Background Medical nutrition therapy is fundamental for diabetes management, however there is a lack of evidence supporting an ideal recommended carbohydrate intake for maintaining optimal glycaemia in individuals living with type 1 diabetes (T1D). Adults with T1D are increasingly drawn to very low carbohydrate (≤ 50 g/day or < 10% total energy intake) and low carbohydrate diets (< 130 g/day or < 26% total energy intake) because of the reported positive impact on both physical health and psychological well-being. Current evidence regarding the effectiveness on glycaemia and the lived experience by adults with T1D when using these diets is limited. This mixed methods systematic review was undertaken to examine the effectiveness of very low and low carbohydrate diets on HbA1c and explore the lived experience of adults with T1D who have followed these dietary regimens.

Methods Seven databases (MEDLINE, Embase, CINAHL, Cochrane CENTRAL, Informit Health Collection, Web of Science, and PsycInfo) were searched from inception to 1 October 2023. Quality assessment of the included studies was undertaken using the JBI’s critical appraisal checklists. Separate quantitative and qualitative synthesis was performed, and findings were integrated for the purpose of comparison and complementarity.

Results Seventeen studies of varying methodologies were included. Findings from quantitative research were inconclusive in determining the effectiveness of very low and low carbohydrate diets on HbA1c levels. Qualitative data synthesis identified four themes [1) Motivation to follow the diet, 2) Health benefits of the diet, 3) Challenges of the diet, and 4) Limited information (participants knowledge, information sources) about the diet] that influenced adherence to very low and low carbohydrate diets. Through the integration of results from selected studies, it was evident that there were conflicting outcomes between quantitative and qualitative studies.

Conclusions There is little evidence to indicate that very low and low carbohydrate diets improve HbA1c in adults with T1D. However, this goes against the reported lived experiences of participants. This review highlights the insufficiency of robust evidence on this topic. Future research involving larger participant samples over longer durations are needed to provide more definitive evidence in relation to the efficacy of these diets and into the enablers and barriers experienced when using a very low or low carbohydrate diet in order to provide support to adults with T1D.

Systematic review registration PROSPERO CRD42023482800.