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Letter to the Editor on "Systematic Review of Diets Enriched in Oleic Acid and Obesity".

We read, with great diligence, the study done by Tutunchi et al. (1) entitled, “The Effects of Diets Enriched in Monounsaturated Oleic Acid on the Management and Prevention of Obesity: A Systematic Review of Human Intervention Studies.” Since we have worked on a similar meta-analysis paper concerning the effects of canola oil intake on anthropometric measurements (2), we are aware of the challenges in systematic reviews regarding the effects of dietary oil intake on body-composition indices. Despite the interesting subject, the current study lacks some important elements of a full systematic review. Inadequate search strategy and, as a result, ignoring a large number of eligible studies are on top of them. The authors stated that all of the human clinical studies evaluating the effects of diets enriched in oleic acid on the obesity measurements were included in this study. However, our initial search revealed that a large number of studies examining the effects of olive oil on body composition indices were ignored (3–19). After a careful review, we noticed that the first keywords set was searched only in the title (“oleic acid,” “olive oil,” “MUFA,” “monounsaturated fatty acid,” and “Mediterranean diet”; the authors’ methods, Eligibility criteria, page 2), whereas the second keywords set (obesityrelated terms) was searched in the title/abstract. We would like to ask the authors about the rationale behind this uncommon approach. This act narrows the search strategy, capturing a limited number of articles (821 papers) in their initial search. This methodological fault has been addressed in other letters to the editor as well (20, 21). Additionally, we observe that the anthropometric measurements might be presented as secondary outcomes; thus, they are not always shown in the title/abstract. In this regard, we would suggest that the authors use the keywords related to “clinical trials” in their search strategy as an independent set of keywords from the first one to find the greatest number of relevant papers. Moreover, the authors did not hand-search the references of the included studies in their systematic review, which is a practical method to cover the limitations that can arise from a defective search strategy (22). Second, there is evidence of bias in including the potential studies. We believe that the used keywords are not satisfactory enough to capture all of the potential literature meeting the study inclusion criteria. The authors have focused on the most popular vegetable oils high in oleic acid, including olive and some other dietary oils, while some other less popular vegetable oils high in oleic acid have been ignored. For instance, the authors overlooked studies that investigated the effects of hazelnut-enriched diets; however, research concerning the fatty acid composition of plant-based oils has indicated that the oleic acid content of hazelnut oil is even greater than in olive oil (23). Thus, at least 9 studies have been missed until 2016 according to the results of a meta-analysis of hazelnut consumption and lipid profile and body weight (24). Surprisingly, the authors missed presenting a key element of a successful systematic review. Quality assessment of the included studies is considered to be a crucial component of a full systematic review (25, 26). Systematic reviews are not only used to identify and critically evaluate the results of the included studies but also to provide the effectiveness of interventions considering the quality of included evidence (27). Numerous tools have been presented for the quality assessment of clinical trials, including Cochrane Collaboration tools (28). Although systematic reviews are considered to be reliable tools to summarize evidence, researchers should take great caution while conducting a systematic review. In conclusion, the current review suffers from some methodological issues, including an incomplete search strategy, missing a large number of publications, and ignoring an appraisal of included studies. In this regard, we would like to ask the authors to address our concerns.