BMJ pulls widely cited apple cider vinegar research

BMJ Group has retracted research suggesting that small daily quantities of apple cider vinegar might help people who are overweight or obese to lose weight.

The small clinical trial was published in the open access journal BMJ Nutrition, Prevention & Health in March 2024 and its findings press released. The study findings generated widespread international attention at the time, and continue to be frequently referred to in media coverage.

The retraction was prompted by concerns raised about the quality of the work, including the approach to statistical analysis of the data; implausible statistical values; the reliability of the raw data; inadequate reporting of methods; and lack of prospective trial registration, which breaches BMJ Group's editorial policy.

Initially, concerns were raised in critiques of the study, some of which were published as letters in the journal. But after review by BMJ Group's content integrity team, the study was referred to statistical experts to evaluate its reliability. This included attempts to replicate the results and examine the authenticity of the underlying data supplied by the authors. 

It wasn't possible for the statisticians to replicate the results and multiple analytical errors were identified. There were also irregularities in the data set, and their report, which is appended to the retraction notice, concluded that the data collected from each participant would require further independent scrutiny. 

The authors said that the identified errors were honest mistakes, but they agree with the decision to retract the study.

Tempting though it is to alert readers to an ostensibly simple and apparently helpful weight loss aid, at present the results of the study are unreliable, and journalists and others should no longer reference or use the results of this study in any future reporting."

Dr. Helen Macdonald, Publication Ethics and Content Integrity Editor, BMJ Group

She added: "This retraction reflects our strategic and proactive approach to investigating concerns raised about the content we publish. We act where necessary in the interests of openness and the importance of correcting the scientific record.

"While we deal with allegations as swiftly as possible, it's very important that due process is followed. Investigations are often complex. This one involved detailed scrutiny of data and correspondence with researchers, institutions, and other experts, for example. Reaching a sound and fair and final decision can therefore take several months."

Commenting on the decision to publish the study despite the lack of trial registration, Professor Martin Kohlmeier, editor in chief of BMJ Nutrition Prevention & Health, explained: "In hindsight, this was the wrong decision to make. But the authors come from a scientific environment that is underrepresented in nutritional research and the journal aims to prioritise high quality evidence, which usually comes from clinical trials. 

"These are relatively unusual in nutritional research as they can be challenging to undertake because of the numbers of participants and time needed to obtain meaningful results."