Research homes in on dietary link to heart ailment in dogs

Much evidence suggests cause-effect in dilated cardiomyopathy, but 'why' is a puzzle

May 3, 2023 (published)
Photo by John Pridham
Toby, age 5½, was so sick with a heart condition last fall that his owner thought he would die. But after being put on medication and switching kibbles, Toby improved. Today, he's back to taking long walks in the woods.

Toby showed up at Dr. David MacDonald's veterinary clinic in rural British Columbia in October seeming near death. The once athletic 5-year-old golden retriever appeared exhausted. His heart rhythm was off. The doctor could barely feel a pulse in the patient's femoral artery (located inside the hind leg). He suspected cardiac disease.

A few weeks later, MacDonald saw a second golden retriever in the same shape. Also common to both, the dogs had been fed exclusively, for four to five years, a particular brand of kibble marketed as grain-free.

MacDonald interpreted the combination of signs and dietary history as textbook instances of canine diet-associated dilated cardiomyopathy, or DCM — a phenomenon he learned about when the U.S. Food and Drug Administration issued an alert in 2018. In the years since, MacDonald heard from colleagues about cases in specialty practices but was startled to witness back-to-back occurrences in his general practice.

"My thought was that if I've seen two classic cases walk through the door in little ol' Courtenay, Vancouver Island, in North America, the number of dogs that have become ill from this must be thousands, if not tens of thousands," he surmised.

The closest thing that exists to an official tally is nowhere near that number, however. As of Nov. 1, the FDA logged 1,382 reports of DCM diagnoses in dogs, dating from Jan. 1, 2014.  

The number is but a loose approximation. The figure could be an overestimate because it includes cases for which detailed medical records weren't submitted to the agency. And presumably, the reports were of suspected diet-associated illness, but that's uncertain. At the same time, the figure could be an underestimate because some reports involved multiple dogs. Also, the FDA didn't count reports involving early signs of heart abnormalities. Moreover, veterinarians are not required to report cases to the government.

In brief

Although the matter is far from resolved, the FDA has gone mum. The agency posted late last year that it "does not intend to release further public updates until there is meaningful new scientific information to share." It's apparently trying to stay out of a proverbial food fight, as pet food companies and suppliers deeply invested in the sales of suspected food formulations push back, sometimes parsing FDA statements to support their position.

Today, foods for dogs and cats marketed as grain-free remain abundantly available. Even companies such as Mars and Nestlé Purina — major players in the corn-, wheat-, barley- and rice-using commercial pet food establishment — sell grain-free options.

In 2019, the year after the FDA alert, sales of grain-free pet foods dipped by 0.3% compared with 2018, according to Statista. But the sector rebounded, growing by 2.9% in 2021 over 2020. Future Market Insights predicts the value of the segment will rise from $38.6 billion in 2022 to $53.2 billion by 2023.

The statistics suggest that consumers overall discount the potential risk — or aren't aware of it or are confused.

Complicating the picture, research suggests that the problem may not be grain-free per se but ingredients used in lieu of conventional grains. Typically, those are pulses, which are the seeds of legumes such as peas, chickpeas and lentils. 

Still, the research is inconclusive. Five years since the FDA flagged aspects of grain-free diets as potentially problematic, multiple studies indicate that such foods, when eaten predominantly and consistently over time, may lead to cardiac changes in some dogs — likely a small percentage. With the risk factors unclear, many veterinarians believe the cause-and-effect picture is compelling enough to steer clear of the implicated foods. But no one has pinpointed what, exactly, about the foods is the issue.

Change the food, heal the patient

DCM is a life-threatening disease in which the myocardium, or muscle tissue of the heart, contracts less strongly, causing it to pump blood less efficiently. To compensate for the muscle weakness, the heart enlarges and may beat faster. The condition can lead to congestive heart failure, characterized by irregular heart rhythm and fluid buildup. Dogs with DCM may be lethargic, cough and pant. They might faint, collapse or suddenly die.

The condition is not rare, but historically, it was seen primarily in genetically predisposed breeds, including Great Danes, Irish wolfhounds and Doberman pinschers. With inherited DCM, the condition is irreversible; its signs might be controlled with medication, but the disease state remains.

Since 2018, some 20 peer-reviewed studies have been published on diet-associated DCM in dogs, and about 10 more on related aspects. Multiple studies have detected physiological differences between dogs fed certain relatively novel food formulations compared with those fed conventional grain-containing formulations. The differences relate to heart function, blood chemistry and metabolites.

The most persuasive evidence that food is to blame is that sick dogs have improved and even recovered after changing diets, documented in at least eight studies.

Read the studies

That's what happened with MacDonald's two patients in Courtenay, B.C. Both dogs, from different households, had been eating an Acana brand of grain-free kibble. Both came to the clinic very ill. Both improved with supportive care and a change of food.

