Abstract: We included 38 studies involving 9445 participants. Among studies that measured diet adherence outcomes between an intervention group and a control/usual care group, 32 out of 123 diet adherence outcomes favoured the intervention group, 4 favoured the control group whereas 62 had no significant difference between groups (assessment was impossible for 25 diet adherence outcomes since data and/or statistical analyses needed for comparison between groups were not provided). Interventions shown to improve at least one diet adherence outcome are: telephone follow-up, video, contract, feedback, nutritional tools and more complex interventions including multiple interventions. However, these interventions also shown no difference in some diet adherence outcomes compared to a control/usual care group making inconclusive results about the most effective intervention to enhance dietary advice. The majority of studies reporting a diet adherence outcome favouring the intervention group compared to the control/usual care group in the short-term also reported no significant effect at later time points. Studies investigating interventions such as a group session, individual session, reminders, restriction and behaviour change techniques reported no diet adherence outcome showing a statistically significant difference favouring the intervention group. Finally, studies were generally of short duration and low quality, and adherence measures varied widely. There is a need for further, long-term, good-quality studies using more standardized and validated measures of adherence to identify the interventions that should be used in practice to enhance adherence to dietary advice in the context of a variety of chronic diseases.

Plain-language summary: We identified 38 studies involving 9445 participants examining several types of interventions for enhancing adherence to dietary advice for preventing and managing many chronic diseases. The main chronic diseases involved were cardiovascular diseases, diabetes, hypertension, and renal diseases. Interventions shown to improve at least one diet adherence outcome are: telephone follow-up, video, contract, feedback, nutritional tools and more complex interventions including multiple interventions. However, these interventions also showed no difference in some diet adherence outcomes compared to a control/usual care group making the results inconclusive about the most effective intervention to enhance dietary advice. Interestingly, all studies including clients with renal diseases reported at least one diet adherence outcome showing a statistically significant difference favouring the intervention group, no matter which intervention was provided. The majority of studies reporting a diet adherence outcome favouring the intervention group compared to the control/usual care group in the short-term also reported no significant effect at later time points. Studies investigating interventions such as a group session, individual session, reminders, restriction and behaviour change techniques reported no diet adherence outcome showing a statistically significant difference favouring the intervention group. Finally, interventions were generally of short duration, studies used different methods for measuring adherence and the quality of the studies was generally low.