS and their microbial DNA content using 16s rRNA Next Generation Sequencing. Based on PA results from in vitro studies, a selection of commercially obtained pure PAs were used for cell mechanistic studies in HUVEC cells, to see their impact on the nitric oxide pathway as a model of endothelial function. Cells treated with PAs or controls were assessed for their nitric oxide levels using diaminofluorescein-2 diacetate (DAF-2DA) fluorescence probe and superoxide production using ferricytochrome c reduction assay. ELISA methods were used to assay for cGMP production, Akt1 and eNOS activation in treated cells. Lastly, scratch assay was performed to assess the impact of treatments on wound healing in HUVEC monolayer using microscopic techniques. Total PAs in raw grains ranged from 22.2.5 ± 34.9 ng/mg to 132.2 ± 12.1 ng/mg. Cooking and digestion significantly affected the PAs in all grains (p < 0.05). Colonic showed release of soluble PA, which peaked at 4-24 h in grains. 3-hydroxybenzaldehyde had a positive correlation with the abundance of Dorea and the mucus degrader Akkermansia (p < 0.05), whereas hydroferulic and isoferulic acids showed a negative correlation with Oscillospira and Ruminococcaceae (p < 0.05), respectively. 4-hydroxybenzoic acid (4HBA) and 3-hydroxybenzoic acid (3HBA) treatments led to significant increase of nitric oxide levels compared to control (p < 0.001). There was significantly less superoxide levels in p-coumaric acid (PCA), 4-HBA and 3-HBA treated cells compared to control treated cells (p<0.01). The results show that PA treatment in cells did not significantly affect cGMP, activated Akt1 and eNOS levels compared to control cells. PA treated cells did not show significant difference in wound gap closure in HUVEC cells compared to control (p > 0.05) but gap closure in PA treated cells were significantly different (p < 0.05) between 6-12 h when compared to positive and negative controls. These data suggest that cooking and in vitro digestion impact the release of bound PAs from the grains studied, and these PA impacted the colonic microbial ecology and vice versa. PA treatment positively affected some indices of endothelial function. These results provide information for the design of an in vivo study to confirm these effects.