The first, Toby, had been active and athletic most of his life. "In our village, we have great mountain bike trail systems ... and we were out on them pretty much every day, and he'd be doing up to 10, 12 kilometers a day," said his owner, John Pridham. "We'd hike some days, I'd bike some days and he'd trot along. He was in great shape."

Last summer, Toby lost his appetite. He began throwing up. Upon finding blood and mucous in the vomitus, Pridham took Toby to the clinic, thinking his dog had a gastrointestinal problem. The first veterinarian who examined Toby thought the same. But medication given to quell vomiting didn't help.

By autumn, Toby was restless, panting nonstop, and had lost a lot of weight. Previously 100 pounds, he'd dropped to 80. His backbone protruded; his stomach distended. Pridham took Toby back to the clinic, this time seeing MacDonald.

"Dr. Dave immediately recognized the problem and said he had congestive heart failure," Pridham recounted. "His lungs were full of fluid. His heart was pounding and large. [The doctor] did an X-ray and ultrasound and said, 'Yeah, it's just mush. ... We might have to put him down.' "

Heavy-hearted, Pridham made an appointment in two days for a checkup, expecting Toby would be euthanized. MacDonald prescribed a heart medication, pimobendan, which helps the heart muscle contract more strongly and aids blood flow, plus a diuretic, furosemide, and a nutritional supplement, taurine. The veterinarian also advised Pridham to stop feeding the Acana grain-free kibble.

To Pridham's happy surprise, Toby had a swift turnaround. "He really changed in those two days," Pridham said. "He really kind of perked up. He lost seven pounds of fluid, and he was breathing better and was more interested in food."

Toby had switched to a Royal Canin therapeutic kibble obtained from the clinic. Today, he is 92 pounds and back on the trail, regularly taking walks of six to eight kilometers (four to five miles). He remains on medication and supplements as his heart continues to heal.

On a list that the FDA shared in 2019 of pet food brands most often named in reports to the agency of DCM in dogs, Acana was at the top, accounting for 67 instances up to that time.

Last week, the veterinary blog AllTradesDVM posted updated figures derived from FDA documents obtained through Freedom of Information Act requests by two other sources. The update shows Acana still at the top of the list of implicated dog food brands, with 183 mentions as of November 2022. Taste of the Wild was second, with 167, and Zignature third, with 161.

Opinions among veterinarians vary

Asked for comment, Champion Petfoods, maker of Acana, replied by email: "The emerging science indicates that non-hereditary DCM is a complex medical condition that may be affected by multiple factors such as genetics, underlying medical conditions and diet. And, to date, this science has not established a causal connection between grain-free diets and DCM. Champion Petfoods believes grain-free foods are safe and nutritionally complete." The company added that it sells grain-inclusive options, too.

Last spring, Champion debuted an Orijen brand Amazing Grains legume-free line. The grains are a blend "including oats, quinoa and chia." The company said it has sold grain-containing formulations for decades.

Other data released by the FDA in 2019 showed golden retrievers as the breed most often represented in reports of DCM to regulators. That fact has remained unchanged. The updated data shows golden retrievers named in 194 reports.

Pridham knew nothing of this until his golden, Toby, nearly died. "He wouldn't have been on that food if I'd heard about it," he said. "I thought it was great — grain-free, it's supposed to be good. At least, that's the fad, anyway. It had good listed ingredients. For over four years, he thrived on it. Until he didn't."

Ingredient nuance

What could be the culprit? In addition to pulses, potatoes and sweet potatoes have been implicated, though to a lesser degree. The issue may be "pulse-full" rather than grain-free. Dr. Lisa Freeman, a veterinary nutritionist at Tufts University Cummings School of Veterinary Medicine, writing in a Feb. 7 post of the blog Petfoodology, noted that "now some grain-inclusive diets contain pulses and can be associated with DCM as well."

Why would pulses harm dogs? That's the unanswered question. Nutrition scientists are digging deep into the biochemistry of pulse digestion, scrutinizing metabolites in search of answers. Sorting through the variables is laborious. Freeman, co-author of about 10 published studies on the subject, writes in the blog: "We have a lot more to learn about their effects on dogs eating diets high in these ingredients (for example, concentration in the diet, effects of processing, and effects of different fractions of peas such as whole peas, pea protein, pea fiber, etc.)."

Cardiology researchers are probing the mystery, as well. One of the more noteworthy studies to date, published in December, compared heart function in 23 outwardly healthy pet dogs eating "nontraditional" kibble with that of 23 outwardly healthy pet dogs eating traditional kibble. All of the dogs had been on their respective kibbles for at least one year and obtained more than 90% of their daily calories from those foods.

