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.