“Eating under protocol”: Acceptability of controlled feeding and vitamin B12 supplementation among pregnant women in Tanzania: a phenomenological qualitative study

Abstract Background Controlled feeding interventions are widely used to assess nutritional effects; however, their success depends not only on biomedical efficacy but also on participant acceptability. In low-resource settings, limited attention has been given to how pregnant women experience such interventions within their social and cultural contexts. This study explored the acceptability and lived experiences of a controlled feeding intervention embedded within a vitamin B12 supplementation trial among pregnant women in Bagamoyo, Tanzania. Methods This was phenomenological qualitative research design to explore and describe the lived experiences of participants involved in a controlled feeding clinical trial. Data was collected using in-depth interviews (IDIs), focus group discussions (FGDs), and key informant interviews (KIIs) with study participants and clinical trial staff. Data were analysed thematically using the Theoretical Framework of Acceptability (TFA), encompassing affective attitude, intervention coherence, perceived effectiveness, burden, opportunity costs, ethicality, and self-efficacy. Results Acceptability was dynamic and evolved over time through participants’ interaction with the intervention and its context. Positive emotional responses were shaped by supportive care and perceived safety, while improved understanding strengthened trust and engagement. Perceived health and nutritional benefits reinforced adherence. However, participants experienced challenges related to dietary restrictions, residential facility stays, and opportunity costs, including family and economic responsibilities. Cultural beliefs influenced early perceptions but diminished with ongoing clarification and positive experiences. Peer support, staff engagement, and experiential learning enhanced participants’ confidence to adapt and remain committed. Conclusions Acceptability of controlled feeding and supplementation interventions is shaped by interconnected emotional, cognitive, and contextual factors that evolve during participation. Designing interventions that prioritise participant support, continuous engagement, and contextual responsiveness is essential for improving feasibility and sustained participation. Integrating acceptability into maternal nutrition research can enhance both implementation and outcomes in similar low-resource settings. Trial registration This qualitative study was embedded within a clinical trial registered at ClinicalTrials.gov (identifier NCT05426395) on 16 June 2022.