Aim To explore how behavioural, normative, and control beliefs shape early dietary implementation after bariatric surgery by exploring patient and dietitian perspectives using the theory of planned behaviour framework.

Methods Semi-structured interviews were conducted with 16 patients who had bariatric surgery within the past 12 months and 24 dietitians involved in postoperative care to explore beliefs about dietary recommendations. Interviews explored five key dietary recommendations with questions guided by the theory of planned behaviour framework. Data were analysed using a deductive and inductive approach.

Results Behavioural beliefs indicated a complex relationship between perceived benefits and cognitive demands, with protein intake and symptom management emerging as key motivators. Normative beliefs highlighted the dual influence of supportive family environments and challenging workplace contexts, particularly regarding modified eating behaviours. Control beliefs demonstrated an integrated pattern where physical constraints, psychological factors, and environmental demands seemed to intersect to influence dietary implementation. Of the five key dietary recommendations, patients faced unique challenges with vitamin supplementation due to the interaction between physical limitations and delayed reward perception. The cognitive burden of managing new dietary behaviours appeared to be a determinant of adoption across belief domains.

Conclusion Early dietary changes after bariatric surgery seem to be influenced by psychological, social, and physical factors beyond education. Future interventions should aim to incorporate strategies to reduce cognitive load, address competing social pressures, and account for the temporal aspects of health-protective behaviours. Theory-based interventions that integrate implementation intention techniques, social support optimisation, and temporal self-regulation strategies may enhance dietary adherence during this critical adaptation phase.