Through echocardiograms, the research team — based at Oregon State University Carlson College of Veterinary Medicine and supported by grants from the university and the American College of Veterinary Internal Medicine — measured how effectively the dogs' hearts pumped blood. They found that, taken together, hearts of the nontraditional diet group pumped less effectively than those of the traditional diet group.

From this, the researchers concluded: "Screening of apparently healthy dogs eating nontraditional diets might allow for early detection of diet-associated DCM."

Dr. Mark Kittleson, a veterinary cardiologist not involved with the research, called it "the best clinical study to date" because it prospectively compared "a reasonably sized group of dogs" on a nontraditional diet with a matched set of dogs on a traditional diet "and did a very comprehensive echo exam on each dog."

Less impressed, Dr. Mark Rishniw, a veterinary cardiologist at Cornell University College of Veterinary Medicine, said that without "before" measurements of each dog's heart function, it's impossible to know whether their heart muscles truly were in the early stages of being compromised.

Rishniw said the results would be more compelling, too, if the study had involved small dogs only. (The subjects ranged from about 37 to 77 pounds.) Kittleson had the same concern. Size matters because hereditary DCM tends to strike larger dogs. "If you had little dogs and these measurements, I'd be really concerned," Rishniw said.

Contrary findings

A study from researchers at the University of Guelph Ontario Veterinary College, published in March, came to a different conclusion. In that research, scientists divided 28 adult Siberian huskies into four groups, each fed for 20 weeks one of four experimental kibbles formulated by Champion Petfoods.

One diet, the control, was composed of 33% corn and zero whole pulses. The other three diets were grain-free and contained 15%, 30% and 45% whole pulses, respectively. The pulses were a combination of green and yellow peas, pinto beans, chickpeas and lentils.

Before and after the feeding trial, the dogs' hearts were examined on echocardiograms, and other health metrics were collected. All looked well. The researchers concluded that a grain-free diet containing up to 45% pulses "is safe for healthy adult dogs to consume when fed for 20 wks."

Acknowledging the participation and funding by Champion Petfoods, the study authors state that "this financial support did not influence the findings or conclusions."

A university news release touted the findings as "reassuring ... for dog owners worried about grain-free diets, which have surged in popularity and now make up almost half of the dog food market in Canada."

Some outside observers weren't reassured. A veterinarian who blogs as Dr. Caitlin Marie on AllTradesDVM noted that the study involved only Siberian huskies, a "performance breed" and "genetically unique population." She highlighted that the diets used in the study were experimental and potentially of higher nutritional quality than what's on store shelves. "While it may be possible to formulate a diet with high amounts of pulse legumes safely, it doesn't mean that diets with high amounts of pulse legumes are currently formulated safely," she wrote.

Dr. Paul Pion, a veterinary cardiologist who, with colleagues including Kittleson, published groundbreaking work in 1987 on DCM in cats, said a 20-week study isn't enough to plumb potential long-term effects of such a diet. Moreover, given the apparently low prevalence of the disease, an absence of changes in 21 dogs is not sufficient to conclude that dogs in general may eat the food risk-free, he said.

Anna Kate Shoveller, a nutritionist and corresponding author on the University of Guelph research team, said by email that the group aimed to control confounding variables, "hence the choice of a single group of homogenous dogs that are not genetically pre-disposed to DCM." She added that the dogs were kept "in the same environment, fed to maintain body weight and managed by the same team for six months to ensure consistency of management."

To the criticism that the experimental diets didn't represent commercial foods, she said they weren't intended to. "[T]hey were meant to be able to remove confounding variables of nutrient density so that we could only test whether pulses are resulting in a reduction in any measure of metabolism and health suggestive of the development of disease."

Regarding the study duration, she said: "While we can always point to more, 20 weeks is a long period of time for a controlled study and the longest controlled study to date. Nutrient deficiencies or toxic compound ingestion (i.e., consumption of anti-nutritional factors such as lectins) would be expected to start to show at least minor changes in one of these outcomes that would be suggestive of an alteration or dysregulation of metabolism."

Pion said an appropriately qualified conclusion for the study would be, " 'There was no effect seen in Siberian husky dogs in 20 weeks.' You can't extrapolate to all dogs from Siberian huskies. You can just say, 'Nothing happened in 20 weeks.' You can't say, 'It's safe.' "

No longer in research, Pion is cofounder and president of the Veterinary Information Network, an international online community for the profession and the parent of the VIN News Service. VIN is supported by its membership and does not accept commercial sponsorships.

In Pion's previous work with cats, he and colleagues discovered that deficiency in taurine, an amino acid, can lead to feline DCM. Following early resistance from the pet food industry, taurine became a standard addition in commercial cat food, eliminating a major cause of the disease — in cats, that is.

What about taurine deficiency in dogs?

