Reliability of studies published as SR/MA on nutrition in cancer prevention - a systematic survey

Background

In several fields of medicine, the quality of studies published as systematic reviews/meta-analyses (SR/MAs) is low. Similar problems may exist for SR/MA on nutrition in cancer prevention. We aimed to assess overall quality and risk of bias (RoB) of studies published as SR/MA on nutritional interventions in cancer prevention with two instruments: AMSTAR 2 ('a measurement tool to assess systematic review 2') and ROBIS ('Risk of Bias in Systematic Reviews') respectively.

Methods

Following a systematic search in 3 databases we included studies identified as SR/MA published between 2010 and 2018 assessing any nutritional interventions in cancer prevention in the general population or among people with cancer risk (Protocol in PROSPERO CRD42019121116). All the steps of study selection and data extraction were done by two independent reviewers with conflicts solved by discussion or by the third reviewer.

Results

We focused on a subsample of 101 SR/MA randomly selected from 737 included SR/MA. Included SR/MA on average searched 2 databases with Medline in 98% and included cohort studies (93%). They focused on specific food (36%), specific nutrients (27%) or beverages (24%, mostly tea and coffee). The assessment using AMSTAR 2 tool indicated that 93% of SR/MA had no pre-specified methodology, in 77% - research questions and inclusion criteria did not include the components of PICO, RoB assessment of primary studies was not used or did not contain all elements (87%) and RoB was not accounted for in the interpretation of the results (75%). Overall, the quality of 97% of studies was assessed as critically low. In the ROBIS tool for 97% of included studies, the overall high risk of bias was detected. The most important methodological flaws in ROBIS were similar to identified in AMSTAR tool.

Conclusions

Poor quality of SR/MA due to flawed methodology may lead to many concerns and mislead public media and consumers.

Key messages

Poor quality of SR/MA due to flawed methodology may lead to many concerns and mislead public media and consumers. The studies published as SR/MAs addressing nutrition for cancer prevention have major flaws, which limit the reliability of their conclusions.

Nutritional “hot topics” in studies published as SR/MA in cancer prevention between 2010 and 2018

Background

The number of systematic reviews and meta analysis (SR/MA) is increasing, also in the field of nutrition.

Objectives

The aim of this work was to analyse the frequency of exploring the effects of specific nutritional interventions in studies published as SR/MA focused on cancer prevention.

Methods

The main study was a systematic survey of SR/MA on nutrition in cancer prevention published between 2010 and 2018 identified through search in 3 databases. The protocol of the study was registered (CRD42019121116 in PROSPERO). All steps of the survey were done by two reviewers independently with any conflicts resolved by discussion or by the third reviewer. The quality and risk of bias were assessed using AMSTAR 2 and ROBIS tools amid subsample of 101 SR/MA. We grouped all dietetic interventions by year and analysed frequency.

Results

Among 737 SR/MA meeting the inclusion criteria, an average number of publications was 83/year ranging from 38 (in 2010) to 118 (in 2015). The most commonly studied food categories were: non-alcoholic beverages (127 publications in total; on average 14/year) and vitamins and/or minerals (157 publications in total; on average 17/ year) with coffee as one of the most popular (on average 7 SR/MA per year). The most commonly analysed vitamins were vitamin D, C, and E, while amid minerals calcium and selenium. Red meat and processed meat were comparably popular with about 4 dedicated publications every year. SR/MA about fruits and vegetables and alcohol appeared on average about 8 times every year. The overall quality of SR/MA assessed in a subsample of randomly selected 101 of included articles was poor (97% were classified as having a low quality).

Conclusions

In SR/MA published between 2010 and 2018, each food category appears many times, while for example coffee - many times every year. Detailed analysis per food item may help identify potential research waste in the field of nutrition in cancer prevention.

Key messages

Currently, coffee is the ‘hottest’ nutritional topic in cancer prevention. The general quality of SR/MA published in the field of cancer prevention is low.