British Journal of Health Psychology (2017) © 2017 The British Psychological Society www.wileyonlinelibrary.com Reducing fat intake using implementation intentions: A meta-analytic review Irene Vil  a* , Isabel Carrero and Raquel Redondo Comillas Pontifical University-ICADE, Madrid, Spain Purpose. To study the efficacy of forming implementation intentions for fat intake reduction as well as possible moderating variables. Methods. Systematic review and meta-analysis of 12 empirical studies (N = 3,323) published in English and Spanish in the Web of Science (Core Collection) and MEDLINE (1990–January 2016) databases. Results. This study found that the efficacy of planning interventions on fat consumption reduction was higher than expected, as a moderate overall effect of implementation intentions was observed (d= 0.488). Moreover, planning for a fat intake reduction seems to be more powerful for men than for women (b = .623; p = .025) and in cases where there is no monitoring during the intervention (d= 0.671 vs. d= 0.231). Conclusions. Previous research was sceptical of the efficacy of planning in the case of avoiding goals in healthy eating. However, our results show that planning is an efficient intervention that can be used by health education programmes to reduce fat intake and, therefore, increase citizen well-being. These results also support the existence of a key variable in the implementation intentions process, that is, goal complexity, and the presence of two moderating variables, that is, gender and monitoring. Statement of contribution What is already known on this subject?  Implementation intentions are action plans subordinate to goal intentions that specify the ‘when, where, and how’ of responses leading to goal attainment.  In healthy eating, the average effect of forming implementation intentions is small to medium, but this efficacy changes depending on the type of intended behaviour. Past evidence shows that the effect size seems to be lower when the intervention aims at reducing unhealthy behaviours versus promoting healthy behaviours. What does this study add?  Forming implementation intentions is an efficient intervention to reduce fat intake with a medium overall effect.  The efficacy of this intervention is increased when men are targeted and when there is no monitoring during the process.  This study introduces a new line of research focused on the study of the effect of planning on complex goals. Healthy eating is becoming an increasingly important behaviour due to problems resulting from poor dietary habits. Prior to 1980, obesity rates were well below 10% *Correspondence should be addressed to Irene Vil a, Alberto Aguilera 23, 28015 Madrid, Spain (email: ivila@comillas.edu). DOI:10.1111/bjhp.12230 1