The story about dogs is more complicated. In cats, taurine is an essential amino acid. Without eating foods rich in taurine, they cannot maintain sufficient levels for their health. Dogs are better able to synthesize taurine from other amino acids but some breeds appear to be cat-like in susceptibility to taurine deficiency.

Veterinary researchers at the University of California, Davis, reported in a study published in 2018 that they were able to reverse DCM in golden retrievers who had been eating grain-free and/or legume-rich diets by changing their food and supplementing with taurine. Goldens are known to be susceptible to taurine deficiency and DCM, and all of the dogs in the study had low blood levels of taurine before treatment.

A few more breeds — cocker spaniels, Newfoundlands, English setters and Labrador retrievers — also have been reported to respond well to taurine. But taurine supplementation does not appear to be the answer to reversing DCM in all breeds.

Still, when the news broke of apparent diet-related DCM in dogs, pet food makers reached for the amino acid in hopes it would do the trick. "Many manufacturers rushed to add taurine to foods that previously did not contain it," said Dr. Leslie Hancock, a veterinarian who has worked for several pet food companies and chairs the nutrition subcommittee at the trade group Pet Food Institute.

Industry involvement in the science

Hancock said many in the industry have put effort and money into solving the puzzle. Sponsors of peer-reviewed, published studies include Mars Petcare, Nestlé Purina Petcare and Champion Petfoods.

VIN News asked Mars whether it is doing further research following its study, Responses in randomised groups of healthy, adult Labrador retrievers fed grain-free diets with high legume inclusion for 30 days display commonalities with dogs with suspected dilated cardiomyopathy, published in April 2022.

Mars replied by email, "Further studies are needed to increase understanding of the relevance of the findings and further details regarding the pathophysiology associated with the development of diet-associated DCM in dogs" but it declined to say whether it would do that research itself.

The company confirmed that it offers grain-free pet foods, saying that it sells a "broad range of products ... to ensure every pet parent has the choice of high-quality, complete and balanced diets for their pet. This also includes grain-free products."

Mars also recently purchased Champion Petfoods. The deal strikes MacDonald, the veterinarian in British Columbia, as perplexing and ironic: His golden retriever patients became sick while on Champion's Acana brand and recovered on Royal Canin, a long-time Mars brand.

"Royal Canin's mottos have been 'incredible in every detail' and 'stubbornly scientific,' " MacDonald observed. "How will Mars dovetail its RC brand with the marketing of grain-free diets for years despite FDA warnings and thousands of gravely sick dogs?"

Mars provided a written general reply in response to MacDonald's question: "We provide healthy, quality, safe pet food that follows or exceeds all FDA standards. We always monitor new findings and work closely with the wider pet food industry. Mars Petcare has offered grain-free and grain-inclusive products for a number of years. We provide high quality, science-backed nutrition and therapeutic health products to meet the individual needs of pets across the world."

VIN News attempted to reach Nestlé Purina, too, for its views on diet-associated DCM. Company officials did not respond to multiple messages by email, telephone and text. Purina states on its website: "While whole grains are an excellent source of healthy energy in most dogs, we understand that some owners prefer to feed food without grains. That's why we've created a nutritious grain-free recipe that's full of the great taste dogs love."

VIN News Service screenshot
Over Zoom, Dr. David MacDonald described two cases of dilated cardiomyopathy in golden retrievers seen last fall at his clinic in British Columbia. While MacDonald has encountered a few instances of DCM in breeds that are genetically predisposed, these two, he said, were "classic cases" of diet-associated illness.

'All a hoax'?

After MacDonald's two very sick golden retriever patients got better with medicine and a change in kibbles, he sent an email heads-up to clients who own goldens to check their dog food.

The reaction startled him. "Some of the responses were surprisingly unreceptive to just a bit of advice," he recounted, crestfallen. Searching the internet, he saw why. "Online, you get two different stories — the story of the cardiomyopathy, and the other [view] on social media that this is all a hoax."

Pridham, Toby's owner, also had strong feelings when he heard about the link between food and DCM, but they weren't directed at the veterinarian. "I was mad at myself for choosing that [food]. I thought it was good!" he said.

After Toby's diagnosis, Pridham took his unfinished bag of Acana kibble to the pet store where he'd bought it. "I said, 'This food is killing him.' "

The store accepted the return, and employees listened politely when Pridham suggested, "You might steer [customers] to another food if you find out they have golden retrievers." Whether they took the advice, he doesn't know.

Pridham also tells Toby's story to other golden retriever owners he meets. He urges them to check their food and consider switching if it contains lentils or other pulses. He notes that treating a dog for DCM is expensive.

"It's $350 a month for a special diet and medications, which isn't insignificant," Pridham said. "I don't begrudge it, but if you can avoid that by changing diets, why wouldn't you?"

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