Easy fixes for complex health problems can be tempting — but they rarely pan out. That seems to be the case for the investigators on one clinical trial who claimed consuming apple cider vinegar caused obese teens and young adults to lose weight.
Their article appeared in March 2024 in BMJ Nutrition, Prevention & Health. The journal is retracting the paper “because the authors’ analyses could not be replicated and multiple errors were identified,” according to the retraction notice.
The retraction, dated September 23, comes more than a year after sleuths pointed out some of these errors and other problems with the analysis.
The paper was covered widely when it was published, though some outlets gave context for the unreliability of this type of research when they mentioned it. It has been cited three times, according to Clarivate’s Web of Science.
The study found that overweight and obese people who consumed apple cider vinegar daily for 12 weeks lost an average of 6 to 8 kilograms and their BMI dropped by an average of 2.7 to 3.0 points.
If the claims were true, apple cider vinegar would be 50 percent more effective than GLP-1 agonists such as Ozempic, said James Heathers, a research integrity consultant and director of the Medical Evidence Project (an initiative of The Center for Scientific Integrity, the parent nonprofit of Retraction Watch).
Rony Abou-Khalil, corresponding author on both the paper and the response, did not respond to our requests for comment. He is listed as the head of the department of biology at Holy Spirit University of Kaslik, in Lebanon.
Heathers started looking into the paper shortly after it was published, and published an analysis describing flaws in the study in May 2024. He described the nearly identical age and BMI distributions within each 30-person experimental group as “unlikely.”
Although researchers want each group to have similar characteristics, randomly assigning participants would usually result in more variation between the groups than seen in the study. The age, height and weight are “almost identical between the randomized [groups of] participants,” he wrote. “This extreme uniformity likely represents a failure of randomization, although it is unclear how this arose.”
He went on to describe other flaws with the statistical analysis in the paper, and pointed out the authors didn’t include their original dataset with the publication.
After reading Heathers’ assessment, Vahid Malbouby at Boise State University and Eric Trexler at Duke University wrote to him citing additional problems with the paper. For example, other studies do not support the claim that apple cider vinegar raises basal metabolic rate even at higher doses than used in the study. The three wrote a letter to the journal detailing their concerns, which they submitted in June 2024 and the journal published in February of this year.
Five rapid responses posted on the article in September and October 2024 questioned the statistical analysis conducted in the paper, as well as problems with the study’s design. One such concern is that each experimental group included participants ages 12 to 25 years. This large range of ages including both children and adults could confound the results, “in that BMI and weight change is very different in a 12 year old who is still likely to be growing and a 25 year old,” according to a response authored by Duane D. Mellor, a dietitian at Aston University in England.
Mellor also pointed out “there is no statement of trial registration, no CONSORT flowchart and now [sic] CONSORT checklist.“ Many publishers, including BMJ, require authors to provide these documents before publishing the research to ensure the validity and transparency of the findings.
In December 2024, the journal published a letter from two of the paper’s original three authors. The authors cited papers to justify their original statistical analysis, the biological plausibility of such a large effect, and the unusual data distributions. They concluded, “we assert that the methodologies employed in our study were appropriate and supported by existing literature.”
Given how long ago serious flaws were identified, “it is disappointing that an expression of concern had not been posted to the article,” said Andrew W. Brown, an associate professor at the University of Arkansas for Medical Sciences and director of biostatistics for the Arkansas Children’s Research Institute, who studies pediatrics, nutrition and obesity. “That left a window of time when a reader may have been misled by the work while it was being evaluated.”
In addition to the paper’s analyses not being replicable, the retraction notice identifies other issues as well: “The authors supplied dataset also demonstrated patterns inconsistent with random allocation of participants to treatment groups, improbably small p-values given the limited number of participants included in the study.”
According to the notice, “the authors state that the discrepancies were honest mistakes that arose from version mismatches, data rounding or formatting differences when exporting from statistical software to reporting spreadsheets. However, the authors agree with the decision to retract the work.”
In a statement, Helen Macdonald, publication ethics and content integrity editor at BMJ Group, said: “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.”
Regarding the fact that the journal published the study without clinical trial registration, Martin Kohlmeier, editor-in-chief of BMJ Nutrition Prevention & Health, said in the same statement, “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.”
Flawed clinical trials are not uncommon in the field of nutrition, Brown told us. “I am glad to see the retraction of this paper to uphold the integrity of the field, but we need to do better in the field of nutrition to register, design, conduct, and report trials to a high standard before publication,” he said.
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