Research paper retraction stirs copper safety debate

Study led by pet food company had found no risk to dog health

Published: July 01, 2024

The retraction of a paper from a prestigious veterinary journal for the first time in its 147-year history is stoking discussion about the reliability of research conducted by large corporations — and whether some dog food contains unsafe levels of copper.

The paper, entitled "Sixteen years of canine hepatic copper concentrations within normal reference ranges in dogs fed a broad range of commercial diets," was published online on March 7 in the Journal of the American Veterinary Medical Association (JAVMA). It was retracted on May 21.

Authored largely by researchers employed by pet food giant Hill's Pet Nutrition, the study found that dogs fed a variety of commercial dog foods throughout their lives had normal copper concentrations in their livers. Copper is an essential nutrient for dogs and other animals, including humans, but an accumulation of excessive amounts of the metal can cause liver disease.

The paper's authors voluntarily requested the retraction after being made aware by the journal that it had received a number of letters to the editor from veterinarians maintaining the paper has major flaws. The VIN News Service knows of four letters, at least three of which are signed by three veterinarians each.

In their retraction notice, the paper's authors said they subsequently discovered new data that had inadvertently been left out of their analysis.

"Unfortunately, with further investigation, new data were discovered that materially change the results of the article as it reads today," the authors said. "As a result of these findings, it is necessary to make corrections to uphold the integrity of scientific research and maintain the trust of our readers and the veterinary community."

The mea culpa was first reported by Retraction Watch, a blog that tracks retractions of scientific papers.

In brief

It's not unusual for research conducted by businesses pertaining to their own products to be published in respected scientific journals, so long as the conflict of interest is clearly disclosed — as was the case with the now-retracted paper, whose four authors include Dr. Leslie Hancock, chief medical officer at Hill's. The paper was peer-reviewed by two unnamed scientists who presumably had no association with Hill's.

The paper's publication and subsequent retraction come as pet food companies face increased scrutiny from some veterinary internal medicine specialists who have been contending for years that high copper levels in some commercial dog foods are sickening pets. The veterinarians have been unable to convince authorities in the United States to impose a maximum copper limit in dog food, or even establish a voluntary copper-reporting framework.

Those veterinarians are among those who wrote to JAVMA about the now-retracted paper. They argue that its flaws can't be explained by missing data alone. And they are disappointed that none of their letters was published by JAVMA, the owner of which, the AVMA, counts Hill's as a sponsor of its annual convention.

The journal's editor-in-chief, Dr. Lisa Fortier, told VIN News that she followed what she considered to be the right process for a "super rare" situation.

"In the process of following contemporary publishing guidelines in issuing the retraction, I also called several other editors who may or may not have dealt with a retraction in the past to get their advice," she said, while noting that there was no requirement to publish the letters to the editor. "In my opinion, and the opinion of other scientists and editors, the process worked. The paper was published, a flaw was found and the paper was retracted."

Still, Fortier said she respected everyone's views. "I understand that not all of the points raised in the letters to the editor were addressed in the retraction."

Consequently, Fortier told VIN News, JAVMA has decided to publish a single letter to the editor that will combine key components of the letters it received. Hill's will be given the opportunity to respond. The merged letter and the company's response will be published in the September issue of JAVMA, online and in print, with a brief introduction by Fortier explaining the retraction process.

Letter writers who spoke to VIN News have mixed views on how the situation has evolved. Some acknowledge the journal is traversing unusual terrain. Others maintain it is belatedly publishing the combined letter in an attempt to save face.

"I think they're only doing this not because it's right but to try to backtrack on mistakes that were made from the beginning," said Dr. Daniel Langlois, an associate professor at Michigan State University's veterinary school. Langlois contends JAVMA should not have allowed the authors to voluntarily retract the "horribly flawed" paper with what he says was an inadequate explanation that didn't address letter writers' concerns.

One aspect of the research that Langlois and other letter writers say is flawed is that its results aren't indicative of dogs in general because 90% of its 336 subjects were research beagles. Among their other concerns is that many of the research subjects were found to have unusually low copper levels in their liver at the end of the study, which, the letter writers contend, points to problems with how copper levels were assessed. Several letter writers, for instance, said that the researchers didn't give liver samples enough time to dry before copper levels were measured.

"There are a lot of scientific flaws in this paper that cannot and will not be addressed by a missing data set," said Dr. Keith Richter, the founder of a specialty hospital in San Diego who is now retired but working on copper-related projects. "They were analyzing faulty data in a faulty way from a faulty population."

Dr. David Twedt, a professor emeritus at Colorado State University's veterinary school, questioned the veracity of the peer-review process. "It's not necessarily the fault of the journal, but I do think the review process was flawed, and the reviewers did not do due diligence in looking at what scientifically was done with this paper."

Richter said that although he's unhappy the letters weren't published sooner, he respects that Fortier was in a tough spot. "I think she's trying to do the right thing, and will take the scientific high road," he said.

Dr. Sharon Center, a professor emeritus at Cornell University's veterinary school, urged patience. "As this is a first-time event for JAVMA and for its relatively new editor-in-chief, we need to give them space to navigate this awkward circumstance: the retraction of a paper authored by employees of a major dog food producer, one author a chief medical officer."

Hill's declined to make any of the paper's authors available for interview, or address any of the specific concerns raised by the letter writers. The company, however, confirmed that it was responding to the combined letter, as well as correcting the paper. "We will submit a revised manuscript sharing the full results of this meaningful study in a timely manner," Hill's said in an email. "We look forward to the opportunity to share corrected data."

VIN News Service commentaries are opinion pieces presenting insights, personal experiences and/or perspectives on topical issues by members of the veterinary community. To submit a commentary for consideration, email

Information and opinions expressed in letters to the editor are those of the author and are independent of the VIN News Service. Letters may be edited for style. We do not verify their content for accuracy.