Endothelial dysfunction is an early marker of cardiovascular disease risk and an important indicator of vascular health (1). Dietary modification has the potential to improve endothelial function (2), however, the long-term success of dietary interventions depends on their acceptability and feasibility. Understanding research participants’ experiences of a dietary intervention can provide insights into factors influencing adherence and inform the sustainability of dietary change. This qualitative phenomenological study aimed to explore participants’ experiences of a 3-month dietetic counselling intervention with a follow-up at 6 months, after following either a high fibre diet (HFibre) or a lower carbohydrate high unsaturated fat (LCHFat) diet designed to improve endothelial function. Fourteen in-depth semi-structured interviews were completed with 15 participants (HFibre n = 7; LCHFat n = 7) via videoconference (Zoom Video Communications, San Jose, CA, USA) following completion of the intervention. Interview questions were informed by the Capability, Opportunity, Motivation, and Behaviour (COM-B) behaviour change framework to explore factors influencing dietary adherence and behavioural modification. Interviews were audio-recorded, transcribed verbatim, and analysed using reflexive thematic analysis as described by Braun and Clarke (3). The primary researcher coded all transcripts. Two additional researchers independently coded half of the transcripts each and developed themes collaboratively. These themes were triangulated and refined with those developed by the primary researcher. Analysis focused on identifying patterns of meaning related to intervention feasibility, acceptability, and potential for clinical implementation. Four major themes were identified: 1) Social environments influenced adherence, with household participation facilitating dietary changes while social occasions often presented challenges; 2) Clear guidance helped participants implement the diet, with study resources, dietitian support, and self-monitoring reducing uncertainty and promoting accountability; 3) Flexibility and pre-planning influenced long-term adherence, with meal preparation and personalised strategies supporting adherence, whereas lifestyle factors including work commitments and travel created barriers; 4) Perceived outcomes and commitment to the study influenced ongoing adherence, with anticipated and realised health benefits strengthening motivation, while unmet expectations reduced it. A sense of responsibility to the broader research project also encouraged adherence. Incorporating behavioural strategies that address these factors may improve the acceptability, feasibility, and sustainability of dietary interventions for vascular health. These findings provide insights to inform the design and implementation of future dietary interventions and clinical practice.